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   -> JavaScript知识库 -> 史上最全的vue.js源码解析最终篇(四) -> 正文阅读

[JavaScript知识库]史上最全的vue.js源码解析最终篇(四)

虽然vue3已经出来很久了,但我觉得vue.js的源码还是非常值得去学习一下。vue.js里面封装的很多工具类在我们平时工作项目中也会经常用到。所以我近期会对vue.js的源码进行解读,分享值得去学习的代码片段,这篇文章将会持续更新。

一、6200~结束代码有哪些内容?:

patch、patchVnode 函数:更新节点
_update:将vnode转换成dom,渲染在视图中
updateDirectives:获取指令钩子,和对不同钩子进行不同处理
e.stopImmediatePropagation():该方法作用在当前节点及事件链的所有后续节点上,目的是在执行完当前事件处理程序后,停止当前节点及所有后续节点的同类事件处理程序的运行
getOuterHTML
解决获取该DOM本身以及包含的HTML代码,没有outerHTML属性会返回undefine的问题
processElement:处理key、ref、内外部插槽、绑定属性、组件的判断
processRef:处理ref
processFor:处理v-for指令
parseFor:解析 v-for 表达式
processIf:处理el的条件 v-if v-else v-else-if
processOnce:处理v-once
processSlotContent:处理传递给组件的插槽内容
processAttrs:处理元素的属性列表 包括指令和非指令

二.6200~结束代码的重点:

1.patch组件的更新
组件更新的过程核心就是新旧 vnode diff,对新旧节点相同以及不同的情况分别做不同的处理。新旧节点不同的更新流程是创建新节点->更新父占位符节点->删除旧节点;而新旧节点相同的更新流程是去获取它们的 children,根据不同情况做不同的更新逻辑。最复杂的情况是新旧节点相同且它们都存在子节点,那么会执行 updateChildren 逻辑,根据是否是sameVnode或者依据key尽可能地复用DOM节点
2.过滤器filter的解析:
parseFilters:解析filter
wrapFilter:转换过滤器格式
3.v-bind的函数:
属性添加函数:getRawBindingAttr
属性获取函数:getBindingAttr
属性操作函数:getAndRemoveAttr
4.vue模板编译
createCompilerCreator
baseCompile
5.vue的编译器:
Vue 的编译器都做了什么?
①将组件的 html 模版解析成 AST 对象
②优化,遍历 AST,为每个节点做静态标记,标记其是否为静态节点,然后进一步标记出静态根节点,这样在后续更新的过程中就可以跳过这些静态节点了;标记静态根用于生成渲染函数阶段,生成静态根节点的渲染函数
③从 AST 生成运行渲染函数,即大家说的 render,其实还有一个,就是 staticRenderFns 数组,里面存放了所有的静态节点的渲染函数

6.过渡动画钩子函数
先执行 beforeEnterHook 钩子,并将 DOM 节点 el 传入,随即判断是否希望通过 css 来控制动画,如果是 true 则,执行 addTransitionClass() 方法为节点加上 startClass 和 activeClass。
然后执行 nextFrame() 进入下一帧,下一帧主要是移除掉上一帧增加好的 class;随即判断过渡是否取消,如未取消,则加上 toClass 过渡类;之后如果用户没有通过 enterHook 钩子函数来控制动画,此时若用户指定了 duration 时间,则执行 setTimeout 进行 duration 时长的延时,否则执行 whenTransitionEnds 决定 cb 的执行时机

三、6200~结束的代码解读:

    function createComponent (vnode, insertedVnodeQueue, parentElm, refElm) {
      var i = vnode.data;
      if (isDef(i)) {
        var isReactivated = isDef(vnode.componentInstance) && i.keepAlive;
        if (isDef(i = i.hook) && isDef(i = i.init)) {
          i(vnode, false /* hydrating */);
        }
        // 调用init钩子后,如果vnode是子组件它应该创建一个子实例并挂载它。这个子组件也设置了占位符vnode的榆树。
        //在这种情况下,我们只需要返回元素就可以了。
        if (isDef(vnode.componentInstance)) {
          initComponent(vnode, insertedVnodeQueue);
          insert(parentElm, vnode.elm, refElm);
          if (isTrue(isReactivated)) {
            reactivateComponent(vnode, insertedVnodeQueue, parentElm, refElm);
          }
          return true
        }
      }
    }

    function initComponent (vnode, insertedVnodeQueue) {
      if (isDef(vnode.data.pendingInsert)) {
        insertedVnodeQueue.push.apply(insertedVnodeQueue, vnode.data.pendingInsert);
        vnode.data.pendingInsert = null;
      }
      vnode.elm = vnode.componentInstance.$el;
      
      if (isPatchable(vnode)) {
        // div#app的创建时会调用invokeCreateHooks
        invokeCreateHooks(vnode, insertedVnodeQueue);
        setScope(vnode);
      } else {
        // 根空的组件。跳过所有元素相关的模块,除了ref
        registerRef(vnode);
        // 确保调用了插入钩子
        insertedVnodeQueue.push(vnode);
      }
    }

    function reactivateComponent (vnode, insertedVnodeQueue, parentElm, refElm) {
      var i;
      // 重新激活的内部转换组件不触发,因为内部节点创建的钩子没有被调用一次。
      var innerNode = vnode;
      while (innerNode.componentInstance) {
        innerNode = innerNode.componentInstance._vnode;
        if (isDef(i = innerNode.data) && isDef(i = i.transition)) {
          for (i = 0; i < cbs.activate.length; ++i) {
            cbs.activate[i](emptyNode, innerNode);
          }
          insertedVnodeQueue.push(innerNode);
          break
        }
      }
      // 与新创建的组件不同,重新激活的keep-alive组件不插入自己
      insert(parentElm, vnode.elm, refElm);
    }
    // 通过insertBefore或者appendChild添加元素
    function insert (parent, elm, ref$$1) {
      if (isDef(parent)) {
        if (isDef(ref$$1)) {
          if (nodeOps.parentNode(ref$$1) === parent) {
            nodeOps.insertBefore(parent, elm, ref$$1);
          }
        } else {
          nodeOps.appendChild(parent, elm);
        }
      }
    }
/*****************************************************************************************************************************/
    // 创建vnode对应 的子节点并添加到vnode对应的DOM上
    function createChildren (vnode, children, insertedVnodeQueue) {
      // 如果children是数组
      if (Array.isArray(children)) {
        {
          // 检查是否存在相同key
          checkDuplicateKeys(children);
        }
        for (var i = 0; i < children.length; ++i) {
          createElm(children[i], insertedVnodeQueue, vnode.elm, null, true, children, i);
        }
      } else if (isPrimitive(vnode.text)) {
         // 如果vnode.是原始值text
        nodeOps.appendChild(vnode.elm, nodeOps.createTextNode(String(vnode.text)));
      }
    }

    function isPatchable (vnode) {
      while (vnode.componentInstance) {
        vnode = vnode.componentInstance._vnode;
      }
      return isDef(vnode.tag)
    }
    // 调用执行所有子组件相关的 create 钩子
    function invokeCreateHooks (vnode, insertedVnodeQueue) {
      // 遍历执行cbs.create里面的函数,触发模块中的钩子函数
      for (var i$1 = 0; i$1 < cbs.create.length; ++i$1) {
        cbs.create[i$1](emptyNode, vnode);
      }
      i = vnode.data.hook; // Reuse variable
      if (isDef(i)) {
        //触发hook中的create钩子函数
        if (isDef(i.create)) { i.create(emptyNode, vnode); }
        //将hook中的insert钩子函数添加到insertedVnodeQueue中
        if (isDef(i.insert)) { insertedVnodeQueue.push(vnode); }
      }
    }

    // 为作用域CSS设置作用域id属性。这是作为一种特殊情况实现的,以避免正常属性修补过程的开销
    function setScope (vnode) {
      var i;
      if (isDef(i = vnode.fnScopeId)) {
        nodeOps.setStyleScope(vnode.elm, i);
      } else {
        var ancestor = vnode;
        while (ancestor) {
          if (isDef(i = ancestor.context) && isDef(i = i.$options._scopeId)) {
            nodeOps.setStyleScope(vnode.elm, i);
          }
          ancestor = ancestor.parent;
        }
      }
      // 对于插槽内容,它们还应该从主机实例获取scopeId.
      if (isDef(i = activeInstance) &&
        i !== vnode.context &&
        i !== vnode.fnContext &&
        isDef(i = i.$options._scopeId)
      ) {
        nodeOps.setStyleScope(vnode.elm, i);
      }
    }
    // 将新节点的数据,一一创建节点,更新Dom
    function addVnodes (parentElm, refElm, vnodes, startIdx, endIdx, insertedVnodeQueue) {
      for (; startIdx <= endIdx; ++startIdx) {
        createElm(vnodes[startIdx], insertedVnodeQueue, parentElm, refElm, false, vnodes, startIdx);
      }
    }
    // 递归调用自己和子节点的remove钩子和destroy钩子
    function invokeDestroyHook (vnode) {
      var i, j;
      var data = vnode.data;
      if (isDef(data)) {
        // destroy钩子函数
        if (isDef(i = data.hook) && isDef(i = i.destroy)) { i(vnode); }
        for (i = 0; i < cbs.destroy.length; ++i) { cbs.destroy[i](vnode); }
      }
      // 递归处理子节点
      if (isDef(i = vnode.children)) {
        for (j = 0; j < vnode.children.length; ++j) {
          invokeDestroyHook(vnode.children[j]);
        }
      }
    }
    //将老节点全部删除
    function removeVnodes (vnodes, startIdx, endIdx) {
      for (; startIdx <= endIdx; ++startIdx) {
        var ch = vnodes[startIdx];
        if (isDef(ch)) {
          if (isDef(ch.tag)) {
            //调用移除事件回调函数并移除节点
            removeAndInvokeRemoveHook(ch);
            // invokeDestroyHook 是执行 module 的 destory 钩子函数以及 vnode 的 destory 钩子函数,并对它的子 vnode 递归调用 invokeDestroyHook 函数
            invokeDestroyHook(ch);
          } else {
            // 就是调用平台的 DOM API 去把真正的 DOM 节点移除
            removeNode(ch.elm);
          }
        }
      }
    }
    // 递归处理了子节点的属性并删除 dom 节点
    function removeAndInvokeRemoveHook (vnode, rm) {
      if (isDef(rm) || isDef(vnode.data)) {
        var i;
        var listeners = cbs.remove.length + 1;
        if (isDef(rm)) {
        //  有一个递归传递的rm回调函数,用于增加侦听器计数
          rm.listeners += listeners;
        } else {
          // directly removing
          rm = createRmCb(vnode.elm, listeners);
        }
        // 递归调用子组件根节点上的钩子
        if (isDef(i = vnode.componentInstance) && isDef(i = i._vnode) && isDef(i.data)) {
          removeAndInvokeRemoveHook(i, rm);
        }
        for (i = 0; i < cbs.remove.length; ++i) {
          cbs.remove[i](vnode, rm);
        }
        if (isDef(i = vnode.data.hook) && isDef(i = i.remove)) {
          i(vnode, rm);
        } else {
          rm();
        }
      } else {
        removeNode(vnode.elm);
      }
    }
    // 如果新老节点都有子节点,但是不相同的时候就会调用 updateChildren,这个函数通过diff算法尽可能的复用先前的DOM节点
    // 以新Node 为标杆 oldNode 为 真实DoM的映射(尽可能的利用已有的oLdNode的资源)最大程度的减少真实dom的操作
    function updateChildren (parentElm, oldCh, newCh, insertedVnodeQueue, removeOnly) {
      var oldStartIdx = 0;
      var newStartIdx = 0;
      var oldEndIdx = oldCh.length - 1;
      var oldStartVnode = oldCh[0];
      var oldEndVnode = oldCh[oldEndIdx];
      var newEndIdx = newCh.length - 1;
      var newStartVnode = newCh[0];
      var newEndVnode = newCh[newEndIdx];
      var oldKeyToIdx, idxInOld, vnodeToMove, refElm;
      // removeOnly是一个特殊标志,仅由<transition group>使用,以确保在离开转换期间被删除的元素保持在正确的相对位置
      var canMove = !removeOnly;
        //检查重复节点
      {
        checkDuplicateKeys(newCh);
      }
      while (oldStartIdx <= oldEndIdx && newStartIdx <= newEndIdx) {
        if (isUndef(oldStartVnode)) {
         
          oldStartVnode = oldCh[++oldStartIdx]; // Vnode has been moved left
        } else if (isUndef(oldEndVnode)) {
          oldEndVnode = oldCh[--oldEndIdx];
          // oldStartVnode和newStartVnode进行比对,如果相似,则进行patch,然后新旧头索引都后移
        } else if (sameVnode(oldStartVnode, newStartVnode)) {
          patchVnode(oldStartVnode, newStartVnode, insertedVnodeQueue, newCh, newStartIdx);
          oldStartVnode = oldCh[++oldStartIdx];
          newStartVnode = newCh[++newStartIdx];
          // oldEndVnode和newEndVnode进行比对,如果相似,则进行patch,然后新旧尾索引前移
        } else if (sameVnode(oldEndVnode, newEndVnode)) {
          patchVnode(oldEndVnode, newEndVnode, insertedVnodeQueue, newCh, newEndIdx);
          oldEndVnode = oldCh[--oldEndIdx];
          newEndVnode = newCh[--newEndIdx];
          // oldStartVnode和newEndVnode进行比对,如果相似,则进行patch,将旧节点移位到最后,然后旧头索引后移,尾索引前移
        } else if (sameVnode(oldStartVnode, newEndVnode)) { // Vnode moved right
          patchVnode(oldStartVnode, newEndVnode, insertedVnodeQueue, newCh, newEndIdx);
          canMove && nodeOps.insertBefore(parentElm, oldStartVnode.elm, nodeOps.nextSibling(oldEndVnode.elm));
          oldStartVnode = oldCh[++oldStartIdx];
          newEndVnode = newCh[--newEndIdx];
          // oldEndVnode和newStartVnode进行比对,处理和上面类似,只不过改为左移
        } else if (sameVnode(oldEndVnode, newStartVnode)) { // Vnode moved left
          patchVnode(oldEndVnode, newStartVnode, insertedVnodeQueue, newCh, newStartIdx);
          canMove && nodeOps.insertBefore(parentElm, oldEndVnode.elm, oldStartVnode.elm);
          oldEndVnode = oldCh[--oldEndIdx];
          newStartVnode = newCh[++newStartIdx];
        } else {
          // 通过createKeyToOldIdx创建key-index的映射,如果新节点在旧节点中不存在,我们将它插入到旧头索引节点前,然后新头索引向后,
          // 如果新节点在就旧节点组中存在,先找到对应的旧节点,然后patch,并将旧节点组中对应节点设置为undefined,代表已经遍历过了,
          // 不再遍历,否则可能存在重复插入的问题
          if (isUndef(oldKeyToIdx)) { oldKeyToIdx = createKeyToOldIdx(oldCh, oldStartIdx, oldEndIdx); }
          idxInOld = isDef(newStartVnode.key)
            ? oldKeyToIdx[newStartVnode.key]
            : findIdxInOld(newStartVnode, oldCh, oldStartIdx, oldEndIdx);
          if (isUndef(idxInOld)) { // New element
            createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx);
          } else {
            vnodeToMove = oldCh[idxInOld];
            if (sameVnode(vnodeToMove, newStartVnode)) {
              patchVnode(vnodeToMove, newStartVnode, insertedVnodeQueue, newCh, newStartIdx);
              oldCh[idxInOld] = undefined;
              canMove && nodeOps.insertBefore(parentElm, vnodeToMove.elm, oldStartVnode.elm);
            } else {
              // same key but different element. treat as new element
              createElm(newStartVnode, insertedVnodeQueue, parentElm, oldStartVnode.elm, false, newCh, newStartIdx);
            }
          }
          newStartVnode = newCh[++newStartIdx];
        }
      }
      if (oldStartIdx > oldEndIdx) {
        refElm = isUndef(newCh[newEndIdx + 1]) ? null : newCh[newEndIdx + 1].elm;
        addVnodes(parentElm, refElm, newCh, newStartIdx, newEndIdx, insertedVnodeQueue);
      } else if (newStartIdx > newEndIdx) {
        removeVnodes(oldCh, oldStartIdx, oldEndIdx);
      }
    }
    // 检查vnode 数组中是否有vnode的key 重复,如果重复则给出警告
    function checkDuplicateKeys (children) {
      var seenKeys = {};
      for (var i = 0; i < children.length; i++) {
        var vnode = children[i];
        var key = vnode.key;
        if (isDef(key)) {
          if (seenKeys[key]) {
            warn(
              ("Duplicate keys detected: '" + key + "'. This may cause an update error."),
              vnode.context
            );
          } else {
            seenKeys[key] = true;
          }
        }
      }
    }
  // sameVnode 是对比新旧节点是否相同的函数
    function findIdxInOld (node, oldCh, start, end) {
      for (var i = start; i < end; i++) {
        var c = oldCh[i];
        if (isDef(c) && sameVnode(node, c)) { return i }
      }
    }
    // patchVnode 函数是对比 两个虚拟 dom 不同的地方, 同时 也是 递归 调用 updateChildren 的 函数
    function patchVnode (
      oldVnode,// 旧节点
      vnode,// 新节点
      insertedVnodeQueue,// 插入节点的队列
      ownerArray,// 节点 数组
      index, // 当前 节点的
      removeOnly // 只有在 patch 函数中被传入,当老节点不是真实的 dom 节点,当新老节点是相同节点的时候
    ) {
       // 如果新节点和旧节点 相等(使用了 同一个地址,直接返回不进行修改)
    // 这里就是 当 props 没有改变的时候,子组件不会做渲染,而是直接复用
      if (oldVnode === vnode) {
        return
      }

      if (isDef(vnode.elm) && isDef(ownerArray)) {
        // clone reused vnode
        vnode = ownerArray[index] = cloneVNode(vnode);
      }

      var elm = vnode.elm = oldVnode.elm;
      //当前节点 是 注释节点(被 v-if )了,或者是一个 异步函数节点,那不执行
      if (isTrue(oldVnode.isAsyncPlaceholder)) {
        if (isDef(vnode.asyncFactory.resolved)) {
          hydrate(oldVnode.elm, vnode, insertedVnodeQueue);
        } else {
          vnode.isAsyncPlaceholder = true;
        }
        return
      }
     // 当前节点 是一个静态节点的时候,或者 标记了 once 的时候,那不执行
      if (isTrue(vnode.isStatic) &&
        isTrue(oldVnode.isStatic) &&
        vnode.key === oldVnode.key &&
        (isTrue(vnode.isCloned) || isTrue(vnode.isOnce))
      ) {
        vnode.componentInstance = oldVnode.componentInstance;
        return
      }

      var i;
      var data = vnode.data;
      // 调用 prepatch 的钩子函数
      if (isDef(data) && isDef(i = data.hook) && isDef(i = i.prepatch)) {
        i(oldVnode, vnode);
      }

      var oldCh = oldVnode.children;
      var ch = vnode.children;
      // 调用 update 钩子函数
      if (isDef(data) && isPatchable(vnode)) {
        // 这里 的 update 钩子函数式 vnode 本身的钩子函数
        for (i = 0; i < cbs.update.length; ++i) { cbs.update[i](oldVnode, vnode); }
        // 这里的 update 钩子函数  是 用户传过来的 钩子函数
        if (isDef(i = data.hook) && isDef(i = i.update)) { i(oldVnode, vnode); }
      }
      if (isUndef(vnode.text)) {
        // 是否存在子节点,然后新老节点的子节点是否相同,否就执行 updateChildren 函数深入对比
        if (isDef(oldCh) && isDef(ch)) {
          if (oldCh !== ch) { updateChildren(elm, oldCh, ch, insertedVnodeQueue, removeOnly); }
        } else if (isDef(ch)) {
          {
            checkDuplicateKeys(ch);
          }
          // 查看新老节点是否存在,新节点存在但是老节点不存在,增加,新节点不存在但是老节点存在,删除
          if (isDef(oldVnode.text)) { nodeOps.setTextContent(elm, ''); }
          addVnodes(elm, null, ch, 0, ch.length - 1, insertedVnodeQueue);
        } else if (isDef(oldCh)) {
          removeVnodes(oldCh, 0, oldCh.length - 1);
        } else if (isDef(oldVnode.text)) {
          nodeOps.setTextContent(elm, '');
        }
      } else if (oldVnode.text !== vnode.text) {
        nodeOps.setTextContent(elm, vnode.text);
      }
      // 检查 新老节点的 text 属性是否一致,否的话,就执行 postpatch 钩子函数更新
      if (isDef(data)) {
        if (isDef(i = data.hook) && isDef(i = i.postpatch)) { i(oldVnode, vnode); }
      }
    }
    // 执行 虚拟 dom 的 insert 钩子函数
    function invokeInsertHook (vnode, queue, initial) {
      // invokeInsertHook函数会循环通过insert方法依次调用收集的hook,在insert方法中就会触发mounted
      if (isTrue(initial) && isDef(vnode.parent)) {
        vnode.parent.data.pendingInsert = queue;
      } else {
        for (var i = 0; i < queue.length; ++i) {
          queue[i].data.hook.insert(queue[i]);
        }
      }
    }

    var hydrationBailed = false;
    
    var isRenderedModule = makeMap('attrs,class,staticClass,staticStyle,key');

    // 将虚拟dom和真是dom进行映射,然后将oldVnode设置为对应的虚拟dom,找到oldVnode.elm的父节点
    // 根据vnode创建一个真实dom节点并插入到该父节点中oldVnode.elm的位置
    function hydrate (elm, vnode, insertedVnodeQueue, inVPre) {
      var i;
      var tag = vnode.tag;
      var data = vnode.data;
      var children = vnode.children;
      inVPre = inVPre || (data && data.pre);
      vnode.elm = elm;

      if (isTrue(vnode.isComment) && isDef(vnode.asyncFactory)) {
        vnode.isAsyncPlaceholder = true;
        return true
      }
      // assert node match
      {
        if (!assertNodeMatch(elm, vnode, inVPre)) {
          return false
        }
      }
      if (isDef(data)) {
        if (isDef(i = data.hook) && isDef(i = i.init)) { i(vnode, true /* hydrating */); }
        if (isDef(i = vnode.componentInstance)) {
          // child component. it should have hydrated its own tree.
          initComponent(vnode, insertedVnodeQueue);
          return true
        }
      }
      if (isDef(tag)) {
        if (isDef(children)) {
          // 空元素,允许客户端拾取并填充子元素
          if (!elm.hasChildNodes()) {
            createChildren(vnode, children, insertedVnodeQueue);
          } else {
            // v-html and domProps: innerHTML
            if (isDef(i = data) && isDef(i = i.domProps) && isDef(i = i.innerHTML)) {
              if (i !== elm.innerHTML) {
                /* istanbul ignore if */
                if (typeof console !== 'undefined' &&
                  !hydrationBailed
                ) {
                  hydrationBailed = true;
                  console.warn('Parent: ', elm);
                  console.warn('server innerHTML: ', i);
                  console.warn('client innerHTML: ', elm.innerHTML);
                }
                return false
              }
            } else {
              // 迭代和比较子列表
              var childrenMatch = true;
              var childNode = elm.firstChild;
              for (var i$1 = 0; i$1 < children.length; i$1++) {
                if (!childNode || !hydrate(childNode, children[i$1], insertedVnodeQueue, inVPre)) {
                  childrenMatch = false;
                  break
                }
                childNode = childNode.nextSibling;
              }
              // 如果childNode不为空,则意味着实际的childNodes列表比虚拟子节点列表长。
              if (!childrenMatch || childNode) {
                /* istanbul ignore if */
                if (typeof console !== 'undefined' &&
                  !hydrationBailed
                ) {
                  hydrationBailed = true;
                  console.warn('Parent: ', elm);
                  console.warn('Mismatching childNodes vs. VNodes: ', elm.childNodes, children);
                }
                return false
              }
            }
          }
        }
        if (isDef(data)) {
          var fullInvoke = false;
          for (var key in data) {
            if (!isRenderedModule(key)) {
              fullInvoke = true;
              invokeCreateHooks(vnode, insertedVnodeQueue);
              break
            }
          }
          if (!fullInvoke && data['class']) {
            // ensure collecting deps for deep class bindings for future updates
            traverse(data['class']);
          }
        }
      } else if (elm.data !== vnode.text) {
        elm.data = vnode.text;
      }
      return true
    }

    function assertNodeMatch (node, vnode, inVPre) {
      if (isDef(vnode.tag)) {
        return vnode.tag.indexOf('vue-component') === 0 || (
          !isUnknownElement$$1(vnode, inVPre) &&
          vnode.tag.toLowerCase() === (node.tagName && node.tagName.toLowerCase())
        )
      } else {
        return node.nodeType === (vnode.isComment ? 8 : 3)
      }
    }

    return function patch (oldVnode, vnode, hydrating, removeOnly) {
      // oldVnode 表示旧的 VNode 节点,它也可以不存在或者是一个 DOM 对象
    // vnode 表示执行 _render 后返回的 VNode 的节点
    // hydrating 表示是否是服务端渲染
    // removeOnly 是给 transition-group 用的
    // 判断 vnode 是否存在,我们的例子是返回了一个vnode的所以跳过
      if (isUndef(vnode)) {
        if (isDef(oldVnode)) { invokeDestroyHook(oldVnode); }
        return
      }

      var isInitialPatch = false;
      var insertedVnodeQueue = [];

      if (isUndef(oldVnode)) {
        // empty mount (likely as component), create new root element
        isInitialPatch = true;
        createElm(vnode, insertedVnodeQueue);
      } else {
        var isRealElement = isDef(oldVnode.nodeType);
        // 第一次执行 patch 函数时传递的 oldVnode 参数是 vm.$el ,也就是要被替换的 DOM 节点
      // 更新调用 patch 函数时传递的 oldVnode 参数是 prevVnode ,所以是一个虚拟 DOM
        if (!isRealElement && sameVnode(oldVnode, vnode)) {
          // patch existing root node
          patchVnode(oldVnode, vnode, insertedVnodeQueue, null, null, removeOnly);
        } else {
          if (isRealElement) {
            // 服务端渲染
            if (oldVnode.nodeType === 1 && oldVnode.hasAttribute(SSR_ATTR)) {
              oldVnode.removeAttribute(SSR_ATTR);
              hydrating = true;
            }
            if (isTrue(hydrating)) {
              if (hydrate(oldVnode, vnode, insertedVnodeQueue)) {
                invokeInsertHook(vnode, insertedVnodeQueue, true);
                return oldVnode
              } else {
                warn(
                  'The client-side rendered virtual DOM tree is not matching ' +
                  'server-rendered content. This is likely caused by incorrect ' +
                  'HTML markup, for example nesting block-level elements inside ' +
                  '<p>, or missing <tbody>. Bailing hydration and performing ' +
                  'full client-side render.'
                );
              }
            }
            // 把 oldVnode 转换成 VNode 对象
          // 在emptyNodeAt中我们将oldVnode传给了oldVnode.elm
            oldVnode = emptyNodeAt(oldVnode);
          }

