flutter UI渲染源码解析之UI Framework绘制（三）

UI Framework绘制

由上文我们知道由window.cc中Window::BeginFrame调用到了framework层onBeginFrame跟onDrawFrame
1.window.dart

  FrameCallback? get onBeginFrame => _onBeginFrame;
  FrameCallback? _onBeginFrame;
  Zone _onBeginFrameZone = Zone.root;
  set onBeginFrame(FrameCallback? callback) {
    _onBeginFrame = callback;
    _onBeginFrameZone = Zone.current;
  }

2.onBeginFrame赋值：binding.dart ensureFrameCallbacksRegistered

@protected
  void ensureFrameCallbacksRegistered() {
    window.onBeginFrame ??= _handleBeginFrame;
    window.onDrawFrame ??= _handleDrawFrame;
  }

3.binding.dart _handleBeginFrame

 void _handleBeginFrame(Duration rawTimeStamp) {
    if (_warmUpFrame) {
      assert(!_ignoreNextEngineDrawFrame);
      _ignoreNextEngineDrawFrame = true;
      return;
    }
    handleBeginFrame(rawTimeStamp);
  }

4.binding.dart handleBeginFrame

void handleBeginFrame(Duration? rawTimeStamp) {
    Timeline.startSync('Frame', arguments: timelineArgumentsIndicatingLandmarkEvent);
    _firstRawTimeStampInEpoch ??= rawTimeStamp;
    _currentFrameTimeStamp = _adjustForEpoch(rawTimeStamp ?? _lastRawTimeStamp);
    if (rawTimeStamp != null)
      _lastRawTimeStamp = rawTimeStamp;

    assert(() {
      _debugFrameNumber += 1;

      if (debugPrintBeginFrameBanner || debugPrintEndFrameBanner) {
        final StringBuffer frameTimeStampDescription = StringBuffer();
        if (rawTimeStamp != null) {
          _debugDescribeTimeStamp(_currentFrameTimeStamp!, frameTimeStampDescription);
        } else {
          frameTimeStampDescription.write('(warm-up frame)');
        }
        _debugBanner = '▄▄▄▄▄▄▄▄ Frame ${_debugFrameNumber.toString().padRight(7)}   ${frameTimeStampDescription.toString().padLeft(18)} ▄▄▄▄▄▄▄▄';
        if (debugPrintBeginFrameBanner)
          debugPrint(_debugBanner);
      }
      return true;
    }());

    assert(schedulerPhase == SchedulerPhase.idle);
    _hasScheduledFrame = false;
    try {
      // TRANSIENT FRAME CALLBACKS
      Timeline.startSync('Animate', arguments: timelineArgumentsIndicatingLandmarkEvent);
      _schedulerPhase = SchedulerPhase.transientCallbacks;
      //遍历_transientCallbacks
      final Map<int, _FrameCallbackEntry> callbacks = _transientCallbacks;
      _transientCallbacks = <int, _FrameCallbackEntry>{};
      callbacks.forEach((int id, _FrameCallbackEntry callbackEntry) {
        if (!_removedIds.contains(id))
          _invokeFrameCallback(callbackEntry.callback, _currentFrameTimeStamp!, callbackEntry.debugStack);
      });
      _removedIds.clear();
    } finally {
      _schedulerPhase = SchedulerPhase.midFrameMicrotasks;
    }
  }

该方法主要功能是遍历_transientCallbacks，执行相应的Animate操作，可通过scheduleFrameCallback()/cancelFrameCallbackWithId()来完成添加和删除成员，再来简单看看这两个方法。

int scheduleFrameCallback(FrameCallback callback, { bool rescheduling = false }) {
  //这里触发了scheduleFrame
    scheduleFrame();
    _nextFrameCallbackId += 1;
    _transientCallbacks[_nextFrameCallbackId] = _FrameCallbackEntry(callback, rescheduling: rescheduling);
    return _nextFrameCallbackId;
  }

  void cancelFrameCallbackWithId(int id) {
    assert(id > 0);
    _transientCallbacks.remove(id);
    _removedIds.add(id);
  }

