[JavaScript知识库]React源码解读之任务调度

React 设计体系如人类社会一般，拨动时间轮盘的那一刻，你便成了穿梭在轮片中的一粒细沙，角逐过程处处都需要亮出你的属性，你重要吗？你无可替代吗？你有特殊权限吗？没有，那不好意思，请继续在轮片中循环。属于你的生命之火殆尽，前来悼念之人很多，这幕，像极了出生时的场景。

干啥玩意儿，这是技术文章不是抒情散文！下面进入正题。

创建的准备上一节已经说明了，主要定义与更新相关的数据结构和变量，计算过期时间等。完成这些准备工作之后，正式进入调度工作，调度过程实现思路是：当与更新或挂载相关api被调用时，就会执行更新的逻辑，更新大致分为以下几个小阶段

scheduleWork

该步骤的主要工作有以下几点

1. 通过 scheduleWorkOnParentPath 方法找到当前 Fiber 的root节点
2. 遍历当前更新节点父节点上的每个节点，对比每个节点的 expirationTime ，如果大于当前节点，则将其值赋值为当前节点的 expirationTime 值。同时，childExpirationTime 的值也是该的逻辑

export function scheduleUpdateOnFiber(
  fiber: Fiber,
  expirationTime: ExpirationTime,
) {
  checkForNestedUpdates();
  warnAboutInvalidUpdatesOnClassComponentsInDEV(fiber);

  const root = markUpdateTimeFromFiberToRoot(fiber, expirationTime);
  if (root === null) {
    warnAboutUpdateOnUnmountedFiberInDEV(fiber);
    return;
  }

  checkForInterruption(fiber, expirationTime);
  recordScheduleUpdate();

  // TODO: computeExpirationForFiber also reads the priority. Pass the
  // priority as an argument to that function and this one.
  const priorityLevel = getCurrentPriorityLevel();

  if (expirationTime === Sync) {
    if (
      // Check if we're inside unbatchedUpdates
      (executionContext & LegacyUnbatchedContext) !== NoContext &&
      // Check if we're not already rendering
      (executionContext & (RenderContext | CommitContext)) === NoContext
    ) {
      // Register pending interactions on the root to avoid losing traced interaction data.
      schedulePendingInteractions(root, expirationTime);

      performSyncWorkOnRoot(root);
    } else {
      ensureRootIsScheduled(root);
      schedulePendingInteractions(root, expirationTime);
      if (executionContext === NoContext) {
        flushSyncCallbackQueue();
      }
    }
  } else {
    ensureRootIsScheduled(root);
    schedulePendingInteractions(root, expirationTime);
  }

    ...
}
export const scheduleWork = scheduleUpdateOnFiber;

如果过期时间等于我们定义的Sync常量对应值，则进一步判断这次更新的状态，如果不是 batchUpdates 什么时候不是这个状态呢？我们前面认识过，比如reder时，判断完这个状态后还需要保证这次的更新渲染已准备好，则开始处理。不过处理之前，还要进行一个操作就是pending interaction，与我们动作相关的内容数据需要保存于 pendingInteractionMap 中。

function scheduleInteractions(root, expirationTime, interactions) {
  if (!enableSchedulerTracing) {
    return;
  }

  if (interactions.size > 0) {
    const pendingInteractionMap = root.pendingInteractionMap;
    const pendingInteractions = pendingInteractionMap.get(expirationTime);
    if (pendingInteractions != null) {
      interactions.forEach(interaction => {
        if (!pendingInteractions.has(interaction)) {
          // Update the pending async work count for previously unscheduled interaction.
          interaction.__count++;
        }

        pendingInteractions.add(interaction);
      });
    } else {
      pendingInteractionMap.set(expirationTime, new Set(interactions));

      // Update the pending async work count for the current interactions.
      interactions.forEach(interaction => {
        interaction.__count++;
      });
    }

    const subscriber = __subscriberRef.current;
    if (subscriber !== null) {
      const threadID = computeThreadID(root, expirationTime);
      subscriber.onWorkScheduled(interactions, threadID);
    }
  }
}

经过以上处理，就能进入 performSyncWorkOnRoot 处理了

function performSyncWorkOnRoot(root) {
  // Check if there's expired work on this root. Otherwise, render at Sync.
  const lastExpiredTime = root.lastExpiredTime;
  const expirationTime = lastExpiredTime !== NoWork ? lastExpiredTime : Sync;
  if (root.finishedExpirationTime === expirationTime) {
    commitRoot(root);
  }
  ...
}

