前言
了解Flutter的同学应该都或多或少知道Flutter中的三棵树(Widget,Element,RenderObject),其中RenderObject负责绘制逻辑，RenderObject中的paint方法类似于Android中View的Draw方法
Flutter状态更新
Flutter中按有无状态更新可分为两类:StatelessWidget(无状态)和StatefulWidget(有状态)，StatefulWidget中创建一个State,在State内部调用setState,等到下一次Vsycn信号过来就会重建更新状态了。
Flutter的渲染流程
在Flutter三棵树中Widget和Element的节点是一一对应，而RenderObject是少于或等于Widget的数量的。当Widget是RenderObjectWidget的派生类的时候才有对应的RenderObject。Element和RenderObject在某些条件下是可以复用的，
Flutter渲染流程
渲染的耗时包括两部分
- 构建流水线任务
- 绘制逻辑
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从上图可以看出Flutter绘制一帧的任务会先构建三棵树然后再去绘制，因为Element复用的原因所以页面刷新的时候Widget和Element的生命方法并不会重复调用（解决构建耗时性能问题），但是在不使用RepaintBoundary的情况下RenderObject中的paint方法会被频繁调用,接下来我们学习一下Flutter是怎么提升绘制性能的。
RepaintBoundary
RepaintBoundary是集继承 SingleChildRenderObjectWidget,也属于RenderObjectWidget的派生类，所以RepaintBoundary也会有对应的RenderObject。
class RepaintBoundary extends SingleChildRenderObjectWidget {
const RepaintBoundary({ Key? key, Widget? child }) : super(key: key, child: child);
factory RepaintBoundary.wrap(Widget child, int childIndex) {
final Key key = child.key != null ? ValueKey<Key>(child.key!) : ValueKey<int>(childIndex);
return RepaintBoundary(key: key, child: child);
}
static List<RepaintBoundary> wrapAll(List<Widget> widgets) => <RepaintBoundary>[
for (int i = 0; i < widgets.length; ++i) RepaintBoundary.wrap(widgets[i], i),
];
@override
RenderRepaintBoundary createRenderObject(BuildContext context) => RenderRepaintBoundary();
}
RepaintBoundary中创建的RenderObject是RenderRepaintBoundary,下面是RenderRepaintBoundary的代码
class RenderRepaintBoundary extends RenderProxyBox {
RenderRepaintBoundary({ RenderBox? child }) : super(child);
//isRepaintBoundary默认是返回false，RenderRepaintBoundary中返回的是true
@override
bool get isRepaintBoundary => true;
//,,,省略无关代码
}
isRepaintBoundary在RenderObject中默认是返回false,RenderRepaintBoundary中返回的是true
RenderObject中isRepaintBoundary的作用
当RenderObject中isRepaintBoundary返回时true时当前节点的RenderObject(以及子节点)的绘制会在新创建Layer完成,这样就和其他Layer做了隔离，因为Layer是可以复用的，这样帧刷新的时候就不需要把每个RenderObject的paint方法都执行一遍。关于Layer的介绍可参考 初识Flutter中的Layer，下面我们是看看isRepaintBoundary返回true时是怎么创建Layer的。
核心代码如下:
void paintChild(RenderObject child, Offset offset) {
//1,isRepaintBoundary = true
if (child.isRepaintBoundary) {
//2,结束当前layer的绘制
stopRecordingIfNeeded();
//3,
_compositeChild(child, offset);
} else {
child._paintWithContext(this, offset);
}
}
//3,合成child
void _compositeChild(RenderObject child, Offset offset) {
// Create a layer for our child, and paint the child into it.