5.binding.dart _handleDrawFrame

  void _handleDrawFrame() {
    if (_ignoreNextEngineDrawFrame) {
      _ignoreNextEngineDrawFrame = false;
      return;
    }
    handleDrawFrame();
  }

6.binding.dart handleDrawFrame

void handleDrawFrame() {
    assert(_schedulerPhase == SchedulerPhase.midFrameMicrotasks);
    Timeline.finishSync(); // end the "Animate" phase
    try {
      // PERSISTENT FRAME CALLBACKS
      _schedulerPhase = SchedulerPhase.persistentCallbacks;
      for (final FrameCallback callback in _persistentCallbacks)
        _invokeFrameCallback(callback, _currentFrameTimeStamp!);

      // POST-FRAME CALLBACKS
      _schedulerPhase = SchedulerPhase.postFrameCallbacks;
      final List<FrameCallback> localPostFrameCallbacks =
          List<FrameCallback>.from(_postFrameCallbacks);
      _postFrameCallbacks.clear();
      for (final FrameCallback callback in localPostFrameCallbacks)
        _invokeFrameCallback(callback, _currentFrameTimeStamp!);
    } finally {
      _schedulerPhase = SchedulerPhase.idle;
      Timeline.finishSync(); // end the Frame
      assert(() {
        if (debugPrintEndFrameBanner)
          debugPrint('▀' * _debugBanner!.length);
        _debugBanner = null;
        return true;
      }());
      _currentFrameTimeStamp = null;
    }
  }

1.遍历_persistentCallbacks，执行相应的回调方法，可通过addPersistentFrameCallback()注册，一旦注册后不可移除，后续每一次frame回调都会执行；
2.遍历_postFrameCallbacks，执行相应的回调方法，可通过addPostFrameCallback()注册，handleDrawFrame()执行完成后会清空_postFrameCallbacks内容。

7.这里我们主要看一下_persistentCallbacks,在flutter app启动过程，也就是执行runApp过程会有WidgetsFlutterBinding初始化过程，WidgetsBinding的initInstances()，根据mixin的顺序，可知此处的super.initInstances() 便是RendererBinding类。

mixin RendererBinding on BindingBase, ServicesBinding, SchedulerBinding, GestureBinding, SemanticsBinding, HitTestable {
  @override
  void initInstances() {
    super.initInstances();
    _instance = this;
    _pipelineOwner = PipelineOwner(
      onNeedVisualUpdate: ensureVisualUpdate,
      onSemanticsOwnerCreated: _handleSemanticsOwnerCreated,
      onSemanticsOwnerDisposed: _handleSemanticsOwnerDisposed,
    );
    window
      ..onMetricsChanged = handleMetricsChanged
      ..onTextScaleFactorChanged = handleTextScaleFactorChanged
      ..onPlatformBrightnessChanged = handlePlatformBrightnessChanged
      ..onSemanticsEnabledChanged = _handleSemanticsEnabledChanged
      ..onSemanticsAction = _handleSemanticsAction;
    initRenderView();
    _handleSemanticsEnabledChanged();
    assert(renderView != null);
    //在这里进行的赋值
    addPersistentFrameCallback(_handlePersistentFrameCallback);
    initMouseTracker();
  }
...
}

8.render/binding.dart _handlePersistentFrameCallback

void _handlePersistentFrameCallback(Duration timeStamp) {
    drawFrame();
    _mouseTracker.schedulePostFrameCheck();
  }