好了，到这里一个expirationTime 为 Sync 的且不是unbatchedUpdates，的调度就完成了，我们发现这条流水线的操作还是容易理解的，好，我们现在进入另一个分支，就是 batchedUpdates

ensureRootIsScheduled(root);
schedulePendingInteractions(root, expirationTime);
if (executionContext === NoContext) {
  // Flush the synchronous work now, unless we're already working or inside
  // a batch. This is intentionally inside scheduleUpdateOnFiber instead of
  // scheduleCallbackForFiber to preserve the ability to schedule a callback
  // without immediately flushing it. We only do this for user-initiated
  // updates, to preserve historical behavior of legacy mode.
  flushSyncCallbackQueue();
}

首先需要确保一点，Root是否已经处理过调度相关工作，通过 ensureRootIsScheduled 方法为root创建调度任务，且一个root只有一个task，假如某个root已经存在了任务，换言之已经调度过，那么我们需要重新为这个task计算一些值。而后同样有一个 schedulePendingInteractions ，用来处理交互引起的更新，方式与上面提到的 pending interaction 类似。

另外，如果executionContext 为NoContext ，则需要刷新用于处理同步更新的回调队列 flushSyncCallbackQueue ，该方法定义在 SchedulerWithReactIntegration.js 中。

如此，周而复始，完成更新的调度过程，最终调用 performSyncWorkOnRoot ，进入下一阶段,相关参考视频讲解：进入学习

performSyncWorkOnRoot

同样的选择题，当前是否能直接去提交更新，yes or no ？

if (root.finishedExpirationTime === expirationTime) {
  // There's already a pending commit at this expiration time.
  // TODO: This is poorly factored. This case only exists for the
  // batch.commit() API.
  commitRoot(root);
}

这种情况是很少的，一般会进入这个判断的else，也就是

...
workLoopSync();
...

function workLoopSync() {
  // Already timed out, so perform work without checking if we need to yield.
  while (workInProgress !== null) {
    workInProgress = performUnitOfWork(workInProgress);
  }
}

又开始了遍历，这个遍历中同样有我们上节分析过一些技巧，比如unitOfWork.alternate 用于节点属性的对比与暂存

function performUnitOfWork(unitOfWork: Fiber): Fiber | null {
  // The current, flushed, state of this fiber is the alternate. Ideally
  // nothing should rely on this, but relying on it here means that we don't
  // need an additional field on the work in progress.
  const current = unitOfWork.alternate;

  startWorkTimer(unitOfWork);
  setCurrentDebugFiberInDEV(unitOfWork);

  let next;
  if (enableProfilerTimer && (unitOfWork.mode & ProfileMode) !== NoMode) {
    startProfilerTimer(unitOfWork);
    next = beginWork(current, unitOfWork, renderExpirationTime);
    stopProfilerTimerIfRunningAndRecordDelta(unitOfWork, true);
  } else {
    next = beginWork(current, unitOfWork, renderExpirationTime);
  }

  resetCurrentDebugFiberInDEV();
  unitOfWork.memoizedProps = unitOfWork.pendingProps;
  if (next === null) {
    // If this doesn't spawn new work, complete the current work.
    next = completeUnitOfWork(unitOfWork);
  }

  ReactCurrentOwner.current = null;
  return next;
}

可以看到执行完相关操作后，随着 beginWork 函数的调用正式进入更新阶段。

beginWork

该部分主要的工作就是更新，更新什么呢？我们第一节讲到 React 不同的组件使用?typeof 指定，针对这些不同类型的组件，定义了各自的处理方法，我们以常用的 ClassComponent 为例。

function beginWork(
  current: Fiber | null,  workInProgress: Fiber,  renderExpirationTime: ExpirationTime,
): Fiber | null {
  const updateExpirationTime = workInProgress.expirationTime;
  ...

而后首先判断当前的更新节点是否为空，若不为空，则执行相关逻辑

...
if (current !== null) {
    const oldProps = current.memoizedProps;
    const newProps = workInProgress.pendingProps;

    if (
      oldProps !== newProps ||
      hasLegacyContextChanged() ||
      // Force a re-render if the implementation changed due to hot reload:
      (__DEV__ ? workInProgress.type !== current.type : false)
    ) {
      // If props or context changed, mark the fiber as having performed work.
      // This may be unset if the props are determined to be equal later (memo).
      didReceiveUpdate = true;
    } else if (updateExpirationTime < renderExpirationTime) {
      didReceiveUpdate = false;
      ...