          // // oldElm拿到原来的 oldVnode
          var oldElm = oldVnode.elm;
          // 拿到父节点body
          var parentElm = nodeOps.parentNode(oldElm);
          // createElm的作用是通过虚拟节点创建真实的 DOM 并插入到它的父节点中,接下来分析 createElm
          createElm(
            vnode,
            insertedVnodeQueue,
            // extremely rare edge case: do not insert if old element is in a
            // leaving transition. Only happens when combining transition +
            // keep-alive + HOCs. (#4590)
            oldElm._leaveCb ? null : parentElm,
            nodeOps.nextSibling(oldElm)
          );

          // 这是父占位节点,是和组件相关的,这里不会执行
          if (isDef(vnode.parent)) {
            var ancestor = vnode.parent;
            var patchable = isPatchable(vnode);
            while (ancestor) {
              for (var i = 0; i < cbs.destroy.length; ++i) {
                cbs.destroy[i](ancestor);
              }
              ancestor.elm = vnode.elm;
              if (patchable) {
                for (var i$1 = 0; i$1 < cbs.create.length; ++i$1) {
                  cbs.create[i$1](emptyNode, ancestor);
                }
                // #6513
                // invoke insert hooks that may have been merged by create hooks.
                // e.g. for directives that uses the "inserted" hook.
                var insert = ancestor.data.hook.insert;
                if (insert.merged) {
                  // start at index 1 to avoid re-invoking component mounted hook
                  for (var i$2 = 1; i$2 < insert.fns.length; i$2++) {
                    insert.fns[i$2]();
                  }
                }
              } else {
                registerRef(ancestor);
              }
              ancestor = ancestor.parent;
            }
          }
        // 判断之前定义的 parentElm 是否存在,有则删除掉对应的节点
        // 我们之前在 const parentElm = nodeOps.parentNode(oldElm) 中拿到了id=app 的父元素body
        // 之后删除<div id="app"></div>完成新旧节点替换工作。
          if (isDef(parentElm)) {
            removeVnodes([oldVnode], 0, 0);
          } else if (isDef(oldVnode.tag)) {
            
            invokeDestroyHook(oldVnode);
          }
        }
      }
      // 我们根据之前递归 createElm 生成的 vnode 插入顺序队列,执行相关的 insert 钩子函数
      invokeInsertHook(vnode, insertedVnodeQueue, isInitialPatch);
      return vnode.elm
    }
  }

  /*  */

  var directives = {
    create: updateDirectives,
    update: updateDirectives,
    destroy: function unbindDirectives (vnode) {
      updateDirectives(vnode, emptyNode);
    }
  };
  // updateDirectives函数的作用:获取指令钩子,和对不同钩子进行不同处理
  function updateDirectives (oldVnode, vnode) {
    if (oldVnode.data.directives || vnode.data.directives) {
      _update(oldVnode, vnode);
    }
  }
  // 主要是将vnode转换成dom,渲染在视图中
  function _update (oldVnode, vnode) {
    //判断旧节点是不是空节点,是的话表示新建/初始化组件
    var isCreate = oldVnode === emptyNode;
     //判断新节点是不是空节点,是的话表示销毁组件
    var isDestroy = vnode === emptyNode;
    //获取旧节点上的所有自定义指令
    var oldDirs = normalizeDirectives$1(oldVnode.data.directives, oldVnode.context);
    //获取新节点上的所有自定义指令
    var newDirs = normalizeDirectives$1(vnode.data.directives, vnode.context);
    //保存inserted钩子函数
    var dirsWithInsert = [];
     //保存componentUpdated钩子函数
    var dirsWithPostpatch = [];

    var key, oldDir, dir;
     //遍历所有新节点上的自定义指令
    for (key in newDirs) {
      oldDir = oldDirs[key];
      dir = newDirs[key];
      //如果旧节点中没有对应的指令,一般都是初始化的时候运行
      if (!oldDir) {
        //对该节点执行指令的bind钩子函数
        callHook$1(dir, 'bind', vnode, oldVnode);
        //dir.def是我们所定义的指令的五个钩子函数的集合
        //如果我们的指令中存在inserted钩子函数
        if (dir.def && dir.def.inserted) {
           //把该指令存入dirsWithInsert中
          dirsWithInsert.push(dir);
        }
      } else {
        //如果旧节点中有对应的指令,一般都是组件更新的时候运行
        //那么这里进行更新操作,运行update钩子(如果有的话)
        //将旧值保存下来,供其他地方使用(仅在 update 和 componentUpdated 钩子中可用)
        dir.oldValue = oldDir.value;
        dir.oldArg = oldDir.arg;
         //对该节点执行指令的update钩子函数
        callHook$1(dir, 'update', vnode, oldVnode);
        //dir.def是我们所定义的指令的五个钩子函数的集合
        //如果我们的指令中存在componentUpdated钩子函数
        if (dir.def && dir.def.componentUpdated) {
          //把该指令存入dirsWithPostpatch中
          dirsWithPostpatch.push(dir);
        }
      }
    }
    // 如果dirsWithInsert的长度不为0,也就是在初始化的时候,且至少有一个指令中有inserted钩子函数
    if (dirsWithInsert.length) {
      //封装回调函数
      var callInsert = function () {
        //遍历所有指令的inserted钩子
        for (var i = 0; i < dirsWithInsert.length; i++) {
          //对节点执行指令的inserted钩子函数
          callHook$1(dirsWithInsert[i], 'inserted', vnode, oldVnode);
        }
      };
      if (isCreate) {
         //如果是新建/初始化组件,使用mergeVNodeHook绑定insert属性,等待后面调用
        mergeVNodeHook(vnode, 'insert', callInsert);
      } else {
         //如果是更新组件,直接调用函数,遍历inserted钩子
        callInsert();
      }
    }
     //如果dirsWithPostpatch的长度不为0,也就是在组件更新的时候,且至少有一个指令中有componentUpdated钩子函数
    if (dirsWithPostpatch.length) {
      //使用mergeVNodeHook绑定postpatch属性,等待后面子组建全部更新完成调用
      mergeVNodeHook(vnode, 'postpatch', function () {
        for (var i = 0; i < dirsWithPostpatch.length; i++) {
          //对节点执行指令的componentUpdated钩子函数
          callHook$1(dirsWithPostpatch[i], 'componentUpdated', vnode, oldVnode);
        }
      });
    }
 //如果不是新建/初始化组件,也就是说是更新组件
    if (!isCreate) {
      //遍历旧节点中的指令
      for (key in oldDirs) {
        //如果新节点中没有这个指令
        if (!newDirs[key]) {
          //从旧节点中解绑,isDestroy表示组件是不是注销了
          //对旧节点执行指令的unbind钩子函数
          callHook$1(oldDirs[key], 'unbind', oldVnode, oldVnode, isDestroy);
        }
      }
    }
  }

  var emptyModifiers = Object.create(null);
  // 遍历本节点所有的指令,逐个从组件中获取,把获取的钩子添加到 遍历到的当前指令上
  function normalizeDirectives$1 (
    dirs,
    vm
  ) {
    var res = Object.create(null);
    if (!dirs) {
      // $flow-disable-line
      return res
    }
    var i, dir;
    for (i = 0; i < dirs.length; i++) {
      dir = dirs[i];
      if (!dir.modifiers) {
        // $flow-disable-line
        dir.modifiers = emptyModifiers;
      }
      res[getRawDirName(dir)] = dir;
      dir.def = resolveAsset(vm.$options, 'directives', dir.name, true);
    }
    // $flow-disable-line
    return res
  }
  // 获取原始名称(包含前缀、参数、修饰符)
  function getRawDirName (dir) {
    return dir.rawName || ((dir.name) + "." + (Object.keys(dir.modifiers || {}).join('.')))
  }
  // 调用指令钩子函数,格式化钩子函数
  //callHook$1有五个参数,第一个参数是指令对象,第二个参数是钩子函数名称,第三个参数新节点,
    //第四个参数是旧节点,第五个参数是是否为注销组件,默认为undefined,只在组件注销时使用
    //在这个函数里,会根据我们传递的钩子函数名称,运行我们自定义组件时,所声明的钩子函数
  function callHook$1 (dir, hook, vnode, oldVnode, isDestroy) {
    var fn = dir.def && dir.def[hook];
    if (fn) {
      try {
        fn(vnode.elm, dir, vnode, oldVnode, isDestroy);
      } catch (e) {
        handleError(e, vnode.context, ("directive " + (dir.name) + " " + hook + " hook"));
      }
    }
  }

  var baseModules = [
    ref,
    directives
  ];

  /* 更新属性 */

  function updateAttrs (oldVnode, vnode) {
    var opts = vnode.componentOptions;
    if (isDef(opts) && opts.Ctor.options.inheritAttrs === false) {
      return
    }
    if (isUndef(oldVnode.data.attrs) && isUndef(vnode.data.attrs)) {
      return
    }
    var key, cur, old;
    var elm = vnode.elm;
    var oldAttrs = oldVnode.data.attrs || {};
    var attrs = vnode.data.attrs || {};
    // 克隆观察到的对象
    if (isDef(attrs.__ob__)) {
      attrs = vnode.data.attrs = extend({}, attrs);
    }

    for (key in attrs) {
      cur = attrs[key];
      old = oldAttrs[key];
      if (old !== cur) {
        setAttr(elm, key, cur, vnode.data.pre);
      }
    }
    //在IE9中,设置type可以重置输入的值
    //IE/Edge在设置最大值之前将进度值降低为1
    if ((isIE || isEdge) && attrs.value !== oldAttrs.value) {
      setAttr(elm, 'value', attrs.value);
    }
    for (key in oldAttrs) {
      if (isUndef(attrs[key])) {
        if (isXlink(key)) {
          elm.removeAttributeNS(xlinkNS, getXlinkProp(key));
        } else if (!isEnumeratedAttr(key)) {
          elm.removeAttribute(key);
        }
      }
    }
  }
  // 设置属性
  function setAttr (el, key, value, isInPre) {
    if (isInPre || el.tagName.indexOf('-') > -1) {
      //设置el元素的key属性为value
      baseSetAttr(el, key, value);
    } else if (isBooleanAttr(key)) {
      // 设置属性为空值
      if (isFalsyAttrValue(value)) {
        el.removeAttribute(key);
      } else {
        //allowfullscreen是iframe的布尔属性,但Flash期望在标签上使用值为"true"
        value = key === 'allowfullscreen' && el.tagName === 'EMBED'
          ? 'true'
          : key;
        el.setAttribute(key, value);
      }
      // 如果key是这三个中的一个:contenteditable,draggable,spellcheck
    } else if (isEnumeratedAttr(key)) {
      el.setAttribute(key, convertEnumeratedValue(key, value));
    } else if (isXlink(key)) {
      if (isFalsyAttrValue(value)) {
        el.removeAttributeNS(xlinkNS, getXlinkProp(key));
      } else {
        el.setAttributeNS(xlinkNS, key, value);
      }
    } else {
      // 直接调用baseSetAttr设置属性
      baseSetAttr(el, key, value);
    }
  }

  function baseSetAttr (el, key, value) {
    //如果value是null或false,则删除属性
    if (isFalsyAttrValue(value)) {
      el.removeAttribute(key);
    } else {
      // 当在textarea设置占位符阻塞第一个输入事件时触发输入事件,并立即移除阻塞器
      if (
        isIE && !isIE9 &&
        el.tagName === 'TEXTAREA' &&
        key === 'placeholder' && value !== '' && !el.__ieph
      ) {
        var blocker = function (e) {
          // 该方法作用在当前节点及事件链的所有后续节点上,目的是在执行完当前事件处理程序后,停止当前节点及所有后续节点的同类事件处理程序的运行
          e.stopImmediatePropagation();
          //移出监听
          el.removeEventListener('input', blocker);
        };
        //添加监听
        el.addEventListener('input', blocker);
        // $flow-disable-line
        el.__ieph = true; /* IE placeholder patched */
      }
      //直接调用原生DOMAPI setAttribute设置属性
      el.setAttribute(key, value);
    }
  }

  var attrs = {
    create: updateAttrs,
    update: updateAttrs
  };

  /* 更新样式 */

  function updateClass (oldVnode, vnode) {
    var el = vnode.elm;
    var data = vnode.data;
    var oldData = oldVnode.data;
    if (
      isUndef(data.staticClass) &&
      isUndef(data.class) && (
        isUndef(oldData) || (
          isUndef(oldData.staticClass) &&
          isUndef(oldData.class)
        )
      )
    ) {
      return
    }

    var cls = genClassForVnode(vnode);

    // handle transition classes
    var transitionClass = el._transitionClasses;
    if (isDef(transitionClass)) {
      cls = concat(cls, stringifyClass(transitionClass));
    }

    // set the class
    if (cls !== el._prevClass) {
      el.setAttribute('class', cls);
      el._prevClass = cls;
    }
  }

  var klass = {
    create: updateClass,
    update: updateClass
  };

  //过滤器解析
  var validDivisionCharRE = /[\w).+\-_$\]]/;
  // filter允许用在两个地方,一个是双括号插值,一个是v-bind表达式后面,如果解析到这两种情况,执行parseFilters解析filter
  function parseFilters (exp) {
    var inSingle = false;
    var inDouble = false;
    var inTemplateString = false;
    var inRegex = false;
    var curly = 0;
    var square = 0;
    var paren = 0;
    var lastFilterIndex = 0;
    var c, prev, i, expression, filters;
    // for循环传入的value字符串
    for (i = 0; i < exp.length; i++) {
      prev = c;
      c = exp.charCodeAt(i);
      if (inSingle) {
        if (c === 0x27 && prev !== 0x5C) { inSingle = false; }
      } else if (inDouble) {
        if (c === 0x22 && prev !== 0x5C) { inDouble = false; }
      } else if (inTemplateString) {
        if (c === 0x60 && prev !== 0x5C) { inTemplateString = false; }
      } else if (inRegex) {
        if (c === 0x2f && prev !== 0x5C) { inRegex = false; }
      } else if (
        c === 0x7C && // pipe
        exp.charCodeAt(i + 1) !== 0x7C &&
        exp.charCodeAt(i - 1) !== 0x7C &&
        !curly && !square && !paren
      ) {
        if (expression === undefined) {
          // first filter, end of expression
          lastFilterIndex = i + 1;
          expression = exp.slice(0, i).trim();
        } else {
          pushFilter();
        }
      } else {
        switch (c) {
          case 0x22: inDouble = true; break         // "
          case 0x27: inSingle = true; break         // '
          case 0x60: inTemplateString = true; break // `
          case 0x28: paren++; break                 // (
          case 0x29: paren--; break                 // )
          case 0x5B: square++; break                // [
          case 0x5D: square--; break                // ]
          case 0x7B: curly++; break                 // {
          case 0x7D: curly--; break                 // }
        }
        if (c === 0x2f) { // /
          var j = i - 1;
          var p = (void 0);
          // find first non-whitespace prev char
          for (; j >= 0; j--) {
            p = exp.charAt(j);
            if (p !== ' ') { break }
          }
          if (!p || !validDivisionCharRE.test(p)) {
            inRegex = true;
          }
        }
      }
    }
    // 如果expression 为空,则说明没有没有filter函数,或者是写法出了问题。某些符号没闭合
    if (expression === undefined) {
      expression = exp.slice(0, i).trim();
    } else if (lastFilterIndex !== 0) {
      pushFilter();
    }
    // 初始化filters变量。将过滤器函数推入filters数组中
    function pushFilter () {
      (filters || (filters = [])).push(exp.slice(lastFilterIndex, i).trim());
      lastFilterIndex = i + 1;
    }
    // 如果有过滤器函数,则结合expression生成最终的表达式
    if (filters) {
      for (i = 0; i < filters.length; i++) {
        expression = wrapFilter(expression, filters[i]);
      }
    }

    return expression
  }
  // 转换过滤器格式,ilter函数结合expression生成最终的表达式
  function wrapFilter (exp, filter) {
    var i = filter.indexOf('(');
    if (i < 0) {
      return ("_f(\"" + filter + "\")(" + exp + ")")
    } else {
      var name = filter.slice(0, i);
      var args = filter.slice(i + 1);
      return ("_f(\"" + name + "\")(" + exp + (args !== ')' ? ',' + args : args))
    }
  }

  /*  */


  // 基础警告
  function baseWarn (msg, range) {
    console.error(("[Vue compiler]: " + msg));
  }
  //循环过滤数组或者对象的值,根据key循环 过滤对象或者数组[key]值,如果不存在则丢弃,如果有相同多个的key值,返回多个值的数组
  function pluckModuleFunction (
    modules,
    key
  ) {
    return modules
      ? modules.map(function (m) { return m[key]; }).filter(function (_) { return _; })
      : []
  }
  //在虚拟dom中添加prop属性
  function addProp (el, name, value, range, dynamic) {
    (el.props || (el.props = [])).push(rangeSetItem({ name: name, value: value, dynamic: dynamic }, range));
    el.plain = false;
  }
//添加attrs属性
  function addAttr (el, name, value, range, dynamic) {
    var attrs = dynamic
      ? (el.dynamicAttrs || (el.dynamicAttrs = []))
      : (el.attrs || (el.attrs = []));
    attrs.push(rangeSetItem({ name: name, value: value, dynamic: dynamic }, range));
    el.plain = false;
  }

  // 添加原始attr(在预转换中使用)
  function addRawAttr (el, name, value, range) {
    el.attrsMap[name] = value;
    el.attrsList.push(rangeSetItem({ name: name, value: value }, range));
  }
 //为虚拟dom 添加一个 指令directives属性 对象
  function addDirective (
    el,
    name,
    rawName,
    value,
    arg,
    isDynamicArg,
    modifiers,
    range
  ) {
    (el.directives || (el.directives = [])).push(rangeSetItem({
      name: name,
      rawName: rawName,
      value: value,
      arg: arg,
      isDynamicArg: isDynamicArg,
      modifiers: modifiers
    }, range));
    el.plain = false;
  }
  // 给修饰符的属性,加上标记
  function prependModifierMarker (symbol, name, dynamic) {
    return dynamic
      ? ("_p(" + name + ",\"" + symbol + "\")")
      : symbol + name // 将事件标记为捕获
  }
 // 为虚拟dom添加events 事件对象属性
  function addHandler (
    el,
    name,
    value,
    modifiers,
    important,
    warn,
    range,
    dynamic
  ) {
    // modifiers 是一个对象,如果传递的参数为空,则给一个冻结的空对象
    modifiers = modifiers || emptyObject;
    // warn prevent and passive modifier
    /* istanbul ignore if */
    if (
      warn &&
      modifiers.prevent && modifiers.passive
    ) {
      warn(
        'passive and prevent can\'t be used together. ' +
        'Passive handler can\'t prevent default event.',
        range
      );
    }
    // 标准化 click.right 和 click.middle,它们实际上不会被真正的触发,从技术讲他们是它们
    // 是特定于浏览器的,但至少目前位置只有浏览器才具有右键和中间键的点击
    if (modifiers.right) {
      if (dynamic) {
        name = "(" + name + ")==='click'?'contextmenu':(" + name + ")";
      } else if (name === 'click') {
         // 动态属性
        name = 'contextmenu';
         // 删除修饰符中的 right 属性
        delete modifiers.right;
      }
    } else if (modifiers.middle) {
      if (dynamic) {
        // 动态属性,name => mouseup 或者 ${name}
        name = "(" + name + ")==='click'?'mouseup':(" + name + ")";
      } else if (name === 'click') {
        name = 'mouseup';
      }
    }
    // 处理 capture、once、passive 这三个修饰符,通过给 name 添加不同的标记来标记这些修饰符
    // 给带有 capture 修饰符的属性,加上 ! 标记
    if (modifiers.capture) {
      delete modifiers.capture;
      name = prependModifierMarker('!', name, dynamic);
    }
    // once 修饰符加 ~ 标记
    if (modifiers.once) {
      delete modifiers.once;
      name = prependModifierMarker('~', name, dynamic);
    }
    // passive 修饰符加 & 标记
    if (modifiers.passive) {
      delete modifiers.passive;
      name = prependModifierMarker('&', name, dynamic);
    }

    var events;
    if (modifiers.native) {
      delete modifiers.native;
      events = el.nativeEvents || (el.nativeEvents = {});
    } else {
      events = el.events || (el.events = {});
    }

    var newHandler = rangeSetItem({ value: value.trim(), dynamic: dynamic }, range);
    if (modifiers !== emptyObject) {
      newHandler.modifiers = modifiers;
    }

    var handlers = events[name];
    /* istanbul ignore if */
    if (Array.isArray(handlers)) {
      important ? handlers.unshift(newHandler) : handlers.push(newHandler);
    } else if (handlers) {
      events[name] = important ? [newHandler, handlers] : [handlers, newHandler];
    } else {
      events[name] = newHandler;
    }

    el.plain = false;
  }
//v-bind的属性添加函数
  function getRawBindingAttr (
    el,
    name
  ) {
    return el.rawAttrsMap[':' + name] ||
      el.rawAttrsMap['v-bind:' + name] ||
      el.rawAttrsMap[name]
  }
//v-bind的属性获取函数
  function getBindingAttr (
    el,
    name,
    getStatic
  ) {
    var dynamicValue =
      getAndRemoveAttr(el, ':' + name) ||
      getAndRemoveAttr(el, 'v-bind:' + name);
    if (dynamicValue != null) {
      return parseFilters(dynamicValue)
    } else if (getStatic !== false) {
      var staticValue = getAndRemoveAttr(el, name);
      if (staticValue != null) {
        return JSON.stringify(staticValue)
      }
    }
  }

  //注意:这只是从数组(attrsList)中删除了属性r,以便它
  //不被processAttrs处理
  //默认情况下,它不从映射(attrsMap)中删除它,因为映射是
  //在代码生成期间需要
  // 移除传进来的属性name,并且返回获取到 属性的值
  function getAndRemoveAttr (
    el,
    name,
    removeFromMap
  ) {
    var val;
    if ((val = el.attrsMap[name]) != null) {
      var list = el.attrsList;
      for (var i = 0, l = list.length; i < l; i++) {
        if (list[i].name === name) {
          list.splice(i, 1);
          break
        }
      }
    }
    if (removeFromMap) {
      delete el.attrsMap[name];
    }
    return val
  }
  //删除属性
  function getAndRemoveAttrByRegex (
    el,
    name
  ) {
    // attrsList 类型为 Array<ASTAttr>
    var list = el.attrsList;
    // 对 attrsList 进行遍历,若有满足 RegExp 的则直接返回当前对应的 attr
    // 若参数 name 传进来的是 slotRE = /^v-slot(:|$)|^#/
    // 那么匹配到 'v-slot' 或者 'v-slot:xxx' 则会返回其对应的 attr
    for (var i = 0, l = list.length; i < l; i++) {
      var attr = list[i];
      if (name.test(attr.name)) {
        list.splice(i, 1);
        return attr
      }
    }
  }

  function rangeSetItem (
    item,
    range
  ) {
    if (range) {
      if (range.start != null) {
        item.start = range.start;
      }
      if (range.end != null) {
        item.end = range.end;
      }
    }
    return item
  }

  /*  */

  /**
   *为虚拟dom添加model属性
   */
  function genComponentModel (
    el,
    value,
    modifiers
  ) {
    var ref = modifiers || {};
    var number = ref.number;
    var trim = ref.trim;

    var baseValueExpression = '$$v';
    var valueExpression = baseValueExpression;
    if (trim) {
      valueExpression =
        "(typeof " + baseValueExpression + " === 'string'" +
        "? " + baseValueExpression + ".trim()" +
        ": " + baseValueExpression + ")";
    }
    if (number) {
      valueExpression = "_n(" + valueExpression + ")";
    }
    var assignment = genAssignmentCode(value, valueExpression);

    el.model = {
      value: ("(" + value + ")"),
      expression: JSON.stringify(value),
      callback: ("function (" + baseValueExpression + ") {" + assignment + "}")
    };
  }

  /**
   * 用于生成v-模型值赋值代码的跨平台代码生成助手
   */
  function genAssignmentCode (
    value,
    assignment
  ) {
    // 对 v-model 对应的 value 做了解析
    var res = parseModel(value);
    if (res.key === null) {
      return (value + "=" + assignment)
    } else {
      return ("$set(" + (res.exp) + ", " + (res.key) + ", " + assignment + ")")
    }
  }

  var len, str, chr, index$1, expressionPos, expressionEndPos;
  //转义字符串对象拆分字符串对象 把后一位key分离出来
  function parseModel (val) {
    val = val.trim();
    len = val.length;

    if (val.indexOf('[') < 0 || val.lastIndexOf(']') < len - 1) {
      index$1 = val.lastIndexOf('.');
      if (index$1 > -1) {
        return {
          exp: val.slice(0, index$1),
          key: '"' + val.slice(index$1 + 1) + '"'
        }
      } else {
        return {
          exp: val,
          key: null
        }
      }
    }

    str = val;
    index$1 = expressionPos = expressionEndPos = 0;

    while (!eof()) {
      chr = next();
      /* istanbul ignore if */
      if (isStringStart(chr)) {
        parseString(chr);
      } else if (chr === 0x5B) {
        parseBracket(chr);
      }
    }

    return {
      exp: val.slice(0, expressionPos),
      key: val.slice(expressionPos + 1, expressionEndPos)
    }
  }
  // 返回字符串第一个字符的 Unicode 编码
  function next () {
    return str.charCodeAt(++index$1)
  }

  function eof () {
    return index$1 >= len
  }

  function isStringStart (chr) {
    return chr === 0x22 || chr === 0x27
  }

  function parseBracket (chr) {
    var inBracket = 1;
    expressionPos = index$1;
    while (!eof()) {
      chr = next();
      if (isStringStart(chr)) {
        parseString(chr);
        continue
      }
      if (chr === 0x5B) { inBracket++; }
      if (chr === 0x5D) { inBracket--; }
      if (inBracket === 0) {
        expressionEndPos = index$1;
        break
      }
    }
  }

  function parseString (chr) {
    var stringQuote = chr;
    while (!eof()) {
      chr = next();
      if (chr === stringQuote) {
        break
      }
    }
  }