if (child._needsPaint) {
//4,如果child需要被绘制(_needsPaint=true代表当前节点或者当前节点子孩子被PipelineOwer标记出需要被重绘)
repaintCompositedChild(child, debugAlsoPaintedParent: true);
} else {
}
final OffsetLayer childOffsetLayer = child._layer! as OffsetLayer;
childOffsetLayer.offset = offset;
appendLayer(child._layer!);
}
//4,
static void repaintCompositedChild(RenderObject child, { bool debugAlsoPaintedParent = false }) {
//重新绘制
_repaintCompositedChild(
child,
debugAlsoPaintedParent: debugAlsoPaintedParent,
);
}
static void _repaintCompositedChild( RenderObject child, {bool debugAlsoPaintedParent = false,
PaintingContext? childContext,}) {
OffsetLayer? childLayer = child._layer as OffsetLayer?;
if (childLayer == null) {
child._layer = childLayer = OffsetLayer();
} else {
childLayer.removeAllChildren();
}
//创建新的PaintingContext，新的PaintingContext会创建新的PictureLayer
childContext ??= PaintingContext(child._layer!, child.paintBounds);
child._paintWithContext(childContext, Offset.zero);
childContext.stopRecordingIfNeeded();
}
流程图如下：
从上述流程可以看出当isRepaintBoundary=false时,就会触发paint的方法，我们假设下图所有RenderObject的isRepaintBoundary=false且其中RenderObject4被标记需要刷新
RenderObject4会自下而上寻找自己的父亲节点，直到找到父节点为isRepaintBoundary=true为止，然后把父节点依次标记需要刷新(_needsPaint = true)，如下图所示：
找到最顶层的父节点,然后执行paint方法，最终的结果就是遍历执行了所有RenderObject的paint方法，如下图所示：
如果RenderObject4的上一级父节点就是isRepaintBoundary=true，那么流程就如下
寻找父节点isRepaintBoundary=true
如果RenderObject4被标记需要刷新，RenderObject1和RenderObject4需要执行paint方法:
假如是RenderObject2,RenderObject3,RenderObject5,RenderObject6,RenderObject7中有一个需要刷新，右边标颜色的节点会执行paint方法
综上流程分析，假设场景是RenderObject4的绘制很耗但是是刷新不频繁，RenderObject5,RenderObject6,RenderObject7的刷新很频繁，我们使用RepaintBoundary对RenderObject4对应的Widget包一层这样可以缩短渲染时绘制阶段的耗时从而降低卡顿问题。
使用到RepaintBoundary的地方
在Flutter framework中的有些Widget就使用到RepaintBoundary了
Flowl
流式布局每个child都是独立的layer渲染
Flow({
Key? key,
required this.delegate,
List<Widget> children = const <Widget>[],
this.clipBehavior = Clip.hardEdge,
}) : assert(delegate != null),
assert(clipBehavior != null),
super(key: key, children: RepaintBoundary.wrapAll(children));
RepaintBoundary中的源码：
factory RepaintBoundary.wrap(Widget child, int childIndex) {
assert(child != null);
final Key key = child.key != null ? ValueKey<Key>(child.key!) : ValueKey<int>(childIndex);
return RepaintBoundary(key: key, child: child);
}
/// Wraps each of the given children in [RepaintBoundary]s.
///
/// The key for each [RepaintBoundary] is derived either from the wrapped
/// child's key (if the wrapped child has a non-null key) or from the wrapped
/// child's index in the list.
static List<RepaintBoundary> wrapAll(List<Widget> widgets) => <RepaintBoundary>[
for (int i = 0; i < widgets.length; ++i) RepaintBoundary.wrap(widgets[i], i),
];
SliverChildBuilderDelegate
SliverChildBuilderDelegate这个是ListView.builder的时候内部会创建SliverChildBuilderDelegate，列表大量item彼此之间独立layer渲染
@override
Widget? build(BuildContext context, int index) {
assert(builder != null);
if (addRepaintBoundaries)
child = RepaintBoundary(child: child);
if (addSemanticIndexes) {
final int? semanticIndex = semanticIndexCallback(child, index);
if (semanticIndex != null)
child = IndexedSemantics(index: semanticIndex + semanticIndexOffset, child: child);
}
if (addAutomaticKeepAlives)
child = AutomaticKeepAlive(child: child);
return KeyedSubtree(child: child, key: key);
}
总结
以上是对RepaintBoundary的作用分析,实则是对RenderObject的原理讲解。希望通过此篇文章帮助到大家提升的对Flutter渲染机制的认识。