9.render/binding.dart drawFrame

  @protected
  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;
    }
  }

该方法由[handleDrawFrame]调用，当需要布置和绘制框架时，引擎会自动调用该方法。
每个框架都包含以下几个阶段：
1.动画阶段：在[Window.onBeginFrame]中注册的[handleBeginFrame]方法，以注册顺序调用在[scheduleFrameCallback]中注册的所有瞬态帧回调。这包括所有正在驱动[AnimationController]对象的[Ticker]实例，这意味着所有活动的[Animation]对象都在此刻打勾。
2.微任务：返回[handleBeginFrame]后，由瞬态帧回调安排的任何微任务都将运行。这通常包括完成此帧的[Ticker]和[AnimationController]的期货回调。
在[handleBeginFrame]之后，将调用向[Window.onDrawFrame]注册的[handleDrawFrame]，该调用将调用所有持久性帧回调，其中最引人注目的是此方法[drawFrame]，其过程如下：
3.布局阶段：对系统中所有脏的[RenderObject]进行布局（请参见[RenderObject.performLayout]）。有关将对象标记为布局不干净的更多详细信息，请参见[RenderObject.markNeedsLayout]。
4.合成位阶段：更新任何脏的[RenderObject]对象上的合成位。请参见[RenderObject.markNeedsCompositingBitsUpdate]。
5.绘制阶段：重新绘制系统中所有脏的[RenderObject]（请参阅[RenderObject.paint]）。这将生成[Layer]树。请参阅[RenderObject.markNeedsPaint]，以获取更多有关将对象标记为脏画的详细信息。
6.合成阶段：层树变成[场景]并发送到GPU。
7.语义阶段：系统中所有脏的[RenderObject]的语义都已更新。这将生成[SemanticsNode]树。有关将对象标记为语义脏的更多详细信息，请参见[RenderObject.markNeedsSemanticsUpdate]。有关步骤3-7的更多详细信息，请参见[PipelineOwner]。
8.完成阶段：在[drawFrame]返回之后，[handleDrawFrame]调用后框架回调（在[addPostFrameCallback]中注册）。一些绑定（例如[WidgetsBinding]）向此列表添加了额外的步骤（例如，请参见[WidgetsBinding.drawFrame]）。

10.object.dart flushLayout

void flushLayout() {
    if (!kReleaseMode) {
      Timeline.startSync('Layout', arguments: timelineArgumentsIndicatingLandmarkEvent);
    }
    assert(() {
      _debugDoingLayout = true;
      return true;
    }());
    try {
    //遍历所有的渲染对象
      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)) {
          //如果对象需要layout则重新layout
          if (node._needsLayout && node.owner == this)
            node._layoutWithoutResize();
        }
      }
    } finally {
      assert(() {
        _debugDoingLayout = false;
        return true;
      }());
      if (!kReleaseMode) {
        Timeline.finishSync();
      }
    }
  }

11.object.dart_layoutWithoutResize

 void _layoutWithoutResize() {
    assert(_relayoutBoundary == this);
    RenderObject debugPreviousActiveLayout;
    assert(!_debugMutationsLocked);
    assert(!_doingThisLayoutWithCallback);
    assert(_debugCanParentUseSize != null);
    assert(() {
      _debugMutationsLocked = true;
      _debugDoingThisLayout = true;
      debugPreviousActiveLayout = _debugActiveLayout;
      _debugActiveLayout = this;
      if (debugPrintLayouts)
        debugPrint('Laying out (without resize) $this');
      return true;
    }());
    try {
      //执行绘制操作，
      performLayout();
      //标记需要更新的
      markNeedsSemanticsUpdate();
    } catch (e, stack) {
      _debugReportException('performLayout', e, stack);
    }
    assert(() {
      _debugActiveLayout = debugPreviousActiveLayout;
      _debugDoingThisLayout = false;
      _debugMutationsLocked = false;
      return true;
    }());
    _needsLayout = false;
    //标记需要绘制的
    markNeedsPaint();
  }

12.object.dart``

void markNeedsSemanticsUpdate() {
    assert(!attached || !owner._debugDoingSemantics);
    if (!attached || owner._semanticsOwner == null) {
      _cachedSemanticsConfiguration = null;
      return;
    }