  /*******************************************************************************************************************************************/

  var warn$1;

  // 在某些情况下,使用的事件必须在运行时确定
//在编译过程中使用了一些保留的标记
  var RANGE_TOKEN = '__r';
  var CHECKBOX_RADIO_TOKEN = '__c';
  //v-model指令的初始化
  // v-model本质上就是一个语法糖。它负责监听用户的输入事件以更新数据,并对一些极端场景进行一些特殊处理
  function model (
    el,
    dir,
    _warn
  ) {
    warn$1 = _warn;
    //值
    var value = dir.value;
    // 修饰符
    var modifiers = dir.modifiers;
    // 标签名
    var tag = el.tag;
    var type = el.attrsMap.type;

    {
      // type="file"的输入是只读的,并设置输入的值
      // value将抛出一个错误
      if (tag === 'input' && type === 'file') {
        warn$1(
          "<" + (el.tag) + " v-model=\"" + value + "\" type=\"file\">:\n" +
          "File inputs are read only. Use a v-on:change listener instead.",
          el.rawAttrsMap['v-model']
        );
      }
    }

    if (el.component) {
      genComponentModel(el, value, modifiers);
      // 组件v-model不需要额外的运行时
      return false
    } else if (tag === 'select') {
      genSelect(el, value, modifiers);
    } else if (tag === 'input' && type === 'checkbox') {
      genCheckboxModel(el, value, modifiers);
    } else if (tag === 'input' && type === 'radio') {
      genRadioModel(el, value, modifiers);
    } else if (tag === 'input' || tag === 'textarea') {
      genDefaultModel(el, value, modifiers);
    } else if (!config.isReservedTag(tag)) {
      genComponentModel(el, value, modifiers);
      // component v-model doesn't need extra runtime
      return false
    } else {
      warn$1(
        "<" + (el.tag) + " v-model=\"" + value + "\">: " +
        "v-model is not supported on this element type. " +
        'If you are working with contenteditable, it\'s recommended to ' +
        'wrap a library dedicated for that purpose inside a custom component.',
        el.rawAttrsMap['v-model']
      );
    }

    // ensure runtime directive metadata
    return true
  }
  //复选框的v-model
  function genCheckboxModel (
    el,
    value,
    modifiers
  ) {
    var number = modifiers && modifiers.number;
    //获取元素值的属性
    var valueBinding = getBindingAttr(el, 'value') || 'null';
    var trueValueBinding = getBindingAttr(el, 'true-value') || 'true';
    var falseValueBinding = getBindingAttr(el, 'false-value') || 'false';
    // 当trueValueBinding为true,在节点添加checked属性
    addProp(el, 'checked',
      "Array.isArray(" + value + ")" +
      "?_i(" + value + "," + valueBinding + ")>-1" + (
        trueValueBinding === 'true'
          ? (":(" + value + ")")
          : (":_q(" + value + "," + trueValueBinding + ")")
      )
    );
    // 在节点添加change事件
    addHandler(el, 'change',
      "var $$a=" + value + "," +
          '$$el=$event.target,' +
          "$$c=$$el.checked?(" + trueValueBinding + "):(" + falseValueBinding + ");" +
      'if(Array.isArray($$a)){' +
        "var $$v=" + (number ? '_n(' + valueBinding + ')' : valueBinding) + "," +
            '$$i=_i($$a,$$v);' +
        "if($$el.checked){$$i<0&&(" + (genAssignmentCode(value, '$$a.concat([$$v])')) + ")}" +
        "else{$$i>-1&&(" + (genAssignmentCode(value, '$$a.slice(0,$$i).concat($$a.slice($$i+1))')) + ")}" +
      "}else{" + (genAssignmentCode(value, '$$c')) + "}",
      null, true
    );
  }
  //单选框的v-model
  function genRadioModel (
    el,
    value,
    modifiers
  ) {
    //获取数字修饰符
    var number = modifiers && modifiers.number;
      //获取元素值的属性
    var valueBinding = getBindingAttr(el, 'value') || 'null';
    // 如果是number类型,则添加checked属性
    valueBinding = number ? ("_n(" + valueBinding + ")") : valueBinding;
    addProp(el, 'checked', ("_q(" + value + "," + valueBinding + ")"));
    //在节点添加change监听事件
    addHandler(el, 'change', genAssignmentCode(value, valueBinding), null, true);
  }
//选择器的v-model
  function genSelect (
    el,
    value,
    modifiers
  ) {
    //获取数字修饰符
    var number = modifiers && modifiers.number;
    var selectedVal = "Array.prototype.filter" +
      ".call($event.target.options,function(o){return o.selected})" +
      ".map(function(o){var val = \"_value\" in o ? o._value : o.value;" +
      "return " + (number ? '_n(val)' : 'val') + "})";

    var assignment = '$event.target.multiple ? $$selectedVal : $$selectedVal[0]';
    var code = "var $$selectedVal = " + selectedVal + ";";
    code = code + " " + (genAssignmentCode(value, assignment));
    addHandler(el, 'change', code, null, true);
  }
  // input标签 和textarea标签
  function genDefaultModel (
    el,
    value,
    modifiers
  ) {
    // 获取type值,比如text,如果未指定则为undefined
    var type = el.attrsMap.type;

    // 如果v-bind:value与v-model冲突,则警告
    //除了v-bind:type的输入
    {
      // 获取动态绑定的value值
      var value$1 = el.attrsMap['v-bind:value'] || el.attrsMap[':value'];
      //获取动态绑定的type值
      var typeBinding = el.attrsMap['v-bind:type'] || el.attrsMap[':type'];
      if (value$1 && !typeBinding) {
        //如果动态绑定了value 且没有绑定type,则报错
        var binding = el.attrsMap['v-bind:value'] ? 'v-bind:value' : ':value';
        warn$1(
          binding + "=\"" + value$1 + "\" conflicts with v-model on the same element " +
          'because the latter already expands to a value binding internally',
          el.rawAttrsMap[binding]
        );
      }
    }

    var ref = modifiers || {};
    //获取lazy修饰符
    var lazy = ref.lazy;
    //获取number修饰符
    var number = ref.number;
     //获取trim修饰符
    var trim = ref.trim;
    var needCompositionGuard = !lazy && type !== 'range';
      //如果有lazy修饰符则绑定为change事件,否则绑定input事件
    var event = lazy
      ? 'change'
      : type === 'range'
        ? RANGE_TOKEN
        : 'input';

    var valueExpression = '$event.target.value';
     //如果有trim修饰符,则在值后面加上trim()
    if (trim) {
      valueExpression = "$event.target.value.trim()";
    }
     //如果有number修饰符,则加上_n函数,就是全局的toNumber函数
    if (number) {
      valueExpression = "_n(" + valueExpression + ")";
    }
 //返回一个表达式
    var code = genAssignmentCode(value, valueExpression);
    if (needCompositionGuard) {
       //如果需要composing配合,则在前面加上一段if语句
      code = "if($event.target.composing)return;" + code;
    }
//添加一个value的prop
    addProp(el, 'value', ("(" + value + ")"));
    //添加event事件
    addHandler(el, event, code, null, true);
    if (trim || number) {
      addHandler(el, 'blur', '$forceUpdate()');
    }
  }

  // 规范化只能在运行时确定的v-model事件标记。
//将事件放在数组的第一个很重要,因为
//整个要点是确保v-model回调函数在之前被调用
// user-attached处理程序。
  function normalizeEvents (on) {
    /* istanbul ignore if */
    if (isDef(on[RANGE_TOKEN])) {
      // IE浏览器 input[type=range]只支持' change '事件
      var event = isIE ? 'change' : 'input';
      on[event] = [].concat(on[RANGE_TOKEN], on[event] || []);
      delete on[RANGE_TOKEN];
    }
    /* istanbul ignore if */
    if (isDef(on[CHECKBOX_RADIO_TOKEN])) {
      on.change = [].concat(on[CHECKBOX_RADIO_TOKEN], on.change || []);
      delete on[CHECKBOX_RADIO_TOKEN];
    }
  }

  var target$1;
  // 创建once的处理事件
  function createOnceHandler$1 (event, handler, capture) {
    // 在闭包中保存当前的目标元素
    var _target = target$1;
    return function onceHandler () {
      var res = handler.apply(null, arguments);
      if (res !== null) {
        // 移出oncede的处理事件
        remove$2(event, onceHandler, capture, _target);
      }
    }
  }

  // Firefox <= 53(特别是ESR 52)具有错误的Event.timeStamp
  //实现,并且不会在事件传播之间触发微任务,因此安全的排除
  var useMicrotaskFix = isUsingMicroTask && !(isFF && Number(isFF[1]) <= 53);

  function add$1 (
    name,
    handler,
    capture,
    passive
  ) {
// 内部点击事件触发补丁,事件处理程序在补丁期间附加到外部元素,并再次触发。这
//发生的原因是浏览器在事件传播之间触发微任务滴答声。解决方案很简单:当附加处理程序时保存时间戳,
//只有传递给它的事件被触发时,处理器才会被触发
//在它被附加之后。
    if (useMicrotaskFix) {
      var attachedTimestamp = currentFlushTimestamp;
      var original = handler;
      handler = original._wrapper = function (e) {
        if (

          e.target === e.currentTarget ||
          // 事件在处理程序附件之后触发
          e.timeStamp >= attachedTimestamp ||
          // 支持有bug事件的环境。时间戳的实现
          // #9462 iOS 9错误:事件。timeStamp在history.pushState之后为0
          // #9681 QtWebEngine事件。timeStamp为负值
          e.timeStamp <= 0 ||
          // #9448如果在多页的另一个文档中触发了保释事件
          //电子/nw.js应用程序,自事件。timeStamp将使用不同的
          //开始参考
          e.target.ownerDocument !== document
        ) {
          return original.apply(this, arguments)
        }
      };
    }
    // 使用addEventListener来实现事件绑定
    target$1.addEventListener(
      name,
      handler,
      supportsPassive
        ? { capture: capture, passive: passive }
        : capture
    );
  }
// 移出事件监听
  function remove$2 (
    name,
    handler,
    capture,
    _target
  ) {
    (_target || target$1).removeEventListener(
      name,
      handler._wrapper || handler,
      capture
    );
  }
//事件更新,先判断没有旧事件,就直接添加新事件,如果有旧事件先移出旧事件,在添加新事件
  function updateDOMListeners (oldVnode, vnode) {
    
    if (isUndef(oldVnode.data.on) && isUndef(vnode.data.on)) {
      return
    }
    var on = vnode.data.on || {};
    var oldOn = oldVnode.data.on || {};
    target$1 = vnode.elm;
    normalizeEvents(on);
    updateListeners(on, oldOn, add$1, remove$2, createOnceHandler$1, vnode.context);
    target$1 = undefined;
  }

  var events = {
    create: updateDOMListeners,
    update: updateDOMListeners
  };

  /*  */

  var svgContainer;

  function updateDOMProps (oldVnode, vnode) {
    if (isUndef(oldVnode.data.domProps) && isUndef(vnode.data.domProps)) {
      return
    }
    var key, cur;
    // 虚拟节点
    var elm = vnode.elm;
    // 第一次渲染的时候没有旧节点,返回{}
    var oldProps = oldVnode.data.domProps || {};
    var props = vnode.data.domProps || {};
    // 克隆观察到的对象
    if (isDef(props.__ob__)) {
       // 当前节点的props
      props = vnode.data.domProps = extend({}, props);
    }
// key就是节点中v-model绑定的值
    for (key in oldProps) {
      // 旧节点存在key而新节点中不存在的话就赋给新节点一个值为空的key
      if (!(key in props)) {
        elm[key] = '';
      }
    }

    for (key in props) {
      // 获取当前节点中key的值
      cur = props[key];
      // ignore children if the node has textContent or innerHTML,
      // as these will throw away existing DOM nodes and cause removal errors
      // on subsequent patches (#3360)
      if (key === 'textContent' || key === 'innerHTML') {
        if (vnode.children) { vnode.children.length = 0; }
        if (cur === oldProps[key]) { continue }
        // #6601 work around Chrome version <= 55 bug where single textNode
        // replaced by innerHTML/textContent retains its parentNode property
        if (elm.childNodes.length === 1) {
          elm.removeChild(elm.childNodes[0]);
        }
      }
      // 传进来的value,执行这一步
      if (key === 'value' && elm.tagName !== 'PROGRESS') {
        // 将值存储为_value
        //非字符串值将被字符串化
        elm._value = cur;
        // 当值相同时,避免重新设置光标位置
        var strCur = isUndef(cur) ? '' : String(cur);
        // 更新数据
        if (shouldUpdateValue(elm, strCur)) {
          elm.value = strCur;
        }
        // 对传进来的不是value的key进行其他操作
      } else if (key === 'innerHTML' && isSVG(elm.tagName) && isUndef(elm.innerHTML)) {
        // IE不支持SVG元素的innerHTML
        svgContainer = svgContainer || document.createElement('div');
        svgContainer.innerHTML = "<svg>" + cur + "</svg>";
        var svg = svgContainer.firstChild;
        while (elm.firstChild) {
          elm.removeChild(elm.firstChild);
        }
        while (svg.firstChild) {
          elm.appendChild(svg.firstChild);
        }
      } else if (
        //如果旧的和新的VDOM状态相同,则跳过更新。
// ' value '被单独处理,因为DOM值可能是临时的
//与VDOM状态不同步,由于焦点,合成和修改。
//这个#4521通过跳过不必要的' checked '更新。
        cur !== oldProps[key]
      ) {
        try {
          elm[key] = cur;
        } catch (e) {}
      }
    }
  }

  // check platforms/web/util/attrs.js acceptValue


  function shouldUpdateValue (elm, checkVal) {
    return (!elm.composing && (
      elm.tagName === 'OPTION' ||
      isNotInFocusAndDirty(elm, checkVal) ||
      isDirtyWithModifiers(elm, checkVal)
    ))
  }

  function isNotInFocusAndDirty (elm, checkVal) {
    // 当文本框(。Number和。trim)失去焦点,其值为不等于更新的值
    var notInFocus = true;
    // #6157
    // 在访问文档时解决IE错误。在iframe中的activeElement
    try { notInFocus = document.activeElement !== elm; } catch (e) {}
    return notInFocus && elm.value !== checkVal
  }

  function isDirtyWithModifiers (elm, newVal) {
    var value = elm.value;
    // v-model运行时注入
    var modifiers = elm._vModifiers;
    if (isDef(modifiers)) {
      if (modifiers.number) {
        return toNumber(value) !== toNumber(newVal)
      }
      if (modifiers.trim) {
        return value.trim() !== newVal.trim()
      }
    }
    return value !== newVal
  }

  var domProps = {
    create: updateDOMProps,
    update: updateDOMProps
  };

  //把style 字符串 转换成对象 比如'width:100px;height:200px;' 转化成 {width:100px,height:200px}
  var parseStyleText = cached(function (cssText) {
    var res = {};
    //匹配字符串中的 ;符号。但是不属于 (;)的 符号 如果是括号中的;不能匹配出来
    var listDelimiter = /;(?![^(]*\))/g;
    //:+任何字符串
    var propertyDelimiter = /:(.+)/;
    cssText.split(listDelimiter).forEach(function (item) {
      if (item) {
        var tmp = item.split(propertyDelimiter);
        tmp.length > 1 && (res[tmp[0].trim()] = tmp[1].trim());
      }
    });
    return res
  });

  // 在同一个vnode上合并静态和动态样式数据
  function normalizeStyleData (data) {
    var style = normalizeStyleBinding(data.style);
    //静态样式在编译过程中被预处理为一个对象
    // and总是一个新鲜对象,所以合并到它是安全的
    return data.staticStyle
      ? extend(data.staticStyle, style)
      : style
  }

  // 将可能的数组/字符串值规范化为Object
  function normalizeStyleBinding (bindingStyle) {
    if (Array.isArray(bindingStyle)) {
      return toObject(bindingStyle)
    }
    if (typeof bindingStyle === 'string') {
      return parseStyleText(bindingStyle)
    }
    return bindingStyle
  }

  /**
   * 父组件样式应该在子组件样式之后
    *以便父组件的样式可以覆盖它
   */
  function getStyle (vnode, checkChild) {
    var res = {};
    var styleData;

    if (checkChild) {
      var childNode = vnode;
      while (childNode.componentInstance) {
        childNode = childNode.componentInstance._vnode;
        if (
          childNode && childNode.data &&
          //  在childNode上合并静态和动态样式数据
          (styleData = normalizeStyleData(childNode.data))
        ) {
          extend(res, styleData);
        }
      }
    }
    // 在vnode上合并静态和动态样式数据
    if ((styleData = normalizeStyleData(vnode.data))) {
      extend(res, styleData);
    }

    var parentNode = vnode;
    while ((parentNode = parentNode.parent)) {
      // 在parentNode上合并静态和动态样式数据
      if (parentNode.data && (styleData = normalizeStyleData(parentNode.data))) {
        extend(res, styleData);
      }
    }
    return res
  }

  /*  */

  var cssVarRE = /^--/;
  var importantRE = /\s*!important$/;
  var setProp = function (el, name, val) {
    /* istanbul ignore if */
    if (cssVarRE.test(name)) {
      el.style.setProperty(name, val);
    } else if (importantRE.test(val)) {
      // 为一个声明了css样式的对象设置一个新的属性值
      el.style.setProperty(hyphenate(name), val.replace(importantRE, ''), 'important');
    } else {
      var normalizedName = normalize(name);
      if (Array.isArray(val)) {
        // 一个一个地设置它们,浏览器只会设置它能识别的
        for (var i = 0, len = val.length; i < len; i++) {
          el.style[normalizedName] = val[i];
        }
      } else {
        el.style[normalizedName] = val;
      }
    }
  };

  var vendorNames = ['Webkit', 'Moz', 'ms'];

  var emptyStyle;
  var normalize = cached(function (prop) {
    emptyStyle = emptyStyle || document.createElement('div').style;
    prop = camelize(prop);
    if (prop !== 'filter' && (prop in emptyStyle)) {
      return prop
    }
    var capName = prop.charAt(0).toUpperCase() + prop.slice(1);
    for (var i = 0; i < vendorNames.length; i++) {
      var name = vendorNames[i] + capName;
      if (name in emptyStyle) {
        return name
      }
    }
  });

  function updateStyle (oldVnode, vnode) {
    var data = vnode.data;
    var oldData = oldVnode.data;

    if (isUndef(data.staticStyle) && isUndef(data.style) &&
      isUndef(oldData.staticStyle) && isUndef(oldData.style)
    ) {
      return
    }

    var cur, name;
    var el = vnode.elm;
    var oldStaticStyle = oldData.staticStyle;
    var oldStyleBinding = oldData.normalizedStyle || oldData.style || {};

    // 如果存在静态样式,则在执行normalizeStyleData时样式绑定已经合并到其中
    var oldStyle = oldStaticStyle || oldStyleBinding;

    var style = normalizeStyleBinding(vnode.data.style) || {};

    //将正则化样式存储在不同的键下如果它是响应式的,确保克隆它
    vnode.data.normalizedStyle = isDef(style.__ob__)
      ? extend({}, style)
      : style;

    var newStyle = getStyle(vnode, true);

    for (name in oldStyle) {
      if (isUndef(newStyle[name])) {
        setProp(el, name, '');
      }
    }
    for (name in newStyle) {
      cur = newStyle[name];
      if (cur !== oldStyle[name]) {
        // Ie9设置为空没有效果,必须使用空字符串
        setProp(el, name, cur == null ? '' : cur);
      }
    }
  }

  var style = {
    create: updateStyle,
    update: updateStyle
  };

  /*  */

  var whitespaceRE = /\s+/;

  /**
   *添加与SVG兼容的类,因为不支持classList
    * SVG元素在IE中
   */
  function addClass (el, cls) {
    /* istanbul ignore if */
    if (!cls || !(cls = cls.trim())) {
      return
    }

    /* istanbul ignore else */
    if (el.classList) {
      if (cls.indexOf(' ') > -1) {
        cls.split(whitespaceRE).forEach(function (c) { return el.classList.add(c); });
      } else {
        el.classList.add(cls);
      }
    } else {
      var cur = " " + (el.getAttribute('class') || '') + " ";
      if (cur.indexOf(' ' + cls + ' ') < 0) {
        el.setAttribute('class', (cur + cls).trim());
      }
    }
  }

  /**
   * 删除与SVG兼容的类,因为不支持classList
    * SVG元素在IE中 
   */
  function removeClass (el, cls) {
    /* istanbul ignore if */
    if (!cls || !(cls = cls.trim())) {
      return
    }

    /* istanbul ignore else */
    if (el.classList) {
      if (cls.indexOf(' ') > -1) {
        cls.split(whitespaceRE).forEach(function (c) { return el.classList.remove(c); });
      } else {
        el.classList.remove(cls);
      }
      if (!el.classList.length) {
        el.removeAttribute('class');
      }
    } else {
      var cur = " " + (el.getAttribute('class') || '') + " ";
      var tar = ' ' + cls + ' ';
      while (cur.indexOf(tar) >= 0) {
        cur = cur.replace(tar, ' ');
      }
      cur = cur.trim();
      if (cur) {
        el.setAttribute('class', cur);
      } else {
        el.removeAttribute('class');
      }
    }
  }

  // 扩展属性
  function resolveTransition (def$$1) {
    if (!def$$1) {
      return
    }
    /* istanbul ignore else */
    if (typeof def$$1 === 'object') {
      var res = {};
      if (def$$1.css !== false) {
        // class对象扩展到空对象res上
        extend(res, autoCssTransition(def$$1.name || 'v'));
      }
      // 将def上的属性扩展到res对象上
      extend(res, def$$1);
      return res
    } else if (typeof def$$1 === 'string') {
      return autoCssTransition(def$$1)
    }
  }
// 生成包含6个需要使用到的class对象
  var autoCssTransition = cached(function (name) {
    return {
      enterClass: (name + "-enter"),
      enterToClass: (name + "-enter-to"),
      enterActiveClass: (name + "-enter-active"),
      leaveClass: (name + "-leave"),
      leaveToClass: (name + "-leave-to"),
      leaveActiveClass: (name + "-leave-active")
    }
  });

  var hasTransition = inBrowser && !isIE9;
  var TRANSITION = 'transition';
  var ANIMATION = 'animation';

  // Transition property/event sniffing
  var transitionProp = 'transition';
  var transitionEndEvent = 'transitionend';
  var animationProp = 'animation';
  var animationEndEvent = 'animationend';
  if (hasTransition) {
    /* istanbul ignore if */
    if (window.ontransitionend === undefined &&
      window.onwebkittransitionend !== undefined
    ) {
      transitionProp = 'WebkitTransition';
      transitionEndEvent = 'webkitTransitionEnd';
    }
    if (window.onanimationend === undefined &&
      window.onwebkitanimationend !== undefined
    ) {
      animationProp = 'WebkitAnimation';
      animationEndEvent = 'webkitAnimationEnd';
    }
  }

  // binding to window is necessary to make hot reload work in IE in strict mode
  var raf = inBrowser
    ? window.requestAnimationFrame
      ? window.requestAnimationFrame.bind(window)
      : setTimeout
    : /* istanbul ignore next */ function (fn) { return fn(); };
    // 参数为 fn,即下一帧需要执行的方法
  function nextFrame (fn) {
    raf(function () {
      raf(fn);
    });
  }

  function addTransitionClass (el, cls) {
    var transitionClasses = el._transitionClasses || (el._transitionClasses = []);
    if (transitionClasses.indexOf(cls) < 0) {
      transitionClasses.push(cls);
      addClass(el, cls);
    }
  }

  function removeTransitionClass (el, cls) {
    if (el._transitionClasses) {
      remove(el._transitionClasses, cls);
    }
    removeClass(el, cls);
  }
  // 通过 getTransitionInfo 获取到 transition 的一些信息,
  // 如 type、timeout、propCount,并为 el 元素绑定上 onEnd。
  function whenTransitionEnds (
    el,
    expectedType,
    cb
  ) {
    var ref = getTransitionInfo(el, expectedType);
    var type = ref.type;
    var timeout = ref.timeout;
    var propCount = ref.propCount;
    if (!type) { return cb() }
    var event = type === TRANSITION ? transitionEndEvent : animationEndEvent;
    var ended = 0;
    var end = function () {
      el.removeEventListener(event, onEnd);
      cb();
    };
    var onEnd = function (e) {
      if (e.target === el) {
        if (++ended >= propCount) {
          end();
        }
      }
    };
    setTimeout(function () {
      if (ended < propCount) {
        end();
      }
    }, timeout + 1);
    el.addEventListener(event, onEnd);
  }

  var transformRE = /\b(transform|all)(,|$)/;

  function getTransitionInfo (el, expectedType) {
    var styles = window.getComputedStyle(el);
    // JSDOM may return undefined for transition properties
    var transitionDelays = (styles[transitionProp + 'Delay'] || '').split(', ');
    var transitionDurations = (styles[transitionProp + 'Duration'] || '').split(', ');
    var transitionTimeout = getTimeout(transitionDelays, transitionDurations);
    var animationDelays = (styles[animationProp + 'Delay'] || '').split(', ');
    var animationDurations = (styles[animationProp + 'Duration'] || '').split(', ');
    var animationTimeout = getTimeout(animationDelays, animationDurations);

    var type;
    var timeout = 0;
    var propCount = 0;
    /* istanbul ignore if */
    if (expectedType === TRANSITION) {
      if (transitionTimeout > 0) {
        type = TRANSITION;
        timeout = transitionTimeout;
        propCount = transitionDurations.length;
      }
    } else if (expectedType === ANIMATION) {
      if (animationTimeout > 0) {
        type = ANIMATION;
        timeout = animationTimeout;
        propCount = animationDurations.length;
      }
    } else {
      timeout = Math.max(transitionTimeout, animationTimeout);
      type = timeout > 0
        ? transitionTimeout > animationTimeout
          ? TRANSITION
          : ANIMATION
        : null;
      propCount = type
        ? type === TRANSITION
          ? transitionDurations.length
          : animationDurations.length
        : 0;
    }
    var hasTransform =
      type === TRANSITION &&
      transformRE.test(styles[transitionProp + 'Property']);
    return {
      type: type,
      timeout: timeout,
      propCount: propCount,
      hasTransform: hasTransform
    }
  }

  function getTimeout (delays, durations) {
    /* istanbul ignore next */
    while (delays.length < durations.length) {
      delays = delays.concat(delays);
    }

    return Math.max.apply(null, durations.map(function (d, i) {
      return toMs(d) + toMs(delays[i])
    }))
  }

