void initInstances() {
    super.initInstances();
    _instance = this;
    _pipelineOwner = PipelineOwner(
      onNeedVisualUpdate: ensureVisualUpdate,
      onSemanticsOwnerCreated: _handleSemanticsOwnerCreated,
      onSemanticsOwnerDisposed: _handleSemanticsOwnerDisposed,
    );
    platformDispatcher
      ..onMetricsChanged = handleMetricsChanged
      ..onTextScaleFactorChanged = handleTextScaleFactorChanged
      ..onPlatformBrightnessChanged = handlePlatformBrightnessChanged
      ..onSemanticsEnabledChanged = _handleSemanticsEnabledChanged
      ..onSemanticsAction = _handleSemanticsAction;
    initRenderView();
    _handleSemanticsEnabledChanged();
    assert(renderView != null);
    addPersistentFrameCallback(_handlePersistentFrameCallback);
    initMouseTracker();
    if (kIsWeb) {
      addPostFrameCallback(_handleWebFirstFrame);
    }
  }

  // 持有需要被布局的對象
  List<RenderObject> _nodesNeedingLayout = <RenderObject>[];
{
    非release包運行代碼忽略
    ...
    try {
      // 如果當前的節(jié)點需要合成
      while (_nodesNeedingLayout.isNotEmpty) {
        final List<RenderObject> dirtyNodes = _nodesNeedingLayout;
        _nodesNeedingLayout = <RenderObject>[];
        for (final RenderObject node in dirtyNodes..sort((RenderObject a, RenderObject b) => a.depth - b.depth)) {
          if (node._needsLayout && node.owner == this)
            node._layoutWithoutResize();
        }
      }
    } finally {
        ... 
    }
}

  void flushCompositingBits() {
    ...
    _nodesNeedingCompositingBitsUpdate.sort((RenderObject a, RenderObject b) => a.depth - b.depth);
    for (final RenderObject node in _nodesNeedingCompositingBitsUpdate) {
      if (node._needsCompositingBitsUpdate && node.owner == this)
        node._updateCompositingBits();
    }
    _nodesNeedingCompositingBitsUpdate.clear();
   ...
  }

  void flushPaint() {
    ...
    try {
      final List<RenderObject> dirtyNodes = _nodesNeedingPaint;
      _nodesNeedingPaint = <RenderObject>[];
      // Sort the dirty nodes in reverse order (deepest first).
      for (final RenderObject node in dirtyNodes..sort((RenderObject a, RenderObject b) => b.depth - a.depth)) {
        ...
        if (node._needsPaint && node.owner == this) {
          if (node._layerHandle.layer!.attached) {
            PaintingContext.repaintCompositedChild(node);
          } else {
            node._skippedPaintingOnLayer();
          }
        }
      }
      ...
    } finally {
      ...
    }
  }

  void initRenderView() {
    renderView = RenderView(configuration: createViewConfiguration(), window: window);
    // 準備第一幀啟動渲染通道. 這里只會調用一次.
    renderView.prepareInitialFrame();
  }

  void scheduleInitialPaint(ContainerLayer rootLayer) {
    _layerHandle.layer = rootLayer;
    owner!._nodesNeedingPaint.add(this);
  }