//主要代码
//第一步
abstract class State<T extends StatefulWidget> with Diagnosticable {
  void setState(VoidCallback fn) {
    final Object? result = fn() as dynamic;
    _element!.markNeedsBuild();
  }
}

//主要代码
//第二步

abstract class Element extends DiagnosticableTree implements BuildContext  {
 BuildOwner? get owner => _owner;
  BuildOwner? _owner;
  void markNeedsBuild() {
  
    if (dirty) {
      return;
    }
    _dirty = true;
    owner!.scheduleBuildFor(this);
  }

  
}

//第三步 BuildOwner 是在WidgetsBinding初始化创建的
class BuildOwner {
  VoidCallback? onBuildScheduled;
  /// Adds an element to the dirty elements list so that it will be rebuilt
  /// when [WidgetsBinding.drawFrame] calls [buildScope].
  void scheduleBuildFor(Element element) {
    if (!_scheduledFlushDirtyElements && onBuildScheduled != null) {
      _scheduledFlushDirtyElements = true;
      onBuildScheduled!();
    }
    _dirtyElements.add(element);
    element._inDirtyList = true;
  }
}

//第四步
mixin WidgetsBinding on BindingBase, ServicesBinding, SchedulerBinding, GestureBinding, RendererBinding, SemanticsBinding {
  @override
  void initInstances() {
    super.initInstances();
    _instance = this;
    _buildOwner = BuildOwner();
    buildOwner!.onBuildScheduled = _handleBuildScheduled;
  }

  void _handleBuildScheduled() {
    ensureVisualUpdate();
  }
mixin SchedulerBinding on BindingBase {
  void ensureVisualUpdate() {
 scheduleFrame(); 
  }
 void scheduleFrame() {

    ensureFrameCallbacksRegistered();//确保帧回调已注册
    platformDispatcher.scheduleFrame();
   
  }
}
  void ensureFrameCallbacksRegistered() {
    platformDispatcher.onBeginFrame ??= _handleBeginFrame;
    platformDispatcher.onDrawFrame ??= _handleDrawFrame;
  }
最终通过widow.scheduleFrame来注册Vsync回调，等带下一个Vsync到来的时候执行handleBeginFrame和handleDrawFrame更新UI