  // Chromium的旧版本(低于61.0.3163.100)采用浮点数
//以一种与区域相关的方式,使用逗号而不是点。
//如果逗号没有被点替换,输入将被舍入(即acting . 作为一个floor函数)导致意外的行为
  function toMs (s) {
    return Number(s.slice(0, -1).replace(',', '.')) * 1000
  }

  /*  */

  function enter (vnode, toggleDisplay) {
    var el = vnode.elm;

    //现在调用leave回调函数
    if (isDef(el._leaveCb)) {
      el._leaveCb.cancelled = true;
      el._leaveCb();
    }

    var data = resolveTransition(vnode.data.transition);
    if (isUndef(data)) {
      return
    }

    /* istanbul ignore if */
    if (isDef(el._enterCb) || el.nodeType !== 1) {
      return
    }

    var css = data.css;
    var type = data.type;
    var enterClass = data.enterClass;
    var enterToClass = data.enterToClass;
    var enterActiveClass = data.enterActiveClass;
    var appearClass = data.appearClass;
    var appearToClass = data.appearToClass;
    var appearActiveClass = data.appearActiveClass;
    var beforeEnter = data.beforeEnter;
    var enter = data.enter;
    var afterEnter = data.afterEnter;
    var enterCancelled = data.enterCancelled;
    var beforeAppear = data.beforeAppear;
    var appear = data.appear;
    var afterAppear = data.afterAppear;
    var appearCancelled = data.appearCancelled;
    var duration = data.duration;

    // activeInstance将始终是管理这个的<transition>组件</transition>
  //转换。要检查的一个边界情况是放置<转场>时
  //作为子组件的根节点。那样的话,我们得核实一下
  // <transition>'s parent for appear check</transition>
    var context = activeInstance;
    var transitionNode = activeInstance.$vnode;
    while (transitionNode && transitionNode.parent) {
      context = transitionNode.context;
      transitionNode = transitionNode.parent;
    }

// 上下文实例没有 mounted 或者 vnode 不是根节点插入的
    var isAppear = !context._isMounted || !vnode.isRootInsert;

    if (isAppear && !appear && appear !== '') {
      return
    }
// 过渡 class 处理
// 如果有定义appear且有对应的appearClass
    var startClass = isAppear && appearClass
    // 执行定义的appearClass
      ? appearClass
      // 否则还是执行enterClass
      : enterClass;
    var activeClass = isAppear && appearActiveClass
      ? appearActiveClass
      : enterActiveClass;
    var toClass = isAppear && appearToClass
      ? appearToClass
      : enterToClass;

// 钩子函数处理
    var beforeEnterHook = isAppear
      ? (beforeAppear || beforeEnter)
      : beforeEnter;
    var enterHook = isAppear
      ? (typeof appear === 'function' ? appear : enter)
      : enter;
    var afterEnterHook = isAppear
      ? (afterAppear || afterEnter)
      : afterEnter;
    var enterCancelledHook = isAppear
      ? (appearCancelled || enterCancelled)
      : enterCancelled;
 // 获取 enter 动画执行时间
    var explicitEnterDuration = toNumber(
      isObject(duration)
        ? duration.enter
        : duration
    );

    if (explicitEnterDuration != null) {
      checkDuration(explicitEnterDuration, 'enter', vnode);
    }

// 用户是否想介入控制 css 动画
    var expectsCSS = css !== false && !isIE9;
    var userWantsControl = getHookArgumentsLength(enterHook);

    var cb = el._enterCb = once(function () {
      // 动画受 css 控制,则移除掉 toClass 和 activeClass
      if (expectsCSS) {
        // 为当前元素 el 增加指定的 class
        removeTransitionClass(el, toClass);
        removeTransitionClass(el, activeClass);
      }
      if (cb.cancelled) {
        if (expectsCSS) {
          removeTransitionClass(el, startClass);
        }
        // 判定过渡是否取消,若取消了,直接移除 startClass 并执行 enterCancelledHook
        enterCancelledHook && enterCancelledHook(el);
      } else {
        afterEnterHook && afterEnterHook(el);
      }
      el._enterCb = null;
    });

    if (!vnode.data.show) {
      // 对 insert 钩子函数进行合并
      mergeVNodeHook(vnode, 'insert', function () {
        var parent = el.parentNode;
        var pendingNode = parent && parent._pending && parent._pending[vnode.key];
        if (pendingNode &&
          pendingNode.tag === vnode.tag &&
          pendingNode.elm._leaveCb
        ) {
          pendingNode.elm._leaveCb();
        }
        enterHook && enterHook(el, cb);
      });
    }

    //执行 beforeEnterHook 钩子,并将 DOM 节点 el 传入
    beforeEnterHook && beforeEnterHook(el);
    // 判断是否希望通过 css 来控制动画,如果是 true 则,执行 addTransitionClass() 方法为节点加上 startClass 和 activeClass
    if (expectsCSS) {
      addTransitionClass(el, startClass);
      addTransitionClass(el, activeClass);
      // 执行 nextFrame() 进入下一帧,下一帧主要是移除掉上一帧增加好的 class
      nextFrame(function () {
        removeTransitionClass(el, startClass);
        // 判断过渡是否取消,如未取消,则加上 toClass 过渡类
        if (!cb.cancelled) {
          addTransitionClass(el, toClass);
          if (!userWantsControl) {
            // 如果用户没有通过 enterHook 钩子函数来控制动画,此时若用户指定了 duration 时间,则执行 setTimeout 进行 duration 时长的延时
            if (isValidDuration(explicitEnterDuration)) {
              setTimeout(cb, explicitEnterDuration);
            } else {
              // 执行 whenTransitionEnds 决定 cb 的执行时机
              whenTransitionEnds(el, type, cb);
            }
          }
        }
      });
    }

    if (vnode.data.show) {
      toggleDisplay && toggleDisplay();
      enterHook && enterHook(el, cb);
    }

    if (!expectsCSS && !userWantsControl) {
      cb();
    }
  }

  function leave (vnode, rm) {
    var el = vnode.elm;

    // 如果进入enter时,_leaveCb没执行,立刻执行
    if (isDef(el._enterCb)) {
      // 执行了_leaveCb的标记
      el._enterCb.cancelled = true;
      // cb._leaveCb执行后会变成null
      el._enterCb();
    }

    var data = resolveTransition(vnode.data.transition);
    if (isUndef(data) || el.nodeType !== 1) {
      return rm()
    }

    /* istanbul ignore if */
    if (isDef(el._leaveCb)) {
      return
    }

    var css = data.css;
    var type = data.type;
    var leaveClass = data.leaveClass;
    var leaveToClass = data.leaveToClass;
    var leaveActiveClass = data.leaveActiveClass;
    var beforeLeave = data.beforeLeave;
    var leave = data.leave;
    var afterLeave = data.afterLeave;
    var leaveCancelled = data.leaveCancelled;
    var delayLeave = data.delayLeave;
    var duration = data.duration;

    var expectsCSS = css !== false && !isIE9;
    // 传入enter钩子
  // 如果钩子里enter函数的参数大于1,说明有传入done函数,表示用户想要自己控制
  // 这也是为什么enter里动画结束后需要调用done函数
    var userWantsControl = getHookArgumentsLength(leave);

    var explicitLeaveDuration = toNumber(
      isObject(duration)
        ? duration.leave
        : duration
    );

    if (isDef(explicitLeaveDuration)) {
      checkDuration(explicitLeaveDuration, 'leave', vnode);
    }
// 这里定义了el._enterCb函数,对应leave里就是el._leaveCb
    var cb = el._leaveCb = once(function () {
      if (el.parentNode && el.parentNode._pending) {
        el.parentNode._pending[vnode.key] = null;
      }
      if (expectsCSS) {
        // 移除v-leave-to
        removeTransitionClass(el, leaveToClass);
        // 移除v-leave-active
        removeTransitionClass(el, leaveActiveClass);
      }
      // 如果在leave的状态里,enter状态的cb函数没执行,则执行enterCancelled钩子
      if (cb.cancelled) {
        if (expectsCSS) {
          removeTransitionClass(el, leaveClass);
        }
        leaveCancelled && leaveCancelled(el);
      } else {
        rm();
        // 否则正常的执行afterEnter钩子
        afterLeave && afterLeave(el);
      }
      el._leaveCb = null;
    });

    if (delayLeave) {
      delayLeave(performLeave);
    } else {
      performLeave();
    }

    function performLeave () {
      // the delayed leave may have already been cancelled
      if (cb.cancelled) {
        return
      }
      // record leaving element
      if (!vnode.data.show && el.parentNode) {
        (el.parentNode._pending || (el.parentNode._pending = {}))[(vnode.key)] = vnode;
      }
      beforeLeave && beforeLeave(el);
      if (expectsCSS) {
        addTransitionClass(el, leaveClass);
        addTransitionClass(el, leaveActiveClass);
        nextFrame(function () {
          removeTransitionClass(el, leaveClass);
          if (!cb.cancelled) {
            addTransitionClass(el, leaveToClass);
            if (!userWantsControl) {
              if (isValidDuration(explicitLeaveDuration)) {
                setTimeout(cb, explicitLeaveDuration);
              } else {
                whenTransitionEnds(el, type, cb);
              }
            }
          }
        });
      }
      leave && leave(el, cb);
      if (!expectsCSS && !userWantsControl) {
        cb();
      }
    }
  }

  // 仅在dev模式下使用
  function checkDuration (val, name, vnode) {
    if (typeof val !== 'number') {
      warn(
        "<transition> explicit " + name + " duration is not a valid number - " +
        "got " + (JSON.stringify(val)) + ".",
        vnode.context
      );
    } else if (isNaN(val)) {
      warn(
        "<transition> explicit " + name + " duration is NaN - " +
        'the duration expression might be incorrect.',
        vnode.context
      );
    }
  }

  function isValidDuration (val) {
    return typeof val === 'number' && !isNaN(val)
  }

  /**
   将转换钩子的参数长度规范化。问题可能是:
* 一个合并的钩子(调用者)与原来的。fns
一个封装的组件方法(check ._length)
* 一个普通函数(.length)
   */
// 用于判断 enterHook
  function getHookArgumentsLength (fn) {
    if (isUndef(fn)) {
      return false
    }
    var invokerFns = fn.fns;
    if (isDef(invokerFns)) {
      return getHookArgumentsLength(
        Array.isArray(invokerFns)
          ? invokerFns[0]
          : invokerFns
      )
    } else {
      return (fn._length || fn.length) > 1
    }
  }
  // 在 create 和 activate 的时候执行 enter()
  function _enter (_, vnode) {
    if (vnode.data.show !== true) {
      enter(vnode);
    }
  }

  var transition = inBrowser ? {
    create: _enter,
    activate: _enter,
    remove: function remove$$1 (vnode, rm) {
      // remove 的时候执行 leave()
      if (vnode.data.show !== true) {
        leave(vnode, rm);
      } else {
        rm();
      }
    }
  } : {};

  var platformModules = [
    attrs,
    klass,
    events,
    domProps,
    style,
    transition
  ];

  /*  */

  //指令模块应该最后应用
//内置模块已经被应用。
  var modules = platformModules.concat(baseModules);

  var patch = createPatchFunction({ nodeOps: nodeOps, modules: modules });

  /**
*不类型检查这个文件,因为flow不喜欢附加
*元素的属性。
*/

  /* istanbul ignore if */
  if (isIE9) {
    // http://www.matts411.com/post/internet-explorer-9-oninput/
    document.addEventListener('selectionchange', function () {
      var el = document.activeElement;
      if (el && el.vmodel) {
        trigger(el, 'input');
      }
    });
  }

  var directive = {
    inserted: function inserted (el, binding, vnode, oldVnode) {
      if (vnode.tag === 'select') {
        // #6903
        if (oldVnode.elm && !oldVnode.elm._vOptions) {
          mergeVNodeHook(vnode, 'postpatch', function () {
            directive.componentUpdated(el, binding, vnode);
          });
        } else {
          setSelected(el, binding, vnode.context);
        }
        el._vOptions = [].map.call(el.options, getValue);
      } else if (vnode.tag === 'textarea' || isTextInputType(el.type)) {
        el._vModifiers = binding.modifiers;
        if (!binding.modifiers.lazy) {
          el.addEventListener('compositionstart', onCompositionStart);
          el.addEventListener('compositionend', onCompositionEnd);
          //当Safari < 10.2 & UIWebView不触发compositionend时
          //在确定构图选择之前切换焦点
          //这也修复了一些浏览器的问题,如iOS Chrome
          //触发“change”而不是“input”自动完成。
          el.addEventListener('change', onCompositionEnd);
          /* istanbul ignore if */
          if (isIE9) {
            el.vmodel = true;
          }
        }
      }
    },

    componentUpdated: function componentUpdated (el, binding, vnode) {
      if (vnode.tag === 'select') {
        setSelected(el, binding, vnode.context);
       //如果v-for渲染的选项发生了变化,
      //该值可能与渲染选项不同步。
      //检测这种情况并过滤掉不再匹配的值
      //在DOM中的选项。
        var prevOptions = el._vOptions;
        var curOptions = el._vOptions = [].map.call(el.options, getValue);
        if (curOptions.some(function (o, i) { return !looseEqual(o, prevOptions[i]); })) {
          //触发改变事件
          //没有找到匹配的选项,至少一个值
          var needReset = el.multiple
            ? binding.value.some(function (v) { return hasNoMatchingOption(v, curOptions); })
            : binding.value !== binding.oldValue && hasNoMatchingOption(binding.value, curOptions);
          if (needReset) {
            trigger(el, 'change');
          }
        }
      }
    }
  };

  function setSelected (el, binding, vm) {
    actuallySetSelected(el, binding, vm);
    /* istanbul ignore if */
    if (isIE || isEdge) {
      setTimeout(function () {
        actuallySetSelected(el, binding, vm);
      }, 0);
    }
  }

  function actuallySetSelected (el, binding, vm) {
    var value = binding.value;
    var isMultiple = el.multiple;
    if (isMultiple && !Array.isArray(value)) {
      warn(
        "<select multiple v-model=\"" + (binding.expression) + "\"> " +
        "expects an Array value for its binding, but got " + (Object.prototype.toString.call(value).slice(8, -1)),
        vm
      );
      return
    }
    var selected, option;
    for (var i = 0, l = el.options.length; i < l; i++) {
      option = el.options[i];
      if (isMultiple) {
        selected = looseIndexOf(value, getValue(option)) > -1;
        if (option.selected !== selected) {
          option.selected = selected;
        }
      } else {
        if (looseEqual(getValue(option), value)) {
          if (el.selectedIndex !== i) {
            el.selectedIndex = i;
          }
          return
        }
      }
    }
    if (!isMultiple) {
      el.selectedIndex = -1;
    }
  }

  function hasNoMatchingOption (value, options) {
    return options.every(function (o) { return !looseEqual(o, value); })
  }

  function getValue (option) {
    return '_value' in option
      ? option._value
      : option.value
  }

  function onCompositionStart (e) {
    e.target.composing = true;
  }

  function onCompositionEnd (e) {
    //防止无缘无故触发输入事件
    if (!e.target.composing) { return }
    e.target.composing = false;
    trigger(e.target, 'input');
  }

  function trigger (el, type) {
    var e = document.createEvent('HTMLEvents');
    e.initEvent(type, true, true);
    el.dispatchEvent(e);
  }

  /*  */

  // 递归地搜索在组件根内部定义的可能转换
  function locateNode (vnode) {
    return vnode.componentInstance && (!vnode.data || !vnode.data.transition)
      ? locateNode(vnode.componentInstance._vnode)
      : vnode
  }
// 控制 el 的 display 属性
  var show = {
    bind: function bind (el, ref, vnode) {
      var value = ref.value;

      vnode = locateNode(vnode);
      var transition$$1 = vnode.data && vnode.data.transition;
      var originalDisplay = el.__vOriginalDisplay =
        el.style.display === 'none' ? '' : el.style.display;
      if (value && transition$$1) {
        vnode.data.show = true;
        enter(vnode, function () {
          el.style.display = originalDisplay;
        });
      } else {
        el.style.display = value ? originalDisplay : 'none';
      }
    },

    update: function update (el, ref, vnode) {
      var value = ref.value;
      var oldValue = ref.oldValue;

      /* istanbul ignore if */
      if (!value === !oldValue) { return }
      vnode = locateNode(vnode);
      var transition$$1 = vnode.data && vnode.data.transition;
      if (transition$$1) {
        vnode.data.show = true;
        if (value) {
          enter(vnode, function () {
            el.style.display = el.__vOriginalDisplay;
          });
        } else {
          leave(vnode, function () {
            el.style.display = 'none';
          });
        }
      } else {
        el.style.display = value ? el.__vOriginalDisplay : 'none';
      }
    },

    unbind: function unbind (
      el,
      binding,
      vnode,
      oldVnode,
      isDestroy
    ) {
      if (!isDestroy) {
        el.style.display = el.__vOriginalDisplay;
      }
    }
  };
// 平台指令
  var platformDirectives = {
    model: directive,
    show: show
  };

  /*过渡Props对象*/

  var transitionProps = {
    name: String,
    appear: Boolean,
    css: Boolean,
    mode: String,
    type: String,
    enterClass: String,
    leaveClass: String,
    enterToClass: String,
    leaveToClass: String,
    enterActiveClass: String,
    leaveActiveClass: String,
    appearClass: String,
    appearActiveClass: String,
    appearToClass: String,
    duration: [Number, String, Object]
  };

  //如果子组件也是一个抽象组件,例如:<keep-alive></keep-alive>
//我们想要递归地检索要渲染的真实组件
  function getRealChild (vnode) {
    var compOptions = vnode && vnode.componentOptions;
    if (compOptions && compOptions.Ctor.options.abstract) {
      return getRealChild(getFirstComponentChild(compOptions.children))
    } else {
      return vnode
    }
  }
  // 从组件实例上获取过渡需要的 data 数据
  function extractTransitionData (comp) {
    var data = {};
    var options = comp.$options;
    // 遍历当前组件的 options.propsData 并赋值给 data
    for (var key in options.propsData) {
      data[key] = comp[key];
    }
    //提取监听器并直接传递给转换方法
    var listeners = options._parentListeners;
    // 遍历父组件的事件并将其赋值给 data
    for (var key$1 in listeners) {
      data[camelize(key$1)] = listeners[key$1];
    }
    return data
  }
 // 占位提示
  function placeholder (h, rawChild) {
    if (/\d-keep-alive$/.test(rawChild.tag)) {
      return h('keep-alive', {
        props: rawChild.componentOptions.propsData
      })
    }
  }
  // 往上一层一层寻找父节点是否也拥有 transition 属性
  function hasParentTransition (vnode) {
    while ((vnode = vnode.parent)) {
      if (vnode.data.transition) {
        return true
      }
    }
  }
// 判断子对象是否相同
  function isSameChild (child, oldChild) {
    return oldChild.key === child.key && oldChild.tag === child.tag
  }

  var isNotTextNode = function (c) { return c.tag || isAsyncPlaceholder(c); };

  var isVShowDirective = function (d) { return d.name === 'show'; };

  var Transition = {
    name: 'transition',
    props: transitionProps,
    abstract: true,

    render: function render (h) {
      var this$1 = this;
      // 首先从默认插槽中获取到 <transition> 包裹的子节点
      var children = this.$slots.default;
      if (!children) {
        return
      }

      // 对其进行 filter 过滤,将文本节点以及空格给过滤掉
      children = children.filter(isNotTextNode);
      /* istanbul ignore if */
      if (!children.length) {
        return
      }

      // 如果不存在子节点则直接 return,如果子节点为多个,则报错,因为 <transition> 组件只能有一个子节点
      if (children.length > 1) {
        warn(
          '<transition> can only be used on a single element. Use ' +
          '<transition-group> for lists.',
          this.$parent
        );
      }

      var mode = this.mode;

      // 判断 mode 是否为 in-out 和 out-in 两种模式,如果不是,直接报错
      if (mode && mode !== 'in-out' && mode !== 'out-in'
      ) {
        warn(
          'invalid <transition> mode: ' + mode,
          this.$parent
        );
      }
      // rawChild 为 <transition> 组件包裹的第一个 vnode 子节点
      var rawChild = children[0];

     //如果这是一个组件的根节点和组件的
    //父容器节点也有过渡。
      if (hasParentTransition(this.$vnode)) {
        return rawChild
      }

      //将转换数据应用到child
      //使用getRealChild()来忽略抽象组件,例如keep-alive
      var child = getRealChild(rawChild);
      /* istanbul ignore if */
      if (!child) {
        return rawChild
      }

      if (this._leaving) {
        return placeholder(h, rawChild)
      }

     //确保一个键对于vnode类型和这个转换是唯一的组件实例。此键将用于删除挂起的离开节点在进入。
      var id = "__transition-" + (this._uid) + "-";
      child.key = child.key == null
        ? child.isComment
          ? id + 'comment'
          : id + child.tag
        : isPrimitive(child.key)
          ? (String(child.key).indexOf(id) === 0 ? child.key : id + child.key)
          : child.key;

      var data = (child.data || (child.data = {})).transition = extractTransitionData(this);
      var oldRawChild = this._vnode;
      var oldChild = getRealChild(oldRawChild);

      //标记v-show过渡模块可以将控制权交给指令
      if (child.data.directives && child.data.directives.some(isVShowDirective)) {
        child.data.show = true;
      }
      // 对新旧 child 进行比较,并对部分钩子函数进行 hook merge 等操作
      if (
        oldChild &&
        oldChild.data &&
        !isSameChild(child, oldChild) &&
        !isAsyncPlaceholder(oldChild) &&
        // 组件根节点是一个注释节点
        !(oldChild.componentInstance && oldChild.componentInstance._vnode.isComment)
      ) {
        //将旧的子转换数据替换为新的子转换数据对于动态转换很重要!
        var oldData = oldChild.data.transition = extend({}, data);
        // 处理过渡模式
        if (mode === 'out-in') {
          // 当leave结束时返回占位符节点和队列更新
          this._leaving = true;
          mergeVNodeHook(oldData, 'afterLeave', function () {
            this$1._leaving = false;
            this$1.$forceUpdate();
          });
          return placeholder(h, rawChild)
        } else if (mode === 'in-out') {
          if (isAsyncPlaceholder(child)) {
            return oldRawChild
          }
          var delayedLeave;
          var performLeave = function () { delayedLeave(); };
          mergeVNodeHook(data, 'afterEnter', performLeave);
          mergeVNodeHook(data, 'enterCancelled', performLeave);
          mergeVNodeHook(oldData, 'delayLeave', function (leave) { delayedLeave = leave; });
        }
      }

      return rawChild
    }
  };

  /*  */

  var props = extend({
    tag: String,
    moveClass: String
  }, transitionProps);

  delete props.mode;

  var TransitionGroup = {
    props: props,

    beforeMount: function beforeMount () {
      var this$1 = this;

      var update = this._update;
      this._update = function (vnode, hydrating) {
        var restoreActiveInstance = setActiveInstance(this$1);
        // force removing pass
        this$1.__patch__(
          this$1._vnode,
          this$1.kept,
          false, // hydrating
          true // removeOnly (!important, avoids unnecessary moves)
        );
        this$1._vnode = this$1.kept;
        restoreActiveInstance();
        update.call(this$1, vnode, hydrating);
      };
    },

    render: function render (h) {
      var tag = this.tag || this.$vnode.data.tag || 'span';
      var map = Object.create(null);
      var prevChildren = this.prevChildren = this.children;
      var rawChildren = this.$slots.default || [];
      var children = this.children = [];
      var transitionData = extractTransitionData(this);

      for (var i = 0; i < rawChildren.length; i++) {
        var c = rawChildren[i];
        if (c.tag) {
          if (c.key != null && String(c.key).indexOf('__vlist') !== 0) {
            children.push(c);
            map[c.key] = c
            ;(c.data || (c.data = {})).transition = transitionData;
          } else {
            var opts = c.componentOptions;
            var name = opts ? (opts.Ctor.options.name || opts.tag || '') : c.tag;
            warn(("<transition-group> children must be keyed: <" + name + ">"));
          }
        }
      }

      if (prevChildren) {
        var kept = [];
        var removed = [];
        for (var i$1 = 0; i$1 < prevChildren.length; i$1++) {
          var c$1 = prevChildren[i$1];
          c$1.data.transition = transitionData;
          c$1.data.pos = c$1.elm.getBoundingClientRect();
          if (map[c$1.key]) {
            kept.push(c$1);
          } else {
            removed.push(c$1);
          }
        }
        this.kept = h(tag, null, kept);
        this.removed = removed;
      }

      return h(tag, null, children)
    },

    updated: function updated () {
      var children = this.prevChildren;
      var moveClass = this.moveClass || ((this.name || 'v') + '-move');
      if (!children.length || !this.hasMove(children[0].elm, moveClass)) {
        return
      }

      // we divide the work into three loops to avoid mixing DOM reads and writes
      // in each iteration - which helps prevent layout thrashing.
      children.forEach(callPendingCbs);
      children.forEach(recordPosition);
      children.forEach(applyTranslation);