此刻略知一二，前后props是否发生更改？根据不同的条件判断为 didReceiveUpdate 赋值。而后根据当前 workInProgress 的tag值判断当前的节点对应组件类型是什么，根据不同类型，进入不同方法进行处理。

switch (workInProgress.tag) {
    ...
}

而后，同样根据该tag，执行更新组件逻辑

case ClassComponent: {
  const Component = workInProgress.type;
  const unresolvedProps = workInProgress.pendingProps;
  const resolvedProps =
        workInProgress.elementType === Component
  ? unresolvedProps
  : resolveDefaultProps(Component, unresolvedProps);
  return updateClassComponent(
    current,
    workInProgress,
    Component,
    resolvedProps,
    renderExpirationTime,
  );
}

reconcileChildren

更新组件过程中，如果还有子节点，需要调度并更新

export function reconcileChildren(
  current: Fiber | null,
  workInProgress: Fiber,
  nextChildren: any,
  renderExpirationTime: ExpirationTime,
) {
  if (current === null) {
    // If this is a fresh new component that hasn't been rendered yet, we
    // won't update its child set by applying minimal side-effects. Instead,
    // we will add them all to the child before it gets rendered. That means
    // we can optimize this reconciliation pass by not tracking side-effects.
    workInProgress.child = mountChildFibers(
      workInProgress,
      null,
      nextChildren,
      renderExpirationTime,
    );
  } else {
    // If the current child is the same as the work in progress, it means that
    // we haven't yet started any work on these children. Therefore, we use
    // the clone algorithm to create a copy of all the current children.

    // If we had any progressed work already, that is invalid at this point so
    // let's throw it out.
    workInProgress.child = reconcileChildFibers(
      workInProgress,
      current.child,
      nextChildren,
      renderExpirationTime,
    );
  }
}

其子节点的 Fiber 调度定义在 ReactChildFiber.js 中，这里不展开了。

commitRoot

轮回中完成以上调度过程，也该到了提交更新的时候了，该方法我们在刚开始就讲到了，那时略过，现在拾起。

function commitRoot(root) {
  const renderPriorityLevel = getCurrentPriorityLevel();
  runWithPriority(
    ImmediatePriority,
    commitRootImpl.bind(null, root, renderPriorityLevel),
  );
  return null;
}

具体的实现在 commitRootImpl 方法中，该方法调用 prepareForCommit 为更新做准备，最终根据更新的类型不同使用不同策略进行更新

let primaryEffectTag =
      effectTag & (Placement | Update | Deletion | Hydrating);
switch (primaryEffectTag) {
    case Placement: {
    commitPlacement(nextEffect);
      // Clear the "placement" from effect tag so that we know that this is
      // inserted, before any life-cycles like componentDidMount gets called.
      // TODO: findDOMNode doesn't rely on this any more but isMounted does
      // and isMounted is deprecated anyway so we should be able to kill this.
      nextEffect.effectTag &= ~Placement;
      break;
    }
  case PlacementAndUpdate: {
    // Placement
    commitPlacement(nextEffect);
    // Clear the "placement" from effect tag so that we know that this is
    // inserted, before any life-cycles like componentDidMount gets called.
    nextEffect.effectTag &= ~Placement;

    // Update
    const current = nextEffect.alternate;
    commitWork(current, nextEffect);
    break;
  }
  case Hydrating: {
    nextEffect.effectTag &= ~Hydrating;
    break;
  }
  case HydratingAndUpdate: {
    nextEffect.effectTag &= ~Hydrating;

    // Update
    const current = nextEffect.alternate;
    commitWork(current, nextEffect);
    break;
  }
  case Update: {
    const current = nextEffect.alternate;
    commitWork(current, nextEffect);
    break;
  }
  case Deletion: {
    commitDeletion(root, nextEffect, renderPriorityLevel);
    break;
  }
}

提交更新相关的处理定义于 ReactFiberCommitWork.js 同样也要借助 tag，做不同策略的处理。

至此完成了任务调度的所有工作，当然在后面的过程，事件相关的处理是只字未提，React最新源码对于事件系统做了很大改动，我们放在后面章节详细讲解。React 源码设计之精妙无法言尽，并且只是略读，完成本系列的粗略讲解后，后续会有更深入源码讲解。读源码为了什么？

1. 理解我们每天使用的框架工作原理
2. 学习作者NB的设计和对于代码极致的追求，运用到自己的项目中