- state对象 调用setState方法后，会调用对应的element对象的markNeedsBuild方法，标记当前element为脏的，
- 调用BuildOwner对象的方法，把element对象 添加到脏列表中_dirtyElements；
注意：RendererBinding，注册了 SchedulerBinding对象的 addPersistentFrameCallback 永久帧回调，接收到回调方法时，调用 drawFrame() ，而RendererBinding和WidgetsBinding都有drawFrame方法；所以先调用WidgetsBinding方法 drawFrame()方法；
- 最终调用 buildOwner.buildScope()方法 ,遍历 _dirtyElements，调用element的rebuild() 触发widget的重新构建
关键代码如下:
//mixin RendererBinding{
void initInstances() {
addPersistentFrameCallback(_handlePersistentFrameCallback);
}
void _handlePersistentFrameCallback(Duration timeStamp) {
drawFrame();
//...
}
void drawFrame() {
assert(renderView != null);
pipelineOwner.flushLayout();
pipelineOwner.flushCompositingBits();
pipelineOwner.flushPaint();
if (sendFramesToEngine) {
renderView.compositeFrame(); // this sends the bits to the GPU
pipelineOwner.flushSemantics(); // this also sends the semantics to the OS.
_firstFrameSent = true;
}
}
}
//
mixin WidgetsBinding {
void drawFrame() {
//...
if (renderViewElement != null)
buildOwner.buildScope(renderViewElement);
super.drawFrame();
}
}
//
class BuildOwner {
void buildScope(Element context, [ VoidCallback callback ]) {
//遍历 _dirtyElements，调用element的rebuild()
try {
_dirtyElements[index].rebuild();
} catch (e, stack) {
}
｝