      // force reflow to put everything in position
      // assign to this to avoid being removed in tree-shaking
      // $flow-disable-line
      this._reflow = document.body.offsetHeight;

      children.forEach(function (c) {
        if (c.data.moved) {
          var el = c.elm;
          var s = el.style;
          addTransitionClass(el, moveClass);
          s.transform = s.WebkitTransform = s.transitionDuration = '';
          el.addEventListener(transitionEndEvent, el._moveCb = function cb (e) {
            if (e && e.target !== el) {
              return
            }
            if (!e || /transform$/.test(e.propertyName)) {
              el.removeEventListener(transitionEndEvent, cb);
              el._moveCb = null;
              removeTransitionClass(el, moveClass);
            }
          });
        }
      });
    },

    methods: {
      hasMove: function hasMove (el, moveClass) {
        /* istanbul ignore if */
        if (!hasTransition) {
          return false
        }
        /* istanbul ignore if */
        if (this._hasMove) {
          return this._hasMove
        }
        // Detect whether an element with the move class applied has
        // CSS transitions. Since the element may be inside an entering
        // transition at this very moment, we make a clone of it and remove
        // all other transition classes applied to ensure only the move class
        // is applied.
        var clone = el.cloneNode();
        if (el._transitionClasses) {
          el._transitionClasses.forEach(function (cls) { removeClass(clone, cls); });
        }
        addClass(clone, moveClass);
        clone.style.display = 'none';
        this.$el.appendChild(clone);
        var info = getTransitionInfo(clone);
        this.$el.removeChild(clone);
        return (this._hasMove = info.hasTransform)
      }
    }
  };
  // 判断每个节点前一帧的过渡动画是否执行完毕,如果没有执行完,则提前执行 _moveCb() 和 _enterCb()
  function callPendingCbs (c) {
    /* istanbul ignore if */
    if (c.elm._moveCb) {
      c.elm._moveCb();
    }
    /* istanbul ignore if */
    if (c.elm._enterCb) {
      c.elm._enterCb();
    }
  }
  // 记录每个节点的新位置
  function recordPosition (c) {
    c.data.newPos = c.elm.getBoundingClientRect();
  }
  // 分别获取节点新旧位置,并计算差值,若存在差值,
  // 则通过设置节点的 transform 属性将需要移动的节点位置偏移到之前的位置,为列表 move 做准备
  function applyTranslation (c) {
    var oldPos = c.data.pos;
    var newPos = c.data.newPos;
    var dx = oldPos.left - newPos.left;
    var dy = oldPos.top - newPos.top;
    if (dx || dy) {
      c.data.moved = true;
      var s = c.elm.style;
      s.transform = s.WebkitTransform = "translate(" + dx + "px," + dy + "px)";
      s.transitionDuration = '0s';
    }
  }

  var platformComponents = {
    Transition: Transition,
    TransitionGroup: TransitionGroup
  };

  /*  */

  // install platform specific utils
  Vue.config.mustUseProp = mustUseProp;
  Vue.config.isReservedTag = isReservedTag;
  Vue.config.isReservedAttr = isReservedAttr;
  Vue.config.getTagNamespace = getTagNamespace;
  Vue.config.isUnknownElement = isUnknownElement;

  // install platform runtime directives & components
  extend(Vue.options.directives, platformDirectives);
  extend(Vue.options.components, platformComponents);

  // install platform patch function
  Vue.prototype.__patch__ = inBrowser ? patch : noop;

  // public mount method
  Vue.prototype.$mount = function (
    el,
    hydrating
  ) {
    el = el && inBrowser ? query(el) : undefined;
    return mountComponent(this, el, hydrating)
  };

  // devtools global hook
  /* istanbul ignore next */
  if (inBrowser) {
    setTimeout(function () {
      if (config.devtools) {
        if (devtools) {
          devtools.emit('init', Vue);
        } else {
          console[console.info ? 'info' : 'log'](
            'Download the Vue Devtools extension for a better development experience:\n' +
            'https://github.com/vuejs/vue-devtools'
          );
        }
      }
      if (config.productionTip !== false &&
        typeof console !== 'undefined'
      ) {
        console[console.info ? 'info' : 'log'](
          "You are running Vue in development mode.\n" +
          "Make sure to turn on production mode when deploying for production.\n" +
          "See more tips at https://vuejs.org/guide/deployment.html"
        );
      }
    }, 0);
  }

  /*  */

  var defaultTagRE = /\{\{((?:.|\r?\n)+?)\}\}/g;
  var regexEscapeRE = /[-.*+?^${}()|[\]\/\\]/g;

  var buildRegex = cached(function (delimiters) {
    var open = delimiters[0].replace(regexEscapeRE, '\\$&');
    var close = delimiters[1].replace(regexEscapeRE, '\\$&');
    return new RegExp(open + '((?:.|\\n)+?)' + close, 'g')
  });



  function parseText (
    text,
    delimiters
  ) {
    var tagRE = delimiters ? buildRegex(delimiters) : defaultTagRE;
    if (!tagRE.test(text)) {
      return
    }
    var tokens = [];
    var rawTokens = [];
    var lastIndex = tagRE.lastIndex = 0;
    var match, index, tokenValue;
    while ((match = tagRE.exec(text))) {
      index = match.index;
      // push text token
      if (index > lastIndex) {
        rawTokens.push(tokenValue = text.slice(lastIndex, index));
        tokens.push(JSON.stringify(tokenValue));
      }
      // tag token
      var exp = parseFilters(match[1].trim());
      tokens.push(("_s(" + exp + ")"));
      rawTokens.push({ '@binding': exp });
      lastIndex = index + match[0].length;
    }
    if (lastIndex < text.length) {
      rawTokens.push(tokenValue = text.slice(lastIndex));
      tokens.push(JSON.stringify(tokenValue));
    }
    return {
      expression: tokens.join('+'),
      tokens: rawTokens
    }
  }

  /*  */

  function transformNode (el, options) {
    var warn = options.warn || baseWarn;
    var staticClass = getAndRemoveAttr(el, 'class');
    if (staticClass) {
      var res = parseText(staticClass, options.delimiters);
      if (res) {
        warn(
          "class=\"" + staticClass + "\": " +
          'Interpolation inside attributes has been removed. ' +
          'Use v-bind or the colon shorthand instead. For example, ' +
          'instead of <div class="{{ val }}">, use <div :class="val">.',
          el.rawAttrsMap['class']
        );
      }
    }
    if (staticClass) {
      el.staticClass = JSON.stringify(staticClass);
    }
    var classBinding = getBindingAttr(el, 'class', false /* getStatic */);
    if (classBinding) {
      el.classBinding = classBinding;
    }
  }

  function genData (el) {
    var data = '';
    if (el.staticClass) {
      data += "staticClass:" + (el.staticClass) + ",";
    }
    if (el.classBinding) {
      data += "class:" + (el.classBinding) + ",";
    }
    return data
  }

  var klass$1 = {
    staticKeys: ['staticClass'],
    transformNode: transformNode,
    genData: genData
  };

  /*  */

  function transformNode$1 (el, options) {
    var warn = options.warn || baseWarn;
    var staticStyle = getAndRemoveAttr(el, 'style');
    if (staticStyle) {
      /* istanbul ignore if */
      {
        var res = parseText(staticStyle, options.delimiters);
        if (res) {
          warn(
            "style=\"" + staticStyle + "\": " +
            'Interpolation inside attributes has been removed. ' +
            'Use v-bind or the colon shorthand instead. For example, ' +
            'instead of <div style="{{ val }}">, use <div :style="val">.',
            el.rawAttrsMap['style']
          );
        }
      }
      el.staticStyle = JSON.stringify(parseStyleText(staticStyle));
    }

    var styleBinding = getBindingAttr(el, 'style', false /* getStatic */);
    if (styleBinding) {
      el.styleBinding = styleBinding;
    }
  }

  function genData$1 (el) {
    var data = '';
    if (el.staticStyle) {
      data += "staticStyle:" + (el.staticStyle) + ",";
    }
    if (el.styleBinding) {
      data += "style:(" + (el.styleBinding) + "),";
    }
    return data
  }

  var style$1 = {
    staticKeys: ['staticStyle'],
    transformNode: transformNode$1,
    genData: genData$1
  };

  /*  */

  var decoder;

  var he = {
    decode: function decode (html) {
      decoder = decoder || document.createElement('div');
      decoder.innerHTML = html;
      return decoder.textContent
    }
  };

  /*  */

  var isUnaryTag = makeMap(
    'area,base,br,col,embed,frame,hr,img,input,isindex,keygen,' +
    'link,meta,param,source,track,wbr'
  );

  // 你可以有意保留的元素(它们会自动关闭)
  var canBeLeftOpenTag = makeMap(
    'colgroup,dd,dt,li,options,p,td,tfoot,th,thead,tr,source'
  );

  // HTML5 tags https://html.spec.whatwg.org/multipage/indices.html#elements-3
  // Phrasing Content https://html.spec.whatwg.org/multipage/dom.html#phrasing-content
  var isNonPhrasingTag = makeMap(
    'address,article,aside,base,blockquote,body,caption,col,colgroup,dd,' +
    'details,dialog,div,dl,dt,fieldset,figcaption,figure,footer,form,' +
    'h1,h2,h3,h4,h5,h6,head,header,hgroup,hr,html,legend,li,menuitem,meta,' +
    'optgroup,option,param,rp,rt,source,style,summary,tbody,td,tfoot,th,thead,' +
    'title,tr,track'
  );

  /**
   * 不需要对该文件进行类型检查,因为它主要是供应商代码。
   */

  // 用于解析标记和属性的正则表达式
  var attribute = /^\s*([^\s"'<>\/=]+)(?:\s*(=)\s*(?:"([^"]*)"+|'([^']*)'+|([^\s"'=<>`]+)))?/;
  var dynamicArgAttribute = /^\s*((?:v-[\w-]+:|@|:|#)\[[^=]+?\][^\s"'<>\/=]*)(?:\s*(=)\s*(?:"([^"]*)"+|'([^']*)'+|([^\s"'=<>`]+)))?/;
  var ncname = "[a-zA-Z_][\\-\\.0-9_a-zA-Z" + (unicodeRegExp.source) + "]*";
  var qnameCapture = "((?:" + ncname + "\\:)?" + ncname + ")";
  var startTagOpen = new RegExp(("^<" + qnameCapture));
  var startTagClose = /^\s*(\/?)>/;
  var endTag = new RegExp(("^<\\/" + qnameCapture + "[^>]*>"));
  var doctype = /^<!DOCTYPE [^>]+>/i;
  // escape -避免在页面内联时作为HTML注释传递
  var comment = /^<!\--/;
  var conditionalComment = /^<!\[/;

  // 特殊元素(可以包含任何内容)
  var isPlainTextElement = makeMap('script,style,textarea', true);
  var reCache = {};

  var decodingMap = {
    '&lt;': '<',
    '&gt;': '>',
    '&quot;': '"',
    '&amp;': '&',
    '&#10;': '\n',
    '&#9;': '\t',
    '&#39;': "'"
  };
  var encodedAttr = /&(?:lt|gt|quot|amp|#39);/g;
  var encodedAttrWithNewLines = /&(?:lt|gt|quot|amp|#39|#10|#9);/g;

  // #5992
  var isIgnoreNewlineTag = makeMap('pre,textarea', true);
  var shouldIgnoreFirstNewline = function (tag, html) { return tag && isIgnoreNewlineTag(tag) && html[0] === '\n'; };

  function decodeAttr (value, shouldDecodeNewlines) {
    var re = shouldDecodeNewlines ? encodedAttrWithNewLines : encodedAttr;
    return value.replace(re, function (match) { return decodingMap[match]; })
  }
  // 处理 template 匹配标签,再传入 start,end,chars 等方法
  function parseHTML (html, options) {
    var stack = [];
    var expectHTML = options.expectHTML;
    var isUnaryTag$$1 = options.isUnaryTag || no;
    var canBeLeftOpenTag$$1 = options.canBeLeftOpenTag || no;
    var index = 0; //解析的索引
    var last, lastTag;// 相当于栈顶指针,不过是用tagName来做栈顶指针的
    while (html) {
      last = html;
      // Make sure we're not in a plaintext content element like script/style
      if (!lastTag || !isPlainTextElement(lastTag)) {
        var textEnd = html.indexOf('<');
        if (textEnd === 0) {
         
          if (comment.test(html)) {
             //说明是注释开头
            var commentEnd = html.indexOf('-->');

            if (commentEnd >= 0) {
              if (options.shouldKeepComment) {
                 // 保存注释 <!--四个字符
                options.comment(html.substring(4, commentEnd), index, index + commentEnd + 3);
              }
              advance(commentEnd + 3);
              continue
            }
          }

          // http://en.wikipedia.org/wiki/Conditional_comment#Downlevel-revealed_conditional_comment
          if (conditionalComment.test(html)) {
            var conditionalEnd = html.indexOf(']>');

            if (conditionalEnd >= 0) {
              advance(conditionalEnd + 2);
              continue
            }
          }

          // Doctype:
          var doctypeMatch = html.match(doctype);
          if (doctypeMatch) {
            advance(doctypeMatch[0].length);
            continue
          }

           /* 把带空格和纯文本放一起讨论 把空格和文本都设成text 传入char */
          // 如果匹配到结尾标签
          var endTagMatch = html.match(endTag);
          if (endTagMatch) {
            var curIndex = index;
            advance(endTagMatch[0].length);
            parseEndTag(endTagMatch[1], curIndex, index);
            continue
          }

          // Start tag:
          var startTagMatch = parseStartTag();
          if (startTagMatch) {
            handleStartTag(startTagMatch);
            if (shouldIgnoreFirstNewline(startTagMatch.tagName, html)) {
              advance(1);
            }
            continue
          }
        }

        var text = (void 0), rest = (void 0), next = (void 0);
        if (textEnd >= 0) {
          rest = html.slice(textEnd);
          while (
            !endTag.test(rest) &&
            !startTagOpen.test(rest) &&
            !comment.test(rest) &&
            !conditionalComment.test(rest)
          ) {
            // < in plain text, be forgiving and treat it as text
            next = rest.indexOf('<', 1);
            if (next < 0) { break }
            textEnd += next;
            rest = html.slice(textEnd);
          }
          text = html.substring(0, textEnd);
        }

        if (textEnd < 0) {
          text = html;
        }

        if (text) {
          advance(text.length);
        }

        if (options.chars && text) {
          options.chars(text, index - text.length, index);
        }
      } else {
        var endTagLength = 0;
        var stackedTag = lastTag.toLowerCase();
        var reStackedTag = reCache[stackedTag] || (reCache[stackedTag] = new RegExp('([\\s\\S]*?)(</' + stackedTag + '[^>]*>)', 'i'));
        var rest$1 = html.replace(reStackedTag, function (all, text, endTag) {
          endTagLength = endTag.length;
          if (!isPlainTextElement(stackedTag) && stackedTag !== 'noscript') {
            text = text
              .replace(/<!\--([\s\S]*?)-->/g, '$1') // #7298
              .replace(/<!\[CDATA\[([\s\S]*?)]]>/g, '$1');
          }
          if (shouldIgnoreFirstNewline(stackedTag, text)) {
            text = text.slice(1);
          }
          if (options.chars) {
            options.chars(text);
          }
          return ''
        });
        index += html.length - rest$1.length;
        html = rest$1;
        parseEndTag(stackedTag, index - endTagLength, index);
      }

      if (html === last) {
        options.chars && options.chars(html);
        if (!stack.length && options.warn) {
          options.warn(("Mal-formatted tag at end of template: \"" + html + "\""), { start: index + html.length });
        }
        break
      }
    }

    // Clean up any remaining tags
    parseEndTag();

    function advance (n) {
      index += n;
      html = html.substring(n);
    }

    function parseStartTag () {
      // 匹配开始标签的左边开头部分
      var start = html.match(startTagOpen);
      if (start) {
        var match = {
          tagName: start[1],
          attrs: [],
          start: index
        };
        advance(start[0].length);
        // 循环解析开始标签上的每一个属性键值对
        var end, attr;
        while (!(end = html.match(startTagClose)) && (attr = html.match(dynamicArgAttribute) || html.match(attribute))) {
          attr.start = index;
          advance(attr[0].length);
          attr.end = index;
          match.attrs.push(attr);
        }
        // 匹配到开始标签的闭合部分,至此开始标签解析结束
        if (end) {
          match.unarySlash = end[1];
          advance(end[0].length);
          match.end = index;
          return match
        }
      }
    }
    // 格式化attr,把标签入栈,方便匹配,调用start函数,创建AST
    function handleStartTag (match) {
      var tagName = match.tagName;
      var unarySlash = match.unarySlash;

      if (expectHTML) {
        if (lastTag === 'p' && isNonPhrasingTag(tagName)) {
          parseEndTag(lastTag);
        }
        if (canBeLeftOpenTag$$1(tagName) && lastTag === tagName) {
          parseEndTag(tagName);
        }
      }
// 判断是否闭合标签, 如果是自闭和的, 那么不入checkStack栈
      var unary = isUnaryTag$$1(tagName) || !!unarySlash;

      var l = match.attrs.length;
      var attrs = new Array(l);
      for (var i = 0; i < l; i++) {
        var args = match.attrs[i];
        var value = args[3] || args[4] || args[5] || '';
        var shouldDecodeNewlines = tagName === 'a' && args[1] === 'href'
          ? options.shouldDecodeNewlinesForHref
          : options.shouldDecodeNewlines;
        attrs[i] = {
          name: args[1],
          value: decodeAttr(value, shouldDecodeNewlines)
        };
        if (options.outputSourceRange) {
          attrs[i].start = args.start + args[0].match(/^\s*/).length;
          attrs[i].end = args.end;
        }
      }
  
      if (!unary) {
        stack.push({ tag: tagName, lowerCasedTag: tagName.toLowerCase(), attrs: attrs, start: match.start, end: match.end });
        lastTag = tagName;
      }

      if (options.start) {
        options.start(tagName, attrs, unary, match.start, match.end);
      }
    }

    function parseEndTag (tagName, start, end) {
      var pos, lowerCasedTagName;
      if (start == null) { start = index; }
      if (end == null) { end = index; }

      // 对endTag通过checkStack进行匹配, 如果匹配到了则出栈, 维护栈顶指针,并调用end方法
      if (tagName) {
        // 开始再checkStack中匹配
        lowerCasedTagName = tagName.toLowerCase();
        for (pos = stack.length - 1; pos >= 0; pos--) {
          if (stack[pos].lowerCasedTag === lowerCasedTagName) {
            break
          }
        }
      } else {
        // 如果没有提供标签名称
        pos = 0;
      }
    }
    // 匹配成功, 但是不一定匹配正确, 只有栈顶元素和你匹配成功才算正确
      if (pos >= 0) {
        // 关闭所有打开的元素,向上堆栈
        for (var i = stack.length - 1; i >= pos; i--) {
          if (i > pos || !tagName &&
            options.warn
          ) {
            options.warn(
              ("tag <" + (stack[i].tag) + "> has no matching end tag."),
              { start: stack[i].start, end: stack[i].end }
            );
          }
          // 匹配成功, 调用end函数把它从AST栈中弹出
          if (options.end) {
            options.end(stack[i].tag, start, end);
          }
        }

        //从堆栈中移除打开的元素
        stack.length = pos;
        lastTag = pos && stack[pos - 1].tag;
      } else if (lowerCasedTagName === 'br') {
        if (options.start) {
          options.start(tagName, [], true, start, end);
        }
      } else if (lowerCasedTagName === 'p') {
        if (options.start) {
          options.start(tagName, [], false, start, end);
        }
        if (options.end) {
          options.end(tagName, start, end);
        }
      }
    }
  }

  /*  */

  var onRE = /^@|^v-on:/;
  var dirRE = /^v-|^@|^:|^#/;
  var forAliasRE = /([\s\S]*?)\s+(?:in|of)\s+([\s\S]*)/;
  var forIteratorRE = /,([^,\}\]]*)(?:,([^,\}\]]*))?$/;
  var stripParensRE = /^\(|\)$/g;
  var dynamicArgRE = /^\[.*\]$/;

  var argRE = /:(.*)$/;
  var bindRE = /^:|^\.|^v-bind:/;
  var modifierRE = /\.[^.\]]+(?=[^\]]*$)/g;

  var slotRE = /^v-slot(:|$)|^#/;

  var lineBreakRE = /[\r\n]/;
  var whitespaceRE$1 = /[ \f\t\r\n]+/g;

  var invalidAttributeRE = /[\s"'<>\/=]/;

  var decodeHTMLCached = cached(he.decode);

  var emptySlotScopeToken = "_empty_";

  // 可配置的状态
  var warn$2;
  var delimiters;
  var transforms;
  var preTransforms;
  var postTransforms;
  var platformIsPreTag;
  var platformMustUseProp;
  var platformGetTagNamespace;
  var maybeComponent;
  // 构造一个 AST 元素节点,每解析一个标签就要生成一个这样的 AST 元素节点,除了根元素,其它节点一般都有一个 parent 元素
  function createASTElement (
    tag,
    attrs,
    parent
  ) {
    return {
      type: 1,
      tag: tag,
      attrsList: attrs,
      attrsMap: makeAttrsMap(attrs),
      rawAttrsMap: {},
      parent: parent,
      children: []
    }
  }

  /**
   * 转换HTML字符串到AST(抽象语法树)
   */
  function parse (
    template,
    options
  ) {
    warn$2 = options.warn || baseWarn;

    platformIsPreTag = options.isPreTag || no;
    platformMustUseProp = options.mustUseProp || no;
    platformGetTagNamespace = options.getTagNamespace || no;
    // 是否是保留标签
    var isReservedTag = options.isReservedTag || no;
    // el.component非空 或者 不是预留的标签  就认为是组件
    maybeComponent = function (el) { return !!(
      el.component ||
      el.attrsMap[':is'] ||
      el.attrsMap['v-bind:is'] ||
      !(el.attrsMap.is ? isReservedTag(el.attrsMap.is) : isReservedTag(el.tag))
    ); };
    // pluckModuleFunction 从options.modules中摘出函数transformNode, 返回值是一个 函数数组
    transforms = pluckModuleFunction(options.modules, 'transformNode');
    preTransforms = pluckModuleFunction(options.modules, 'preTransformNode');
    postTransforms = pluckModuleFunction(options.modules, 'postTransformNode');

    delimiters = options.delimiters;
    // 栈 未闭合的元素先压栈 等待闭合标签再出栈
    var stack = [];
    var preserveWhitespace = options.preserveWhitespace !== false;
    var whitespaceOption = options.whitespace;
    // 根节点, 只设置一次
    var root;
    // 父节点, 一开始为空
    var currentParent;
    var inVPre = false;
    var inPre = false;
    var warned = false;
    // 辅助警告提示类,由于在生成模板过程是在循环体里面,为避免重复警告提示,定义这个只要提示一次就不再提示的警告方法
    function warnOnce (msg, range) {
      if (!warned) {
        warned = true;
        warn$2(msg, range);
      }
    }

    function closeElement (element) {
      // 若当前元素不是pre元素,则删除元素尾部的空白文本节点
      trimEndingWhitespace(element);
       // 如果当前标签没有v-pre并且没有编译过,则编译一下
      if (!inVPre && !element.processed) {
        element = processElement(element, options);
      }
      // 树管理
      if (!stack.length && element !== root) {
        // 允许根元素带有v-if, v-else-if和v-else
        if (root.if && (element.elseif || element.else)) {
          {
            checkRootConstraints(element);
          }
          addIfCondition(root, {
            exp: element.elseif,
            block: element
          });
        } else {
          warnOnce(
            "Component template should contain exactly one root element. " +
            "If you are using v-if on multiple elements, " +
            "use v-else-if to chain them instead.",
            { start: element.start }
          );
        }
      }
      // 如果不是根节点且不是script或style之类被禁止的标签的话
      if (currentParent && !element.forbidden) {
        // 如果当前标签绑定有v-else-if或v-else,则需要解析一下
        if (element.elseif || element.else) {
          processIfConditions(element, currentParent);
        } else {
          // 如果当前标签是一个作用域插槽
          if (element.slotScope) {
            //范围槽
            //将它保留在子列表中,以便v-else(-if)条件可以
            //找到它作为prev节点。
            var name = element.slotTarget || '"default"'
            ;(currentParent.scopedSlots || (currentParent.scopedSlots = {}))[name] = element;
          }
          // 把当前元素加入到父级元素的子节点列表中,从而创建AST的父子层级关系
          currentParent.children.push(element);
          // 同时也将当前节点的父级节点标记为当前的父级节点
          element.parent = currentParent;
        }
      }

      //最后的孩子清理
      //过滤出限定范围的槽
      element.children = element.children.filter(function (c) { return !(c).slotScope; });
      // 再次删除末尾的空白节点
      trimEndingWhitespace(element);

      // 检查之前的状态
      if (element.pre) {
        inVPre = false;
      }
      if (platformIsPreTag(element.tag)) {
        inPre = false;
      }
      // 应用转化后
      for (var i = 0; i < postTransforms.length; i++) {
        postTransforms[i](element, options);
      }
    }
    // 若当前元素不是pre元素,则删除元素尾部的空白文本节点
    function trimEndingWhitespace (el) {
      // 删除末尾的空白节点
      if (!inPre) {
        var lastNode;
        while (
          // 节点存在
          (lastNode = el.children[el.children.length - 1]) &&
          // 是文本节点
          lastNode.type === 3 &&
          // 文本节点的内容是空白符
          lastNode.text === ' '
        ) {
          // 弹出该元素
          el.children.pop();
        }
      }
    }
    // 检查当前元素是否符合作为根元素的要求
    function checkRootConstraints (el) {
      // 通过判断 el.tag === 'slot' || el.tag === 'template' 来判断根元素是否是slot 标签或 template 标签
      if (el.tag === 'slot' || el.tag === 'template') {
        warnOnce(
          "Cannot use <" + (el.tag) + "> as component root element because it may " +
          'contain multiple nodes.',
          { start: el.start }
        );
      }
      // 判断当前元素是否使用了 v-for 指令,因为v-for 指令会渲染多个节点所以根元素是不允许使用 v-for 指令的
      if (el.attrsMap.hasOwnProperty('v-for')) {
        warnOnce(
          'Cannot use v-for on stateful component root element because ' +
          'it renders multiple elements.',
          el.rawAttrsMap['v-for']
        );
      }
    }
    // 对模板解析
    parseHTML(template, {
      warn: warn$2,
      expectHTML: options.expectHTML,
      isUnaryTag: options.isUnaryTag,
      canBeLeftOpenTag: options.canBeLeftOpenTag,
      // 是否应该对属性值进行一次编码
      shouldDecodeNewlines: options.shouldDecodeNewlines,
      // 是否应该对a标签的href进行一次编码
      shouldDecodeNewlinesForHref: options.shouldDecodeNewlinesForHref,
      // 是否需要保留注释
      shouldKeepComment: options.comments,
      outputSourceRange: options.outputSourceRange,
      // 解析过程中的回调函数,生成 AST
      start: function start (tag, attrs, unary, start$1, end) {
        // 检查名称空间。
        //如果有父节点,则继承父节点
        var ns = (currentParent && currentParent.ns) || platformGetTagNamespace(tag);

