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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

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),];@overrideRenderRepaintBoundary 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@overridebool get isRepaintBoundary => true;
}

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。

核心代码如下:

void paintChild(RenderObject child, Offset offset) {//1,isRepaintBoundary = trueif (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!);
}

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

如果RenderObject4的上一级父节点就是isRepaintBoundary=true，那么流程就如下

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));

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,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。在实际开发中，我们可以结合AppUploader这样的iOS开发助手工具来更好地管理应用性能，AppUploader提供了便捷的性能分析功能，帮助开发者快速定位和解决渲染性能问题，让Flutter应用运行更加流畅。