        // 处理IE svg错误
        if (isIE && ns === 'svg') {
          attrs = guardIESVGBug(attrs);
        }
        // 当解析到标签开始位置时会执行这个钩子函数,将标签名和对应的属性传过来
        var element = createASTElement(tag, attrs, currentParent);
        if (ns) {
          element.ns = ns;
        }

        {
          if (options.outputSourceRange) {
            element.start = start$1;
            element.end = end;
            element.rawAttrsMap = element.attrsList.reduce(function (cumulated, attr) {
              cumulated[attr.name] = attr;
              return cumulated
            }, {});
          }
           // 检测非法属性并提示
          attrs.forEach(function (attr) {
            if (invalidAttributeRE.test(attr.name)) {
              warn$2(
                "Invalid dynamic argument expression: attribute names cannot contain " +
                "spaces, quotes, <, >, / or =.",
                {
                  start: attr.start + attr.name.indexOf("["),
                  end: attr.start + attr.name.length
                }
              );
            }
          });
        }
        // 如果当前标签是一个<style>...</style>或<script></script>、<script type="type/javascript"></script>的话
        // 提示用户这是一个在模板中被禁止使用的标签,因为模板仅仅只是用来描述状态与页面的呈现的,不应该包含样式和脚本标签
        if (isForbiddenTag(element) && !isServerRendering()) {
          element.forbidden = true;
          warn$2(
            'Templates should only be responsible for mapping the state to the ' +
            'UI. Avoid placing tags with side-effects in your templates, such as ' +
            "<" + tag + ">" + ', as they will not be parsed.',
            { start: element.start }
          );
        }

        // apply pre-transforms
        for (var i = 0; i < preTransforms.length; i++) {
          element = preTransforms[i](element, options) || element;
        }
        // 如果inVPre为false,可能还没有解析当前标签是否标记了v-pre
        if (!inVPre) {
          processPre(element);
          // 如果解析过后发现elem上标记有pre=true,说明标签确实标记了v-pre
          if (element.pre) {
            inVPre = true;
          }
        }
         // 当然,除了vue的指令v-pre之外,我们html也自带一个pre标签,
            // 如果标签名是pre,那也要将inPre标记为true
        if (platformIsPreTag(element.tag)) {
          inPre = true;
        }
        if (inVPre) {
          // 如果一个标签被标记了v-pre,那我们只需要把attrList中剩余的属性复制到elem的attrs中去即可
          // 因为attrList中的其他属性都在刚刚进行预处理的时候已经处理并从attrList中删除了
          processRawAttrs(element);
        } else if (!element.processed) {
          // 如果还有没有处理的结构指令,如v-for、v-if等,就处理一下
          processFor(element);
          processIf(element);
          processOnce(element);
        }
        // 如果不存在根节点,则当前节点就是根节点
        if (!root) {
          root = element;
          {
            checkRootConstraints(root);
          }
        }
        // 判断当前节点是不是一个自闭标签,如果是一个自闭标签,那么直接结束当前标签解析
      // 如果是不是自闭标签,我们需要记录下当前节点当做是下个节点的父级元素,并加这个元素压入栈中
        if (!unary) {
          currentParent = element;
          stack.push(element);
        } else {
          closeElement(element);
        }
      },

      end: function end (tag, start, end$1) {
         // 当前标签已经解析结束了,将标签从栈中弹出
        var element = stack[stack.length - 1];
         // 此时栈顶元素便是我们下一个元素的父级
        stack.length -= 1;
        currentParent = stack[stack.length - 1];
        if (options.outputSourceRange) {
          element.end = end$1;
        }
        // 关闭标签
        closeElement(element);
      },

      chars: function chars (text, start, end) {
          // 如果不存在父级节点,那么我们可以得知,
            // 这个解析出来的文本,要么就是在根节点之外,要么,压根就没有根节点,所给的tpl直接就是一段文本
        if (!currentParent) {
          {
            // 如果解析出来的文本跟传入的模板完全相同,那么,说明直传进来一个文本内容,警告提示
            if (text === template) {
              warnOnce(
                'Component template requires a root element, rather than just text.',
                { start: start }
              );
            } else if ((text = text.trim())) {
              warnOnce(
                ("text \"" + text + "\" outside root element will be ignored."),
                { start: start }
              );
            }
          }
          return
        }
        // 在IE浏览器中的textarea的placeholder有一个bug,浏览器会将placeholder的内容会被作为textarea的文本节点放入到textarea中
        if (isIE &&
          currentParent.tag === 'textarea' &&
          currentParent.attrsMap.placeholder === text
        ) {
          return
        }
        var children = currentParent.children;
        if (inPre || text.trim()) {
          text = isTextTag(currentParent) ? text : decodeHTMLCached(text);
        } else if (!children.length) {
          // 在开始标记之后删除只包含空格的节点
          text = '';
        } else if (whitespaceOption) {
          if (whitespaceOption === 'condense') {
            // 在压缩模式下,如果空白节点包含
            //换行,否则压缩为一个空格
            text = lineBreakRE.test(text) ? '' : ' ';
          } else {
            text = ' ';
          }
        } else {
          // 其他情况:看看是不是需要保留空格
          text = preserveWhitespace ? ' ' : '';
        }
        if (text) {
           // 如果不是在pre标签中且删除空白的选项是condense,则删除文本中的换行符
          if (!inPre && whitespaceOption === 'condense') {
            // 将连续的空格压缩为单个空格
            text = text.replace(whitespaceRE$1, ' ');
          }
          var res;
          var child;
           // 如果当前节点没有v-pre属性且是一个空白符并且可以解析出动态变量
          if (!inVPre && text !== ' ' && (res = parseText(text, delimiters))) {
            child = {
              type: 2,
              expression: res.expression,
              tokens: res.tokens,
              text: text
            };
          } else if (text !== ' ' || !children.length || children[children.length - 1].text !== ' ') {
            child = {
              type: 3,
              text: text
            };
          }
          if (child) {
            if (options.outputSourceRange) {
              child.start = start;
              child.end = end;
            }
            // 将创建的文本节点或表达式节点加入到父级节点的children中
            children.push(child);
          }
        }
      },
      comment: function comment (text, start, end) {
        // 禁止将任何东西作为同胞添加到根节点
        // 只有在根节点下创建注释才有效,只要不在根节点内部的注释都会被忽略
        if (currentParent) {
          var child = {
            type: 3,
            text: text,
            isComment: true
          };
          if (options.outputSourceRange) {
            child.start = start;
            child.end = end;
          }
          currentParent.children.push(child);
        }
      }
    });
    return root
  }
// 解析v-pre
  function processPre (el) {
    if (getAndRemoveAttr(el, 'v-pre') != null) {
      el.pre = true;
    }
  }
  // 将属性作为无绑定属性做解析
  function processRawAttrs (el) {
    var list = el.attrsList;
    var len = list.length;
    if (len) {
      var attrs = el.attrs = new Array(len);
      for (var i = 0; i < len; i++) {
        attrs[i] = {
          name: list[i].name,
          value: JSON.stringify(list[i].value)
        };
        if (list[i].start != null) {
          attrs[i].start = list[i].start;
          attrs[i].end = list[i].end;
        }
      }
    } else if (!el.pre) {
      // 没有属性的pre块中的非根节点
      el.plain = true;
    }
  }
  // 处理:key、ref、内外部插槽、绑定属性、组件的判断
  function processElement (
    element,
    options
  ) {
    processKey(element);
    //确定这是否是一个普通元素
    //删除结构属性
    element.plain = (
      !element.key &&
      !element.scopedSlots &&
      !element.attrsList.length
    );
 // 获取元素的ref属性值
    processRef(element);
    // 处理插槽
    processSlotContent(element);
    // 处理<slot></slot>标签, 获取slot标签的name属性值
    processSlotOutlet(element);
    // 处理<component>标签的 is 和 inline-template属性
    processComponent(element);
    // 循环transforms数组,逐个对element进行转化处理
    for (var i = 0; i < transforms.length; i++) {
      // 对element实施转换处理
      element = transforms[i](element, options) || element;
    }
    // 处理元素的属性列表 包括指令和非指令
    processAttrs(element);
    return element
  }
// 获取el的key值
  function processKey (el) {
    // 获取key的表达式
    var exp = getBindingAttr(el, 'key');
    if (exp) {
      {
        // 非生产环境
        if (el.tag === 'template') {
          // template上不能带key
          warn$2(
            "<template> cannot be keyed. Place the key on real elements instead.",
            // 获取el的key属性
            getRawBindingAttr(el, 'key')
          );
        }
        if (el.for) {
          // v-for的解析结果中包含这两个迭代器
          var iterator = el.iterator2 || el.iterator1;
          var parent = el.parent;
          // el父元素是<transition-group>, 它是vue内置的组件
          if (iterator && iterator === exp && parent && parent.tag === 'transition-group') {
             // <transition-group> 的子节点不能用v-for的index作为key
            warn$2(
              "Do not use v-for index as key on <transition-group> children, " +
              "this is the same as not using keys.",
              getRawBindingAttr(el, 'key'),
              true /* tip */
            );
          }
        }
      }
      // 为el.key赋值
      el.key = exp;
    }
  }
  // 处理ref
  // 如果在普通的 DOM 元素上使用, 引用指向的就是 DOM 元素; 如果用在子组件上, 引用就指向组件
  function processRef (el) {
     // 获取el的ref属性值
    var ref = getBindingAttr(el, 'ref');
    if (ref) {
      el.ref = ref;
      // 如果el或其父节点有for, el.refInFor为true
      el.refInFor = checkInFor(el);
    }
  }
  // 处理v-for指令
  function processFor (el) {
    var exp;
    if ((exp = getAndRemoveAttr(el, 'v-for'))) {
      // 解析v-for表达式
      var res = parseFor(exp);
      if (res) {
         // 将源对象res中的属性混入目标对象el
        extend(el, res);
      } else {
        // 无效的 v-for 表达式
        warn$2(
          ("Invalid v-for expression: " + exp),
          el.rawAttrsMap['v-for']
        );
      }
    }
  }


  // 解析 v-for 表达式 解析出 迭代器 别名 循环的变量
  function parseFor (exp) {
    // 匹配正则
    var inMatch = exp.match(forAliasRE);
    if (!inMatch) { return }
    var res = {};
    res.for = inMatch[2].trim();
    var alias = inMatch[1].trim().replace(stripParensRE, '');
    var iteratorMatch = alias.match(forIteratorRE);
    if (iteratorMatch) {
      res.alias = alias.replace(forIteratorRE, '').trim();
      res.iterator1 = iteratorMatch[1].trim();
      if (iteratorMatch[2]) {
        res.iterator2 = iteratorMatch[2].trim();
      }
    } else {
      res.alias = alias;
    }
    return res
  }
// 处理el的条件 v-if v-else v-else-if 
  function processIf (el) {
    var exp = getAndRemoveAttr(el, 'v-if');
    if (exp) {
      el.if = exp;
      // 为el增加if条件
      addIfCondition(el, {
        exp: exp,
        block: el
      });
    } else {
      if (getAndRemoveAttr(el, 'v-else') != null) {
        el.else = true;
      }
      var elseif = getAndRemoveAttr(el, 'v-else-if');
      if (elseif) {
        el.elseif = elseif;
      }
    }
  }
// 处理if条件(el有 v-else 或 v-else-if)
  function processIfConditions (el, parent) {
    // 寻找el的前一个元素
    var prev = findPrevElement(parent.children);
    if (prev && prev.if) {
      // 为pre添加if条件
      addIfCondition(prev, {
        exp: el.elseif,
        block: el
      });
    } else {
      // v-else-if v-else 没有对应的 v-if
      warn$2(
        "v-" + (el.elseif ? ('else-if="' + el.elseif + '"') : 'else') + " " +
        "used on element <" + (el.tag) + "> without corresponding v-if.",
        el.rawAttrsMap[el.elseif ? 'v-else-if' : 'v-else']
      );
    }
  }
// 返回children数组中后边第一个type为1的元素
// 逐步往children数组中添加子节点,数组最后一个元素就是当前处理元素的前一个节点
  function findPrevElement (children) {
    var i = children.length;
    while (i--) {
      if (children[i].type === 1) {
        return children[i]
      } else {
        if (children[i].text !== ' ') {
          // v-if 与 v-else / v-else-if之间的文本将被忽略
          warn$2(
            "text \"" + (children[i].text.trim()) + "\" between v-if and v-else(-if) " +
            "will be ignored.",
            children[i]
          );
        }
        // 从数组后边删除一个元素
        children.pop();
      }
    }
  }
// 为el元素增加 if条件
  function addIfCondition (el, condition) {
    if (!el.ifConditions) {
      // 初始为空数组
      el.ifConditions = [];
    }
    // 为数组添加新元素
    el.ifConditions.push(condition);
  }
// 处理v-once, 设置el.once属性
  function processOnce (el) {
    var once$$1 = getAndRemoveAttr(el, 'v-once');
    if (once$$1 != null) {
      el.once = true;
    }
  }

  // 处理传递给组件的插槽内容
  // v-slot取代了 被废弃但未被移除的 slot 和 slot-scope
  function processSlotContent (el) {
    var slotScope;
    // scope只能用在<template>标签中 slot-scope可以用在其他普通元素
    if (el.tag === 'template') {
      // 匹配绑定在 template 上的 v-slot 指令,这里会匹配到对应 v-slot 的 attr(类型为 ASTAttr)
      slotScope = getAndRemoveAttr(el, 'scope');
   // 若 slotBinding 存在,则继续进行 slotName 的正则匹配
  // 随即将匹配出来的 name 赋值给 slotTarget,dynamic 赋值给 slotTargetDynamic
  // slotScope 赋值为 slotBinding.value 或者 '_empty_'
      if (slotScope) {
        warn$2(
          "the \"scope\" attribute for scoped slots have been deprecated and " +
          "replaced by \"slot-scope\" since 2.5. The new \"slot-scope\" attribute " +
          "can also be used on plain elements in addition to <template> to " +
          "denote scoped slots.",
          el.rawAttrsMap['scope'],
          true
        );
      }
      el.slotScope = slotScope || getAndRemoveAttr(el, 'slot-scope');
    } else if ((slotScope = getAndRemoveAttr(el, 'slot-scope'))) {
      /* istanbul ignore if */
      if (el.attrsMap['v-for']) {
        warn$2(
          "Ambiguous combined usage of slot-scope and v-for on <" + (el.tag) + "> " +
          "(v-for takes higher priority). Use a wrapper <template> for the " +
          "scoped slot to make it clearer.",
          el.rawAttrsMap['slot-scope'],
          true
        );
      }
      el.slotScope = slotScope;
    }

    // slot="xxx"
    var slotTarget = getBindingAttr(el, 'slot');
    if (slotTarget) {
      el.slotTarget = slotTarget === '""' ? '"default"' : slotTarget;
      el.slotTargetDynamic = !!(el.attrsMap[':slot'] || el.attrsMap['v-bind:slot']);
     //将slot保存为本地shadow DOM compat的属性
    //仅用于非作用域槽
      if (el.tag !== 'template' && !el.slotScope) {
        addAttr(el, 'slot', slotTarget, getRawBindingAttr(el, 'slot'));
      }
    }

    // v-slot语法
    {
      if (el.tag === 'template') {
         // v-slot on component 表示默认插槽
        var slotBinding = getAndRemoveAttrByRegex(el, slotRE);
        if (slotBinding) {
          {
            if (el.slotTarget || el.slotScope) {
              warn$2(
                "Unexpected mixed usage of different slot syntaxes.",
                el
              );
            }
            if (el.parent && !maybeComponent(el.parent)) {
              warn$2(
                "<template v-slot> can only appear at the root level inside " +
                "the receiving component",
                el
              );
            }
          }
          var ref = getSlotName(slotBinding);
          var name = ref.name;
          var dynamic = ref.dynamic;
          el.slotTarget = name;
          el.slotTargetDynamic = dynamic;
          // 将其强制放入限定范围的槽中进行性能测试
          el.slotScope = slotBinding.value || emptySlotScopeToken;
        }
      } else {
        // 组件上的V-slot表示默认槽位
        var slotBinding$1 = getAndRemoveAttrByRegex(el, slotRE);
        // 将组件的 children 添加到其默认插槽中去
        if (slotBinding$1) {
          {
            if (!maybeComponent(el)) {
              warn$2(
                "v-slot can only be used on components or <template>.",
                slotBinding$1
              );
            }
            if (el.slotScope || el.slotTarget) {
              warn$2(
                "Unexpected mixed usage of different slot syntaxes.",
                el
              );
            }
            if (el.scopedSlots) {
              warn$2(
                "To avoid scope ambiguity, the default slot should also use " +
                "<template> syntax when there are other named slots.",
                slotBinding$1
              );
            }
          }
          // 将组件的子组件添加到其默认槽中
          var slots = el.scopedSlots || (el.scopedSlots = {});
          var ref$1 = getSlotName(slotBinding$1);
          var name$1 = ref$1.name;
           // 匹配拿到 slotBinding 中 name,dynamic 的值
          var dynamic$1 = ref$1.dynamic;
          // 获取 slots 中 key 对应匹配出来 name 的 slot
          // 然后再其下面创建一个标签名为 template 的 ASTElement,attrs 为空数组,parent 为当前节点
          var slotContainer = slots[name$1] = createASTElement('template', [], el);
           // 这里 name、dynamic 统一赋值给 slotContainer 的 slotTarget、slotTargetDynamic,而不是 el
          slotContainer.slotTarget = name$1;
          slotContainer.slotTargetDynamic = dynamic$1;
           // 将当前节点的 children 添加到 slotContainer 的 children 属性中
          slotContainer.children = el.children.filter(function (c) {
            if (!c.slotScope) {
              c.parent = slotContainer;
              return true
            }
          });
          slotContainer.slotScope = slotBinding$1.value || emptySlotScopeToken;
          // 移除从scopedSlots返回的子元素
          el.children = [];
          // 标记el非普通,这样数据就生成了
          el.plain = false;
        }
      }
    }
  }

  function getSlotName (binding) {
     // 'v-slot:item' 匹配获取到 'item'
    var name = binding.name.replace(slotRE, '');
    if (!name) {
      if (binding.name[0] !== '#') {
        name = 'default';
      } else {
        warn$2(
          "v-slot shorthand syntax requires a slot name.",
          binding
        );
      }
    }
    // 返回一个 key 包含 name,dynamic 的对象
  // 'v-slot:[item]' 匹配然后 replace 后获取到 name = '[item]'
  // 进而进行动态参数进行匹配 dynamicArgRE.test(name) 结果为 true
    return dynamicArgRE.test(name)
      // // 截取变量,如 '[item]' 截取后变成 'item'
      ? { name: name.slice(1, -1), dynamic: true }
      : { name: ("\"" + name + "\""), dynamic: false }
  }

  //处理插槽
  function processSlotOutlet (el) {
    if (el.tag === 'slot') {
      el.slotName = getBindingAttr(el, 'name');
      // <slot>标签不能有key值
      if (el.key) {
        warn$2(
          "`key` does not work on <slot> because slots are abstract outlets " +
          "and can possibly expand into multiple elements. " +
          "Use the key on a wrapping element instead.",
          getRawBindingAttr(el, 'key')
        );
      }
    }
  }
  // 处理<component>标签,它可以有两个属性 is 和 inline-template
  // 当 inline-template 这个特殊的 attribute 出现在一个子组件上时,
// 这个组件将会使用其里面的内容作为模板, 而不是将其作为被分发(插槽)的内容。这使得模板的撰写工作更加灵活。
  function processComponent (el) {
    var binding;
    // 获取is属性的值, 赋值给el.component
    if ((binding = getBindingAttr(el, 'is'))) {
      el.component = binding;
    }
    // 获取inline-template属性的值, 辅助给el.inlineTemplate
    if (getAndRemoveAttr(el, 'inline-template') != null) {
      el.inlineTemplate = true;
    }
  }
// 处理元素的属性列表 包括指令和非指令
  function processAttrs (el) {
    var list = el.attrsList;
    var i, l, name, rawName, value, modifiers, syncGen, isDynamic;
    //遍历每个属性
    for (i = 0, l = list.length; i < l; i++) {
      name = rawName = list[i].name;
      value = list[i].value;
      // 如果该属性以v-、@或:开头,表示这是Vue内部指令
      if (dirRE.test(name)) {
        // mark element as dynamic
        el.hasBindings = true;
        // //获取修饰符,比如:{native: true,prevent: true}
        modifiers = parseModifiers(name.replace(dirRE, ''));
        // support .foo shorthand syntax for the .prop modifier
        if (modifiers) {
          name = name.replace(modifierRE, '');
        }
        // /bindRD等于/^:|^v-bind:/ ,即该属性是v-bind指令时
        if (bindRE.test(name)) { // v-bind
          // 去掉指令特性,获取特性名
          name = name.replace(bindRE, '');
           //对一些表达式做解析
          value = parseFilters(value);
          isDynamic = dynamicArgRE.test(name);
          if (isDynamic) {
            name = name.slice(1, -1);
          }
          if (
            value.trim().length === 0
          ) {
            warn$2(
              ("The value for a v-bind expression cannot be empty. Found in \"v-bind:" + name + "\"")
            );
          }
          //是否绑定到DOM对象上
          if (modifiers) {
            if (modifiers.prop && !isDynamic) {
              name = camelize(name);
              if (name === 'innerHtml') { name = 'innerHTML'; }
            }
            if (modifiers.camel && !isDynamic) {
              name = camelize(name);
            }
            if (modifiers.sync) {
              syncGen = genAssignmentCode(value, "$event");
              if (!isDynamic) {
                addHandler(
                  el,
                  ("update:" + (camelize(name))),
                  syncGen,
                  null,
                  false,
                  warn$2,
                  list[i]
                );
                if (hyphenate(name) !== camelize(name)) {
                  addHandler(
                    el,
                    ("update:" + (hyphenate(name))),
                    syncGen,
                    null,
                    false,
                    warn$2,
                    list[i]
                  );
                }
              } else {
                // handler w/ dynamic event name
                addHandler(
                  el,
                  ("\"update:\"+(" + name + ")"),
                  syncGen,
                  null,
                  false,
                  warn$2,
                  list[i],
                  true // dynamic
                );
              }
            }
          }
          //如果isProp为true
          if ((modifiers && modifiers.prop) || (
            !el.component && platformMustUseProp(el.tag, el.attrsMap.type, name)
          )) {
            //则调用addProp()
            addProp(el, name, value, list[i], isDynamic);
          } else {
            //否则调用addAttr(),addAttr()函数用于在AST对象上新增一个attrs属性
            addAttr(el, name, value, list[i], isDynamic);
          }
        } else if (onRE.test(name)) { // v-on
           //onRE等于/^@|^v-on:/,即该属性是v-on指令时
          name = name.replace(onRE, '');
          isDynamic = dynamicArgRE.test(name);
          if (isDynamic) {
            name = name.slice(1, -1);
          }
          addHandler(el, name, value, modifiers, false, warn$2, list[i], isDynamic);
        } else { // 普通指令
          name = name.replace(dirRE, '');
          // parse arg
          var argMatch = name.match(argRE);
          var arg = argMatch && argMatch[1];
          isDynamic = false;
          if (arg) {
            name = name.slice(0, -(arg.length + 1));
            if (dynamicArgRE.test(arg)) {
              arg = arg.slice(1, -1);
              isDynamic = true;
            }
          }
          addDirective(el, name, rawName, value, arg, isDynamic, modifiers, list[i]);
          if (name === 'model') {
            checkForAliasModel(el, value);
          }
        }
      } else {
        // literal attribute
        {
          var res = parseText(value, delimiters);
          if (res) {
            warn$2(
              name + "=\"" + value + "\": " +
              'Interpolation inside attributes has been removed. ' +
              'Use v-bind or the colon shorthand instead. For example, ' +
              'instead of <div id="{{ val }}">, use <div :id="val">.',
              list[i]
            );
          }
        }
        addAttr(el, name, JSON.stringify(value), list[i]);
        // #6887 firefox doesn't update muted state if set via attribute
        // even immediately after element creation
        if (!el.component &&
            name === 'muted' &&
            platformMustUseProp(el.tag, el.attrsMap.type, name)) {
          addProp(el, name, 'true', list[i]);
        }
      }
    }
  }
  // 检查当前元素el或其父元素是否有for指令
  function checkInFor (el) {
    var parent = el;
    while (parent) {
      if (parent.for !== undefined) {
        return true
      }
      //  往父元素查询
      parent = parent.parent;
    }
    return false
  }
 // 解析出所有的修饰符, 组成一个对象 .prevent
  function parseModifiers (name) {
    var match = name.match(modifierRE);
    if (match) {
      var ret = {};
      // 循环处理所有匹配的字符串
      match.forEach(function (m) { ret[m.slice(1)] = true; });
      return ret
    }
  }
// 简历属性map对象, 同时验重
  function makeAttrsMap (attrs) {
    var map = {};
    for (var i = 0, l = attrs.length; i < l; i++) {
      if (
        map[attrs[i].name] && !isIE && !isEdge
      ) {
         // 非生产环境 && 有次属性名 && 不是IE && 不是Edge, 给出错误信息
        warn$2('duplicate attribute: ' + attrs[i].name, attrs[i]);
      }
      // 建立属性map对象
      map[attrs[i].name] = attrs[i].value;
    }
    return map
  }

  // script标签如果没有设置type, 默认是"text/javascript",浏览器会按照js去解析, 如果是其他type,忽略处理,等待页面加载完成后,
// 模板引擎获取到标签内容,然后使用数据对其进行渲染再输出到页面上
  function isTextTag (el) {
    return el.tag === 'script' || el.tag === 'style'
  }
// <style>标签  type 为空或者为 text/javascript 的 <script>标签
  function isForbiddenTag (el) {
    return (
      el.tag === 'style' ||
      (el.tag === 'script' && (
        !el.attrsMap.type ||
        el.attrsMap.type === 'text/javascript'
      ))
    )
  }

  var ieNSBug = /^xmlns:NS\d+/;
  var ieNSPrefix = /^NS\d+:/;

  // 对一些属性做特殊处理  监视IE浏览器的SVG bug
  function guardIESVGBug (attrs) {
    var res = [];
    // 遍历属性数组
    for (var i = 0; i < attrs.length; i++) {
      var attr = attrs[i];
       // 属性名替换
      if (!ieNSBug.test(attr.name)) {
        attr.name = attr.name.replace(ieNSPrefix, '');
        res.push(attr);
      }
    }
    return res
  }
// 检查是否针对v-for的别名变量绑定了 v-modal 这样不合法
  function checkForAliasModel (el, value) {
    var _el = el;
    while (_el) {
      if (_el.for && _el.alias === value) {
        warn$2(
          "<" + (el.tag) + " v-model=\"" + value + "\">: " +
          "You are binding v-model directly to a v-for iteration alias. " +
          "This will not be able to modify the v-for source array because " +
          "writing to the alias is like modifying a function local variable. " +
          "Consider using an array of objects and use v-model on an object property instead.",
          el.rawAttrsMap['v-model']
        );
      }
      // 从子节点往上逐层判断
      _el = _el.parent;
    }
  }

  /* // 处理checkbox、radio,input等需要预处理的标签 */
  function preTransformNode (el, options) {
    if (el.tag === 'input') {
      var map = el.attrsMap;
      if (!map['v-model']) {
        return
      }

      var typeBinding;
      if (map[':type'] || map['v-bind:type']) {
        typeBinding = getBindingAttr(el, 'type');
      }
      if (!map.type && !typeBinding && map['v-bind']) {
        typeBinding = "(" + (map['v-bind']) + ").type";
      }

      if (typeBinding) {
        var ifCondition = getAndRemoveAttr(el, 'v-if', true);
        var ifConditionExtra = ifCondition ? ("&&(" + ifCondition + ")") : "";
        var hasElse = getAndRemoveAttr(el, 'v-else', true) != null;
        var elseIfCondition = getAndRemoveAttr(el, 'v-else-if', true);
        // 1. checkbox
        var branch0 = cloneASTElement(el);
        // process for on the main node
        processFor(branch0);
        addRawAttr(branch0, 'type', 'checkbox');
        processElement(branch0, options);
        branch0.processed = true; // prevent it from double-processed
        branch0.if = "(" + typeBinding + ")==='checkbox'" + ifConditionExtra;
        addIfCondition(branch0, {
          exp: branch0.if,
          block: branch0
        });
        // 2. add radio else-if condition
        var branch1 = cloneASTElement(el);
        getAndRemoveAttr(branch1, 'v-for', true);
        addRawAttr(branch1, 'type', 'radio');
        processElement(branch1, options);
        addIfCondition(branch0, {
          exp: "(" + typeBinding + ")==='radio'" + ifConditionExtra,
          block: branch1
        });
        // 3. other
        var branch2 = cloneASTElement(el);
        getAndRemoveAttr(branch2, 'v-for', true);
        addRawAttr(branch2, ':type', typeBinding);
        processElement(branch2, options);
        addIfCondition(branch0, {
          exp: ifCondition,
          block: branch2
        });

        if (hasElse) {
          branch0.else = true;
        } else if (elseIfCondition) {
          branch0.elseif = elseIfCondition;
        }

        return branch0
      }
    }
  }
  // 通过 createASTElement 函数再创建出一个元素描述对象
  function cloneASTElement (el) {
    return createASTElement(el.tag, el.attrsList.slice(), el.parent)
  }

  var model$1 = {
    preTransformNode: preTransformNode
  };

  var modules$1 = [
    klass$1,
    style$1,
    model$1
  ];

  /*  */

  function text (el, dir) {
    if (dir.value) {
      addProp(el, 'textContent', ("_s(" + (dir.value) + ")"), dir);
    }
  }

  /*  */

  function html (el, dir) {
    if (dir.value) {
      addProp(el, 'innerHTML', ("_s(" + (dir.value) + ")"), dir);
    }
  }

  var directives$1 = {
    model: model,
    text: text,
    html: html
  };

  /*  */

  var baseOptions = {
    expectHTML: true,
    modules: modules$1,
    directives: directives$1,
    isPreTag: isPreTag,
    isUnaryTag: isUnaryTag,
    mustUseProp: mustUseProp,
    canBeLeftOpenTag: canBeLeftOpenTag,
    isReservedTag: isReservedTag,
    getTagNamespace: getTagNamespace,
    staticKeys: genStaticKeys(modules$1)
  };

  /*  */

  var isStaticKey;
  var isPlatformReservedTag;

  var genStaticKeysCached = cached(genStaticKeys$1);

  /**
   * Goal of the optimizer: walk the generated template AST tree
   * and detect sub-trees that are purely static, i.e. parts of
   * the DOM that never needs to change.
   *
   * Once we detect these sub-trees, we can:
   *
   * 1. Hoist them into constants, so that we no longer need to
   *    create fresh nodes for them on each re-render;
   * 2. Completely skip them in the patching process.
   */
  function optimize (root, options) {
    if (!root) { return }
    isStaticKey = genStaticKeysCached(options.staticKeys || '');
    isPlatformReservedTag = options.isReservedTag || no;
    // first pass: mark all non-static nodes.
    markStatic$1(root);
    // second pass: mark static roots.
    markStaticRoots(root, false);
  }

  function genStaticKeys$1 (keys) {
    return makeMap(
      'type,tag,attrsList,attrsMap,plain,parent,children,attrs,start,end,rawAttrsMap' +
      (keys ? ',' + keys : '')
    )
  }

  function markStatic$1 (node) {
    node.static = isStatic(node);
    if (node.type === 1) {
      // do not make component slot content static. this avoids
      // 1. components not able to mutate slot nodes
      // 2. static slot content fails for hot-reloading
      if (
        !isPlatformReservedTag(node.tag) &&
        node.tag !== 'slot' &&
        node.attrsMap['inline-template'] == null
      ) {
        return
      }
      for (var i = 0, l = node.children.length; i < l; i++) {
        var child = node.children[i];
        markStatic$1(child);
        if (!child.static) {
          node.static = false;
        }
      }
      if (node.ifConditions) {
        for (var i$1 = 1, l$1 = node.ifConditions.length; i$1 < l$1; i$1++) {
          var block = node.ifConditions[i$1].block;
          markStatic$1(block);
          if (!block.static) {
            node.static = false;
          }
        }
      }
    }
  }

  function markStaticRoots (node, isInFor) {
    if (node.type === 1) {
      if (node.static || node.once) {
        node.staticInFor = isInFor;
      }
      // For a node to qualify as a static root, it should have children that
      // are not just static text. Otherwise the cost of hoisting out will
      // outweigh the benefits and it's better off to just always render it fresh.
      if (node.static && node.children.length && !(
        node.children.length === 1 &&
        node.children[0].type === 3
      )) {
        node.staticRoot = true;
        return
      } else {
        node.staticRoot = false;
      }
      if (node.children) {
        for (var i = 0, l = node.children.length; i < l; i++) {
          markStaticRoots(node.children[i], isInFor || !!node.for);
        }
      }
      if (node.ifConditions) {
        for (var i$1 = 1, l$1 = node.ifConditions.length; i$1 < l$1; i$1++) {
          markStaticRoots(node.ifConditions[i$1].block, isInFor);
        }
      }
    }
  }

  function isStatic (node) {
    if (node.type === 2) { // expression
      return false
    }
    if (node.type === 3) { // text
      return true
    }
    return !!(node.pre || (
      !node.hasBindings && // no dynamic bindings
      !node.if && !node.for && // not v-if or v-for or v-else
      !isBuiltInTag(node.tag) && // not a built-in
      isPlatformReservedTag(node.tag) && // not a component
      !isDirectChildOfTemplateFor(node) &&
      Object.keys(node).every(isStaticKey)
    ))
  }

  function isDirectChildOfTemplateFor (node) {
    while (node.parent) {
      node = node.parent;
      if (node.tag !== 'template') {
        return false
      }
      if (node.for) {
        return true
      }
    }
    return false
  }

  /*  */

  var fnExpRE = /^([\w$_]+|\([^)]*?\))\s*=>|^function(?:\s+[\w$]+)?\s*\(/;
  var fnInvokeRE = /\([^)]*?\);*$/;
  var simplePathRE = /^[A-Za-z_$][\w$]*(?:\.[A-Za-z_$][\w$]*|\['[^']*?']|\["[^"]*?"]|\[\d+]|\[[A-Za-z_$][\w$]*])*$/;

  // KeyboardEvent.keyCode aliases
  var keyCodes = {
    esc: 27,
    tab: 9,
    enter: 13,
    space: 32,
    up: 38,
    left: 37,
    right: 39,
    down: 40,
    'delete': [8, 46]
  };

  // KeyboardEvent.key aliases
  var keyNames = {
    // #7880: IE11 and Edge use `Esc` for Escape key name.
    esc: ['Esc', 'Escape'],
    tab: 'Tab',
    enter: 'Enter',
    // #9112: IE11 uses `Spacebar` for Space key name.
    space: [' ', 'Spacebar'],
    // #7806: IE11 uses key names without `Arrow` prefix for arrow keys.
    up: ['Up', 'ArrowUp'],
    left: ['Left', 'ArrowLeft'],
    right: ['Right', 'ArrowRight'],
    down: ['Down', 'ArrowDown'],
    // #9112: IE11 uses `Del` for Delete key name.
    'delete': ['Backspace', 'Delete', 'Del']
  };

  // #4868: modifiers that prevent the execution of the listener
  // need to explicitly return null so that we can determine whether to remove
  // the listener for .once
  var genGuard = function (condition) { return ("if(" + condition + ")return null;"); };

  var modifierCode = {
    stop: '$event.stopPropagation();',
    prevent: '$event.preventDefault();',
    self: genGuard("$event.target !== $event.currentTarget"),
    ctrl: genGuard("!$event.ctrlKey"),
    shift: genGuard("!$event.shiftKey"),
    alt: genGuard("!$event.altKey"),
    meta: genGuard("!$event.metaKey"),
    left: genGuard("'button' in $event && $event.button !== 0"),
    middle: genGuard("'button' in $event && $event.button !== 1"),
    right: genGuard("'button' in $event && $event.button !== 2")
  };

  function genHandlers (
    events,
    isNative
  ) {
    var prefix = isNative ? 'nativeOn:' : 'on:';
    var staticHandlers = "";
    var dynamicHandlers = "";
    for (var name in events) {
      var handlerCode = genHandler(events[name]);
      if (events[name] && events[name].dynamic) {
        dynamicHandlers += name + "," + handlerCode + ",";
      } else {
        staticHandlers += "\"" + name + "\":" + handlerCode + ",";
      }
    }
    staticHandlers = "{" + (staticHandlers.slice(0, -1)) + "}";
    if (dynamicHandlers) {
      return prefix + "_d(" + staticHandlers + ",[" + (dynamicHandlers.slice(0, -1)) + "])"
    } else {
      return prefix + staticHandlers
    }
  }

  function genHandler (handler) {
    if (!handler) {
      return 'function(){}'
    }

    if (Array.isArray(handler)) {
      return ("[" + (handler.map(function (handler) { return genHandler(handler); }).join(',')) + "]")
    }

    var isMethodPath = simplePathRE.test(handler.value);
    var isFunctionExpression = fnExpRE.test(handler.value);
    var isFunctionInvocation = simplePathRE.test(handler.value.replace(fnInvokeRE, ''));

    if (!handler.modifiers) {
      if (isMethodPath || isFunctionExpression) {
        return handler.value
      }
      return ("function($event){" + (isFunctionInvocation ? ("return " + (handler.value)) : handler.value) + "}") // inline statement
    } else {
      var code = '';
      var genModifierCode = '';
      var keys = [];
      for (var key in handler.modifiers) {
        if (modifierCode[key]) {
          genModifierCode += modifierCode[key];
          // left/right
          if (keyCodes[key]) {
            keys.push(key);
          }
        } else if (key === 'exact') {
          var modifiers = (handler.modifiers);
          genModifierCode += genGuard(
            ['ctrl', 'shift', 'alt', 'meta']
              .filter(function (keyModifier) { return !modifiers[keyModifier]; })
              .map(function (keyModifier) { return ("$event." + keyModifier + "Key"); })
              .join('||')
          );
        } else {
          keys.push(key);
        }
      }
      if (keys.length) {
        code += genKeyFilter(keys);
      }
      // Make sure modifiers like prevent and stop get executed after key filtering
      if (genModifierCode) {
        code += genModifierCode;
      }
      var handlerCode = isMethodPath
        ? ("return " + (handler.value) + ".apply(null, arguments)")
        : isFunctionExpression
          ? ("return (" + (handler.value) + ").apply(null, arguments)")
          : isFunctionInvocation
            ? ("return " + (handler.value))
            : handler.value;
      return ("function($event){" + code + handlerCode + "}")
    }
  }
  // genKeyFilter用于生成一段过滤的字符串
  function genKeyFilter (keys) {
    return (
      // make sure the key filters only apply to KeyboardEvents
      // #9441: can't use 'keyCode' in $event because Chrome autofill fires fake
      // key events that do not have keyCode property...
      "if(!$event.type.indexOf('key')&&" +
      (keys.map(genFilterCode).join('&&')) + ")return null;"
    )
  }

  function genFilterCode (key) {
    var keyVal = parseInt(key, 10);
    if (keyVal) {
      return ("$event.keyCode!==" + keyVal)
    }
    var keyCode = keyCodes[key];
    var keyName = keyNames[key];
    return (
      "_k($event.keyCode," +
      (JSON.stringify(key)) + "," +
      (JSON.stringify(keyCode)) + "," +
      "$event.key," +
      "" + (JSON.stringify(keyName)) +
      ")"
    )
  }

  /*  */

  function on (el, dir) {
    if (dir.modifiers) {
      warn("v-on without argument does not support modifiers.");
    }
    el.wrapListeners = function (code) { return ("_g(" + code + "," + (dir.value) + ")"); };
  }

  /*  */

  function bind$1 (el, dir) {
    el.wrapData = function (code) {
      return ("_b(" + code + ",'" + (el.tag) + "'," + (dir.value) + "," + (dir.modifiers && dir.modifiers.prop ? 'true' : 'false') + (dir.modifiers && dir.modifiers.sync ? ',true' : '') + ")")
    };
  }
 /****************************************************************************************************************************************** */ 
  // CodegenState 里的工具函数
  var baseDirectives = {
    on: on,
    bind: bind$1,
    cloak: noop
  };

 




  // CodegenState函数里面包含转化时需要的属性和方法
  var CodegenState = function CodegenState (options) {
    this.options = options;
    this.warn = options.warn || baseWarn;
    this.transforms = pluckModuleFunction(options.modules, 'transformCode');
    //dataGenFns是对静态类和静态样式的处理
    this.dataGenFns = pluckModuleFunction(options.modules, 'genData');
    //directives是对指令的相关操作
    this.directives = extend(extend({}, baseDirectives), options.directives);
    //保留标签标志
    var isReservedTag = options.isReservedTag || no;
    //不是保留标签就可能是组件
    this.maybeComponent = function (el) { return !!el.component || !isReservedTag(el.tag); };
    //onceId使用v-once的递增id
    this.onceId = 0;
    //staticRenderFns是对静态根节点的处理
    this.staticRenderFns = [];
    this.pre = false;
  };



  function generate (
    ast,
    options
  ) {
    // 实例化 CodegenState 对象,生成代码的时候需要用到其中的一些东西
    var state = new CodegenState(options);
    // 生成字符串格式的代码
    var code = ast ? (ast.tag === 'script' ? 'null' : genElement(ast, state)) : '_c("div")';
    return {
      render: ("with(this){return " + code + "}"),
      staticRenderFns: state.staticRenderFns
    }
  }

  function genElement (el, state) {
    if (el.parent) {
      el.pre = el.pre || el.parent.pre;
    }

    if (el.staticRoot && !el.staticProcessed) {
      // 处理静态根节点,生成节点的渲染函数
      return genStatic(el, state)
    } else if (el.once && !el.onceProcessed) {
      // 处理带有 v-once 指令的节点
      return genOnce(el, state)
    } else if (el.for && !el.forProcessed) {
      // 处理节点上的 v-for 指令  
      return genFor(el, state)
    } else if (el.if && !el.ifProcessed) {
      // 处理带有 v-if 指令的节点
      return genIf(el, state)
      // 当前节点不是 template 标签也不是插槽和带有 v-pre 指令的节点时
    } else if (el.tag === 'template' && !el.slotTarget && !state.pre) {
      // 生成所有子节点的渲染函数,返回一个数组
      return genChildren(el, state) || 'void 0'
    } else if (el.tag === 'slot') {
      // 生成插槽的渲染函数
      return genSlot(el, state)
    } else {
      // component or element
      var code;
      if (el.component) {
        // 处理动态组件和普通元素(自定义组件、原生标签)
        code = genComponent(el.component, el, state);
      } else {
        var data;
        // 自定义组件和原生标签
        if (!el.plain || (el.pre && state.maybeComponent(el))) {
          // 非普通元素或者带有 v-pre 指令的组件,处理节点的所有属性,返回一个 JSON 字符串
          data = genData$2(el, state);
        }
        // 处理子节点,得到所有子节点字符串格式的代码组成的数组
        var children = el.inlineTemplate ? null : genChildren(el, state, true);
        // 得到最终的字符串格式的代码
        code = "_c('" + (el.tag) + "'" + (data ? ("," + data) : '') + (children ? ("," + children) : '') + ")";
      }
       // 如果提供了 transformCode 方法, 
    // 则最终的 code 会经过各个模块(module)的该方法处理
      for (var i = 0; i < state.transforms.length; i++) {
        code = state.transforms[i](el, code);
      }
      return code
    }
  }

  // 处理静态根节点,生成节点的渲染函数
  function genStatic (el, state) {
    el.staticProcessed = true;
    var originalPreState = state.pre;
    if (el.pre) {
      state.pre = el.pre;
    }
     // 将当前静态节点的渲染函数放到 staticRenderFns 数组中
    state.staticRenderFns.push(("with(this){return " + (genElement(el, state)) + "}"));
    state.pre = originalPreState;
    // 返回一个可执行函数 _m(idx, true or '') 
    return ("_m(" + (state.staticRenderFns.length - 1) + (el.staticInFor ? ',true' : '') + ")")
  }

  // 处理带有 v-once 指令的节点
  function genOnce (el, state) {
    el.onceProcessed = true;
    if (el.if && !el.ifProcessed) {
      // 当前节点存在 v-if 指令,得到一个三元表达式,condition ? render1 : render2
      return genIf(el, state)
    } else if (el.staticInFor) {
      var key = '';
      var parent = el.parent;
      while (parent) {
        // 当前节点是一个包含在 v-for 指令内部的静态节点,得到 `_o(_c(tag, data, children), number, key)`
        if (parent.for) {
          key = parent.key;
          break
        }
        parent = parent.parent;
      }
      if (!key) {
        // 当前节点就是一个单纯的 v-once 节点,得到 `_m(idx, true of '')`
        state.warn(
          "v-once can only be used inside v-for that is keyed. ",
          el.rawAttrsMap['v-once']
        );
        return genElement(el, state)
      }
      return ("_o(" + (genElement(el, state)) + "," + (state.onceId++) + "," + key + ")")
    } else {
      return genStatic(el, state)
    }
  }
  // 处理带有 v-if 指令的节点,最终得到一个三元表达式:condition ? render1 : render2
  function genIf (
    el,
    state,
    altGen,
    altEmpty
  ) {
    // 标记当前节点的 v-if 指令已经被处理过了,避免无效的递归
    el.ifProcessed = true; // avoid recursion
    return genIfConditions(el.ifConditions.slice(), state, altGen, altEmpty)
  }

  function genIfConditions (
    conditions,
    state,
    altGen,
    altEmpty
  ) {
    //  长度若为空,则直接返回一个空节点渲染函数
    if (!conditions.length) {
      return altEmpty || '_e()'
    }
// 从 conditions 数组中拿出第一个条件对象 { exp, block }
    var condition = conditions.shift();
    // 返回结果是一个三元表达式字符串,condition ? 渲染函数1 : 渲染函数2
    if (condition.exp) {
      // 如果 condition.exp 条件成立,则得到一个三元表达式,
    // 如果条件不成立,则通过递归的方式找 conditions 数组中下一个元素,
    // 直到找到条件成立的元素,然后返回一个三元表达式
      return ("(" + (condition.exp) + ")?" + (genTernaryExp(condition.block)) + ":" + (genIfConditions(conditions, state, altGen, altEmpty)))
    } else {
      return ("" + (genTernaryExp(condition.block)))
    }

    // v-if with v-once should generate code like (a)?_m(0):_m(1)
    function genTernaryExp (el) {
      return altGen
        ? altGen(el, state)
        : el.once
          ? genOnce(el, state)
          : genElement(el, state)
    }
  }
  // 处理节点上的 v-for 指令  
  function genFor (
    el,
    state,
    altGen,
    altHelper
  ) {
    // 处理节点上的 v-for 指令  
    var exp = el.for;
    var alias = el.alias;
     // iterator 为 v-for = "(item ,idx) in obj" 时
    var iterator1 = el.iterator1 ? ("," + (el.iterator1)) : '';
    var iterator2 = el.iterator2 ? ("," + (el.iterator2)) : '';
  // 提示,v-for 指令在组件上时必须使用 key
    if (state.maybeComponent(el) &&
      el.tag !== 'slot' &&
      el.tag !== 'template' &&
      !el.key
    ) {
      state.warn(
        "<" + (el.tag) + " v-for=\"" + alias + " in " + exp + "\">: component lists rendered with " +
        "v-for should have explicit keys. " +
        "See https://vuejs.org/guide/list.html#key for more info.",
        el.rawAttrsMap['v-for'],
        true /* tip */
      );
    }
// 标记当前节点上的 v-for 指令已经被处理过了
    el.forProcessed = true; // avoid recursion
    return (altHelper || '_l') + "((" + exp + ")," +
      "function(" + alias + iterator1 + iterator2 + "){" +
        "return " + ((altGen || genElement)(el, state)) +
      '})'
  }

  function genData$2 (el, state) {
    var data = '{';

    // directives first.
    // directives may mutate the el's other properties before they are generated.
    var dirs = genDirectives(el, state);
    if (dirs) { data += dirs + ','; }

    // key
    if (el.key) {
      data += "key:" + (el.key) + ",";
    }
    // ref
    if (el.ref) {
      data += "ref:" + (el.ref) + ",";
    }
    if (el.refInFor) {
      data += "refInFor:true,";
    }
    // pre
    if (el.pre) {
      data += "pre:true,";
    }
    // record original tag name for components using "is" attribute
    if (el.component) {
      data += "tag:\"" + (el.tag) + "\",";
    }
    // module data generation functions
    for (var i = 0; i < state.dataGenFns.length; i++) {
      data += state.dataGenFns[i](el);
    }
    // attributes
    if (el.attrs) {
      data += "attrs:" + (genProps(el.attrs)) + ",";
    }
    // DOM props
    if (el.props) {
      data += "domProps:" + (genProps(el.props)) + ",";
    }
    // event handlers
    if (el.events) {
      data += (genHandlers(el.events, false)) + ",";
    }
    if (el.nativeEvents) {
      data += (genHandlers(el.nativeEvents, true)) + ",";
    }
    // slot target
    // only for non-scoped slots
    if (el.slotTarget && !el.slotScope) {
      data += "slot:" + (el.slotTarget) + ",";
    }
    // scoped slots
    if (el.scopedSlots) {
      data += (genScopedSlots(el, el.scopedSlots, state)) + ",";
    }
    // component v-model
    if (el.model) {
      data += "model:{value:" + (el.model.value) + ",callback:" + (el.model.callback) + ",expression:" + (el.model.expression) + "},";
    }
    // inline-template
    if (el.inlineTemplate) {
      var inlineTemplate = genInlineTemplate(el, state);
      if (inlineTemplate) {
        data += inlineTemplate + ",";
      }
    }
    data = data.replace(/,$/, '') + '}';
    // v-bind dynamic argument wrap
    // v-bind with dynamic arguments must be applied using the same v-bind object
    // merge helper so that class/style/mustUseProp attrs are handled correctly.
    if (el.dynamicAttrs) {
      data = "_b(" + data + ",\"" + (el.tag) + "\"," + (genProps(el.dynamicAttrs)) + ")";
    }
    // v-bind data wrap
    if (el.wrapData) {
      data = el.wrapData(data);
    }
    // v-on data wrap
    if (el.wrapListeners) {
      data = el.wrapListeners(data);
    }
    return data
  }
  // 运行指令的编译方法,如果指令存在运行时任务,则返回 directives: [{ name, rawName, value, arg, modifiers }, ...}] 
  function genDirectives (el, state) {
     // 获取指令数组
    var dirs = el.directives;
     // 没有指令则直接结束
    if (!dirs) { return }
    // 指令的处理结果
    var res = 'directives:[';
    // 标记,用于标记指令是否需要在运行时完成的任务,比如 v-model 的 input 事件
    var hasRuntime = false;
    var i, l, dir, needRuntime;
     // 遍历指令数组
    for (i = 0, l = dirs.length; i < l; i++) {
      dir = dirs[i];
      needRuntime = true;
      // 获取节点当前指令的处理方法,比如 web 平台的 v-html、v-text、v-model
      var gen = state.directives[dir.name];
      if (gen) {
         // 执行指令的编译方法,如果指令还需要运行时完成一部分任务,则返回 true,比如 v-model
        needRuntime = !!gen(el, dir, state.warn);
      }
      if (needRuntime) {
         // 表示该指令在运行时还有任务
        hasRuntime = true;
        res += "{name:\"" + (dir.name) + "\",rawName:\"" + (dir.rawName) + "\"" + (dir.value ? (",value:(" + (dir.value) + "),expression:" + (JSON.stringify(dir.value))) : '') + (dir.arg ? (",arg:" + (dir.isDynamicArg ? dir.arg : ("\"" + (dir.arg) + "\""))) : '') + (dir.modifiers ? (",modifiers:" + (JSON.stringify(dir.modifiers))) : '') + "},";
      }
    }
    if (hasRuntime) {
      // 也就是说,只有指令存在运行时任务时,才会返回 res
      return res.slice(0, -1) + ']'
    }
  }
  // 处理内联模版
  function genInlineTemplate (el, state) {
    var ast = el.children[0];
    if (el.children.length !== 1 || ast.type !== 1) {
      state.warn(
        'Inline-template components must have exactly one child element.',
        { start: el.start }
      );
    }
    if (ast && ast.type === 1) {
      var inlineRenderFns = generate(ast, state.options);
      return ("inlineTemplate:{render:function(){" + (inlineRenderFns.render) + "},staticRenderFns:[" + (inlineRenderFns.staticRenderFns.map(function (code) { return ("function(){" + code + "}"); }).join(',')) + "]}")
    }
  }
  // 处理作用域插槽模版
  function genScopedSlots (
    el,
    slots,
    state
  ) {
   //默认情况下,作用域槽被认为是“稳定的”,这允许子槽
//只有作用域槽的组件跳过来自父组件的强制更新。
//但在某些情况下,我们不得不放弃这种优化
//例如,如果槽包含动态名称,有v-if或v-for…
    var needsForceUpdate = el.for || Object.keys(slots).some(function (key) {
      var slot = slots[key];
      return (
        slot.slotTargetDynamic ||
        slot.if ||
        slot.for ||
        containsSlotChild(slot) // is passing down slot from parent which may be dynamic
      )
    });

    // 如果一个有作用域槽的组件位于条件分支中,
    //可以重用相同的组件,但不同的组件
    //编译的槽位内容。为了避免这种情况,我们生成一个基于
    //所有槽位内容的生成代码。
    var needsKey = !!el.if;

    // 或者当它在另一个作用域槽或v-for中(反应性可能是由于中间作用域变量而断开连接)
    // #9438, #9506
    // 通过正确分析作用域内绑定,可以进一步优化这一点并跳过不使用范围变量的强制更新。
    if (!needsForceUpdate) {
      var parent = el.parent;
      while (parent) {
        if (
          (parent.slotScope && parent.slotScope !== emptySlotScopeToken) ||
          parent.for
        ) {
          needsForceUpdate = true;
          break
        }
        if (parent.if) {
          needsKey = true;
        }
        parent = parent.parent;
      }
    }

    var generatedSlots = Object.keys(slots)
      .map(function (key) { return genScopedSlot(slots[key], state); })
      .join(',');

    return ("scopedSlots:_u([" + generatedSlots + "]" + (needsForceUpdate ? ",null,true" : "") + (!needsForceUpdate && needsKey ? (",null,false," + (hash(generatedSlots))) : "") + ")")
  }

  function hash(str) {
    var hash = 5381;
    var i = str.length;
    while(i) {
      hash = (hash * 33) ^ str.charCodeAt(--i);
    }
    return hash >>> 0
  }

  function containsSlotChild (el) {
    if (el.type === 1) {
      if (el.tag === 'slot') {
        return true
      }
      return el.children.some(containsSlotChild)
    }
    return false
  }

  function genScopedSlot (
    el,
    state
  ) {
    var isLegacySyntax = el.attrsMap['slot-scope'];
    if (el.if && !el.ifProcessed && !isLegacySyntax) {
      return genIf(el, state, genScopedSlot, "null")
    }
    if (el.for && !el.forProcessed) {
      return genFor(el, state, genScopedSlot)
    }
    var slotScope = el.slotScope === emptySlotScopeToken
      ? ""
      : String(el.slotScope);
      // 函数参数为标签上slot-scope属性对应的值
    var fn = "function(" + slotScope + "){" +
      "return " + (el.tag === 'template'
        ? el.if && isLegacySyntax
          ? ("(" + (el.if) + ")?" + (genChildren(el, state) || 'undefined') + ":undefined")
          : genChildren(el, state) || 'undefined'
        : genElement(el, state)) + "}";
    // 反向代理v-slot没有作用域
    var reverseProxy = slotScope ? "" : ",proxy:true";
    // key为插槽的名称,fn为生成的函数代码
    return ("{key:" + (el.slotTarget || "\"default\"") + ",fn:" + fn + reverseProxy + "}")
  }
  // 生成所有子节点的渲染函数,返回一个数组
  function genChildren (
    el,
    state,
    checkSkip,
    altGenElement,
    altGenNode
  ) {
    // 所有子节点
    var children = el.children;
    if (children.length) {
      // 第一个子节点
      var el$1 = children[0];
      // optimize single v-for
      if (children.length === 1 &&
        el$1.for &&
        el$1.tag !== 'template' &&
        el$1.tag !== 'slot'
      ) {
         // 优化,只有一个子节点 && 子节点的上有 v-for 指令 && 子节点的标签不为 template 或者 slot
      // 优化的方式是直接调用 genElement 生成该节点的渲染函数,不需要走下面的循环然后调用 genCode 最后得到渲染函数
        var normalizationType = checkSkip
          ? state.maybeComponent(el$1) ? ",1" : ",0"
          : "";
        return ("" + ((altGenElement || genElement)(el$1, state)) + normalizationType)
      }
      // 获取节点规范化类型,返回一个 number
      var normalizationType$1 = checkSkip
        ? getNormalizationType(children, state.maybeComponent)
        : 0;
        // 函数,生成代码的一个函数
      var gen = altGenNode || genNode;
       // 返回一个数组,数组的每个元素都是一个子节点的渲染函数
      return ("[" + (children.map(function (c) { return gen(c, state); }).join(',')) + "]" + (normalizationType$1 ? ("," + normalizationType$1) : ''))
    }
  }

  // 确定子数组所需的规范化。
// 0:不需要标准化
// 1:需要简单的规范化(可能是1级深嵌套数组)
// 2:需要完全标准化
  function getNormalizationType (
    children,
    maybeComponent
  ) {
    var res = 0;
    for (var i = 0; i < children.length; i++) {
      var el = children[i];
      if (el.type !== 1) {
        continue
      }
      if (needsNormalization(el) ||
          (el.ifConditions && el.ifConditions.some(function (c) { return needsNormalization(c.block); }))) {
        res = 2;
        break
      }
      if (maybeComponent(el) ||
          (el.ifConditions && el.ifConditions.some(function (c) { return maybeComponent(c.block); }))) {
        res = 1;
      }
    }
    return res
  }

  function needsNormalization (el) {
    return el.for !== undefined || el.tag === 'template' || el.tag === 'slot'
  }
  // 生成节点的渲染函数
  function genNode (node, state) {
    if (node.type === 1) {
      return genElement(node, state)
    } else if (node.type === 3 && node.isComment) {
      return genComment(node)
    } else {
      return genText(node)
    }
  }
// 生成文本的渲染函数
  function genText (text) {
    return ("_v(" + (text.type === 2
      ? text.expression // no need for () because already wrapped in _s()
      : transformSpecialNewlines(JSON.stringify(text.text))) + ")")
  }
// 生成Comment文本的渲染函数
  function genComment (comment) {
    return ("_e(" + (JSON.stringify(comment.text)) + ")")
  }
  // 生成插槽的渲染函数,得到 _t(slotName, children, attrs, bind)
  function genSlot (el, state) {
     // 插槽名称
    var slotName = el.slotName || '"default"';
     // 生成所有的子节点
    var children = genChildren(el, state);
    // 结果字符串,_t(slotName, children, attrs, bind)
    var res = "_t(" + slotName + (children ? (",function(){return " + children + "}") : '');
    var attrs = el.attrs || el.dynamicAttrs
      ? genProps((el.attrs || []).concat(el.dynamicAttrs || []).map(function (attr) { return ({
          // 槽道具是驼峰的
          name: camelize(attr.name),
          value: attr.value,
          dynamic: attr.dynamic
        }); }))
      : null;
    var bind$$1 = el.attrsMap['v-bind'];
    if ((attrs || bind$$1) && !children) {
      res += ",null";
    }
    if (attrs) {
      res += "," + attrs;
    }
    if (bind$$1) {
      res += (attrs ? '' : ',null') + "," + bind$$1;
    }
    return res + ')'
  }

  //  生成动态组件的渲染函数
  function genComponent (
    componentName,
    el,
    state
  ) {
    // 所有的子节点
    var children = el.inlineTemplate ? null : genChildren(el, state, true);
     // componentName 是 is 属性的值
    return ("_c(" + componentName + "," + (genData$2(el, state)) + (children ? ("," + children) : '') + ")")
  }
  // 拼凑AST对象的属性或DOM属性
  function genProps (props) {
    var staticProps = "";
    var dynamicProps = "";
    //遍历props
    for (var i = 0; i < props.length; i++) {
      //对应的值
      var prop = props[i];
      var value = transformSpecialNewlines(prop.value);
      if (prop.dynamic) {
        dynamicProps += (prop.name) + "," + value + ",";
      } else {
        staticProps += "\"" + (prop.name) + "\":" + value + ",";
      }
    }
    staticProps = "{" + (staticProps.slice(0, -1)) + "}";
    if (dynamicProps) {
      return ("_d(" + staticProps + ",[" + (dynamicProps.slice(0, -1)) + "])")
    } else {
      return staticProps
    }
  }

  // 它们被浏览器理解为换行,而在Javascript的字符串表达式中是不允许换行的,从而导致错误
  function transformSpecialNewlines (text) {
    return text
      .replace(/\u2028/g, '\\u2028')
      .replace(/\u2029/g, '\\u2029')
  }

  /*  */



  // 这些关键字不应该出现在表达式中,而应该出现在操作符中
//允许使用typeof, instanceof和in
  var prohibitedKeywordRE = new RegExp('\\b' + (
    'do,if,for,let,new,try,var,case,else,with,await,break,catch,class,const,' +
    'super,throw,while,yield,delete,export,import,return,switch,default,' +
    'extends,finally,continue,debugger,function,arguments'
  ).split(',').join('\\b|\\b') + '\\b');

  // 这些一元操作符不应该用作属性/方法名
  var unaryOperatorsRE = new RegExp('\\b' + (
    'delete,typeof,void'
  ).split(',').join('\\s*\\([^\\)]*\\)|\\b') + '\\s*\\([^\\)]*\\)');

  // 在表达式中剥离字符串
  var stripStringRE = /'(?:[^'\\]|\\.)*'|"(?:[^"\\]|\\.)*"|`(?:[^`\\]|\\.)*\$\{|\}(?:[^`\\]|\\.)*`|`(?:[^`\\]|\\.)*`/g;

  //检测模板中有问题的表达式
  function detectErrors (ast, warn) {
    if (ast) {
      checkNode(ast, warn);
    }
  }
  // 检查Node
  function checkNode (node, warn) {
    if (node.type === 1) {
      for (var name in node.attrsMap) {
        if (dirRE.test(name)) {
          var value = node.attrsMap[name];
          if (value) {
            var range = node.rawAttrsMap[name];
            if (name === 'v-for') {
              checkFor(node, ("v-for=\"" + value + "\""), warn, range);
            } else if (name === 'v-slot' || name[0] === '#') {
              checkFunctionParameterExpression(value, (name + "=\"" + value + "\""), warn, range);
            } else if (onRE.test(name)) {
              checkEvent(value, (name + "=\"" + value + "\""), warn, range);
            } else {
              checkExpression(value, (name + "=\"" + value + "\""), warn, range);
            }
          }
        }
      }
      if (node.children) {
        for (var i = 0; i < node.children.length; i++) {
          checkNode(node.children[i], warn);
        }
      }
    } else if (node.type === 2) {
      checkExpression(node.expression, node.text, warn, node);
    }
  }
  // 检查 Event
  function checkEvent (exp, text, warn, range) {
    var stripped = exp.replace(stripStringRE, '');
    var keywordMatch = stripped.match(unaryOperatorsRE);
    if (keywordMatch && stripped.charAt(keywordMatch.index - 1) !== '$') {
      warn(
        "avoid using JavaScript unary operator as property name: " +
        "\"" + (keywordMatch[0]) + "\" in expression " + (text.trim()),
        range
      );
    }
    checkExpression(exp, text, warn, range);
  }
  // 检查 For 循环
  function checkFor (node, text, warn, range) {
    checkExpression(node.for || '', text, warn, range);
    checkIdentifier(node.alias, 'v-for alias', text, warn, range);
    checkIdentifier(node.iterator1, 'v-for iterator', text, warn, range);
    checkIdentifier(node.iterator2, 'v-for iterator', text, warn, range);
  }
// 检查 Identifier
  function checkIdentifier (
    ident,
    type,
    text,
    warn,
    range
  ) {
    if (typeof ident === 'string') {
      try {
        new Function(("var " + ident + "=_"));
      } catch (e) {
        warn(("invalid " + type + " \"" + ident + "\" in expression: " + (text.trim())), range);
      }
    }
  }
  // 检查表达式
  function checkExpression (exp, text, warn, range) {
    try {
      new Function(("return " + exp));
    } catch (e) {
      // prohibitedKeywordRE:正则校验:禁止关键字
       // stripStringRE:正则校验:脚本字符串
      var keywordMatch = exp.replace(stripStringRE, '').match(prohibitedKeywordRE);
      if (keywordMatch) {
        warn(
          "avoid using JavaScript keyword as property name: " +
          "\"" + (keywordMatch[0]) + "\"\n  Raw expression: " + (text.trim()),
          range
        );
      } else {
        warn(
          "invalid expression: " + (e.message) + " in\n\n" +
          "    " + exp + "\n\n" +
          "  Raw expression: " + (text.trim()) + "\n",
          range
        );
      }
    }
  }
  // 检查函数表达式
  function checkFunctionParameterExpression (exp, text, warn, range) {
    try {
      new Function(exp, '');
    } catch (e) {
      warn(
        "invalid function parameter expression: " + (e.message) + " in\n\n" +
        "    " + exp + "\n\n" +
        "  Raw expression: " + (text.trim()) + "\n",
        range
      );
    }
  }

  /*  */

  var range = 2;

  function generateCodeFrame (
    source,
    start,
    end
  ) {
    if ( start === void 0 ) start = 0;
    if ( end === void 0 ) end = source.length;

    var lines = source.split(/\r?\n/);
    var count = 0;
    var res = [];
    for (var i = 0; i < lines.length; i++) {
      count += lines[i].length + 1;
      if (count >= start) {
        for (var j = i - range; j <= i + range || end > count; j++) {
          if (j < 0 || j >= lines.length) { continue }
          res.push(("" + (j + 1) + (repeat$1(" ", 3 - String(j + 1).length)) + "|  " + (lines[j])));
          var lineLength = lines[j].length;
          if (j === i) {
            // push underline
            var pad = start - (count - lineLength) + 1;
            var length = end > count ? lineLength - pad : end - start;
            res.push("   |  " + repeat$1(" ", pad) + repeat$1("^", length));
          } else if (j > i) {
            if (end > count) {
              var length$1 = Math.min(end - count, lineLength);
              res.push("   |  " + repeat$1("^", length$1));
            }
            count += lineLength + 1;
          }
        }
        break
      }
    }
    return res.join('\n')
  }

  function repeat$1 (str, n) {
    var result = '';
    if (n > 0) {
      while (true) { // eslint-disable-line
        if (n & 1) { result += str; }
        n >>>= 1;
        if (n <= 0) { break }
        str += str;
      }
    }
    return result
  }
 // 构建函数
  function createFunction (code, errors) {
    try {
      return new Function(code)
    } catch (err) {
      errors.push({ err: err, code: code });
      return noop
    }
  }

  function createCompileToFunctionFn (compile) {
     // 创建一个空对象
    var cache = Object.create(null);

    return function compileToFunctions (
      template, // 字符串模板 template 也就是经过那些操作获取之后的template
      options, // 参数
      vm // 虚拟dom
    ) {
      // 把options和空对象混合,也就是获取options,相当于浅拷贝一份options
      options = extend({}, options);
      // 获取警告
      var warn$$1 = options.warn || warn;
        // 删除options.warn属性
      delete options.warn;

      /* istanbul ignore if */
      {
        // 检测可能的CSP限制
        try {
          new Function('return 1');
        } catch (e) {
          if (e.toString().match(/unsafe-eval|CSP/)) {
            warn$$1(
              'It seems you are using the standalone build of Vue.js in an ' +
              'environment with Content Security Policy that prohibits unsafe-eval. ' +
              'The template compiler cannot work in this environment. Consider ' +
              'relaxing the policy to allow unsafe-eval or pre-compiling your ' +
              'templates into render functions.'
            );
          }
        }
      }

     // options中是否有delimiters 一般的delimiters都是delimiters: ['${', '}']这种
      var key = options.delimiters
      // 如果有delimiters  String(['${', '}'])返回 '${,}' + template
        ? String(options.delimiters) + template
         // 如果没有delimiters
        : template;
      if (cache[key]) {
        // 如果cache对象中有此key属性,直接返回
        return cache[key]
      }

      // // 执行compiled  传入template和options参数
      var compiled = compile(template, options);

      // 检查编译错误/技巧
      {
        if (compiled.errors && compiled.errors.length) {
          if (options.outputSourceRange) {
            compiled.errors.forEach(function (e) {
              warn$$1(
                "Error compiling template:\n\n" + (e.msg) + "\n\n" +
                generateCodeFrame(template, e.start, e.end),
                vm
              );
            });
          } else {
            warn$$1(
              "Error compiling template:\n\n" + template + "\n\n" +
              compiled.errors.map(function (e) { return ("- " + e); }).join('\n') + '\n',
              vm
            );
          }
        }
        if (compiled.tips && compiled.tips.length) {
          if (options.outputSourceRange) {
            compiled.tips.forEach(function (e) { return tip(e.msg, vm); });
          } else {
            compiled.tips.forEach(function (msg) { return tip(msg, vm); });
          }
        }
      }

      // 将代码转换为函数
      var res = {};
      var fnGenErrors = [];
      res.render = createFunction(compiled.render, fnGenErrors);
      res.staticRenderFns = compiled.staticRenderFns.map(function (code) {
        return createFunction(code, fnGenErrors)
      });

      //检查函数生成错误。
      //只有当编译器本身存在错误时才会发生这种情况。
      //主要用于代码生成开发
      /* istanbul ignore if */
      {
        if ((!compiled.errors || !compiled.errors.length) && fnGenErrors.length) {
          warn$$1(
            "Failed to generate render function:\n\n" +
            fnGenErrors.map(function (ref) {
              var err = ref.err;
              var code = ref.code;

              return ((err.toString()) + " in\n\n" + code + "\n");
          }).join('\n'),
            vm
          );
        }
      }

      return (cache[key] = res)
    }
  }

  /* vue编译器 */
  // createCompilerCreator是个高阶函数,接受一个函数baseCompile,返回了一个函数createCompiler,createCompiler函数里又有一个compile函数,
  // 里面调用了baseCompile和最初传入的baseOptions,最后返回compile函数和compileToFunctions函数
  function createCompilerCreator (baseCompile) {
    return function createCompiler (baseOptions) {
      function compile (
        template,
        options
      ) {
        // 将参数baseOptions和传入的options进行合并得到finalOptions
        var finalOptions = Object.create(baseOptions);
        var errors = [];
        var tips = [];

        var warn = function (msg, range, tip) {
          (tip ? tips : errors).push(msg);
        };

        if (options) {
          if (options.outputSourceRange) {
            // match方法可在字符串内检索指定的值,或找到一个或多个正则表达式的匹配
            var leadingSpaceLength = template.match(/^\s*/)[0].length;

            warn = function (msg, range, tip) {
              var data = { msg: msg };
              if (range) {
                if (range.start != null) {
                  data.start = range.start + leadingSpaceLength;
                }
                if (range.end != null) {
                  data.end = range.end + leadingSpaceLength;
                }
              }
              (tip ? tips : errors).push(data);
            };
          }
          // 合并baseOptions和options的模块
          if (options.modules) {
            finalOptions.modules =
              (baseOptions.modules || []).concat(options.modules);
          }
          // 合并baseOptions和options的指令
          if (options.directives) {
            finalOptions.directives = extend(
              Object.create(baseOptions.directives || null),
              options.directives
            );
          }
          // 复制options选项
          for (var key in options) {
            if (key !== 'modules' && key !== 'directives') {
              finalOptions[key] = options[key];
            }
          }
        }

        finalOptions.warn = warn;

        var compiled = baseCompile(template.trim(), finalOptions);
        {
          detectErrors(compiled.ast, warn);
        }
        compiled.errors = errors;
        compiled.tips = tips;
        return compiled
      }

      return {
        compile: compile,
        compileToFunctions: createCompileToFunctionFn(compile)
      }
    }
  }

  // createCompilerCreator 函数的返回值并且传递了 baseCompile 函数作为参数
  // 通过 createCompilerCreator 函数创建编译器
  var createCompiler = createCompilerCreator(function baseCompile (
    template,
    options
  ) {
    var ast = parse(template.trim(), options);
     // 优化,遍历 AST,为每个节点做静态标记
  // 标记每个节点是否为静态节点,然后进一步标记出静态根节点
  // 这样在后续更新中就可以跳过这些静态节点了
  // 标记静态根,用于生成渲染函数阶段,生成静态根节点的渲染函数
    if (options.optimize !== false) {
      optimize(ast, options);
    }
    // 代码生成,将 ast 转换成可执行的 render 函数的字符串形式
    var code = generate(ast, options);
    return {
      ast: ast,
      render: code.render,
      staticRenderFns: code.staticRenderFns
    }
  });

  /*  */

  var ref$1 = createCompiler(baseOptions);
  var compile = ref$1.compile;
  // 把模板字符串template编译成渲染函数render
  var compileToFunctions = ref$1.compileToFunctions;

  /*  */

  // 检查当前浏览器是否在属性值中编码字符
  var div;
  function getShouldDecode (href) {
    div = div || document.createElement('div');
    div.innerHTML = href ? "<a href=\"\n\"/>" : "<div a=\"\n\"/>";
    return div.innerHTML.indexOf('&#10;') > 0
  }

  // IE会在属性值中编码换行符,而其他浏览器不会
  // 解码换行符
  var shouldDecodeNewlines = inBrowser ? getShouldDecode(false) : false;
  // 解码href换行符
  var shouldDecodeNewlinesForHref = inBrowser ? getShouldDecode(true) : false;

  // vue会进行缓存,有缓存则读缓存结果,没有缓存则运行回调函数
  var idToTemplate = cached(function (id) {
    var el = query(id);
    return el && el.innerHTML
  });

  var mount = Vue.prototype.$mount;
  // $mount 方法支持传入 2 个参数,第一个是 el,它表示挂载的元素,
  // 第二个参数是和服务端渲染相关,在浏览器环境下不需要传第二个参数
  Vue.prototype.$mount = function (
    el,
    hydrating
  ) {
    el = el && query(el);
    if (el === document.body || el === document.documentElement) {
      warn(
        "Do not mount Vue to <html> or <body> - mount to normal elements instead."
      );
      return this
    }

    var options = this.$options;
    // 如果没有定义 render 方法,则会把 el 或者 template 字符串转换成 render 方法
    if (!options.render) {
      //取template值
      var template = options.template;
      if (template) {
        if (typeof template === 'string') {
          // charAt() 方法可返回指定位置的字符,判断第一个字符是否为"#",template为模板字符串或者 '#tmp'形式
          if (template.charAt(0) === '#') {
            // document.querySelector('#tmp').innerHTML形式
            template = idToTemplate(template);
            /* istanbul ignore if */
            if (!template) {
              warn(
                ("Template element not found or is empty: " + (options.template)),
                this
              );
            }
          }
          //document.querySelector('#tmp')形式
        } else if (template.nodeType) {
          template = template.innerHTML;
        } else {
          {
            warn('invalid template option:' + template, this);
          }
           // template选项无效
          return this
        }
      } else if (el) {
         //如果没定义template 则把el的outerHTML作为模板
        template = getOuterHTML(el);
      }
      if (template) {
        /* istanbul ignore if */
        if (config.performance && mark) {
          mark('compile');
        }
        //将template编译为render function
        var ref = compileToFunctions(template, {
          outputSourceRange: "development" !== 'production',
          shouldDecodeNewlines: shouldDecodeNewlines,
          shouldDecodeNewlinesForHref: shouldDecodeNewlinesForHref,
          delimiters: options.delimiters,
          comments: options.comments
        }, this);
        var render = ref.render;
        var staticRenderFns = ref.staticRenderFns;
        options.render = render;
        options.staticRenderFns = staticRenderFns;

        /* istanbul ignore if */
        if (config.performance && mark) {
          mark('compile end');
          measure(("vue " + (this._name) + " compile"), 'compile', 'compile end');
        }
      }
    }
    return mount.call(this, el, hydrating)
  };

  /**
   * 获取元素的outerHTML,同时处理IE中的SVG元素
   */
  function getOuterHTML (el) {
    // 如果元素的outerHTML存在
    if (el.outerHTML) {
      // 返回该元素及本身的所有HTML代码
      return el.outerHTML
    } else {
       // 如果该元素的outerHTML属性不存在,创建一个DIV
      var container = document.createElement('div');
       // 向DIV中添加这个el
      container.appendChild(el.cloneNode(true));
      // 此时返回container的innerHTML就可以得到该元素即本身的HTML代码
      return container.innerHTML
    }
  }

  Vue.compile = compileToFunctions;

  return Vue;

}));
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