    // Dirty the semantics tree starting at `this` until we have reached a
    // RenderObject that is a semantics boundary. All semantics past this
    // RenderObject are still up-to date. Therefore, we will later only rebuild
    // the semantics subtree starting at the identified semantics boundary.

    final bool wasSemanticsBoundary = _semantics != null && _cachedSemanticsConfiguration?.isSemanticBoundary == true;
    _cachedSemanticsConfiguration = null;
    bool isEffectiveSemanticsBoundary = _semanticsConfiguration.isSemanticBoundary && wasSemanticsBoundary;
    RenderObject node = this;

    while (!isEffectiveSemanticsBoundary && node.parent is RenderObject) {
      if (node != this && node._needsSemanticsUpdate)
        break;
      node._needsSemanticsUpdate = true;

      node = node.parent as RenderObject;
      isEffectiveSemanticsBoundary = node._semanticsConfiguration.isSemanticBoundary;
      if (isEffectiveSemanticsBoundary && node._semantics == null) {
        return;
      }
    }
    if (node != this && _semantics != null && _needsSemanticsUpdate) {
      owner._nodesNeedingSemantics.remove(this);
    }
    if (!node._needsSemanticsUpdate) {
      node._needsSemanticsUpdate = true;
      if (owner != null) {
        assert(node._semanticsConfiguration.isSemanticBoundary || node.parent is! RenderObject);
        //将需要更新的node记录下来
        owner._nodesNeedingSemantics.add(node);
        owner.requestVisualUpdate();
      }
    }
  }

13.object.dart ``

void markNeedsPaint() {
    assert(owner == null || !owner.debugDoingPaint);
    if (_needsPaint)
      return;
    _needsPaint = true;
    if (isRepaintBoundary) {
      assert(() {
        if (debugPrintMarkNeedsPaintStacks)
          debugPrintStack(label: 'markNeedsPaint() called for $this');
        return true;
      }());
   
      assert(_layer is OffsetLayer);
      if (owner != null) {
        //将需要重绘的node记录下来
        owner._nodesNeedingPaint.add(this);
        owner.requestVisualUpdate();
      }
    } else if (parent is RenderObject) {
      final RenderObject parent = this.parent as RenderObject;
      parent.markNeedsPaint();
      assert(parent == this.parent);
    } else {
      assert(() {
        if (debugPrintMarkNeedsPaintStacks)
          debugPrintStack(label: 'markNeedsPaint() called for $this (root of render tree)');
        return true;
      }());
  
      if (owner != null)
        owner.requestVisualUpdate();
    }
  }

14.object.dartflushCompositingBits

 void flushCompositingBits() {
    if (!kReleaseMode) {
      Timeline.startSync('Compositing bits');
    }
    _nodesNeedingCompositingBitsUpdate.sort((RenderObject a, RenderObject b) => a.depth - b.depth);
    for (final RenderObject node in _nodesNeedingCompositingBitsUpdate) {
      //node _needsCompositingBitsUpdate=true时进行合成操作
      if (node._needsCompositingBitsUpdate && node.owner == this)
        node._updateCompositingBits();
    }
    _nodesNeedingCompositingBitsUpdate.clear();
    if (!kReleaseMode) {
      Timeline.finishSync();
    }
  }

15.object.dart_updateCompositingBits:更新合成位，根据子view来判断当前的view是否需要标记paint

  void _updateCompositingBits() {
    if (!_needsCompositingBitsUpdate)
      return;
    final bool oldNeedsCompositing = _needsCompositing;
    _needsCompositing = false;
    visitChildren((RenderObject child) {
    //遍历子view若子view中有需要更新的，那么当前的view也需要更新
      child._updateCompositingBits();
      if (child.needsCompositing)
        _needsCompositing = true;
    });
    if (isRepaintBoundary || alwaysNeedsCompositing)
      _needsCompositing = true;
    if (oldNeedsCompositing != _needsCompositing)
    //若子view有更新的标记当前的view需要绘制
      markNeedsPaint();
    _needsCompositingBitsUpdate = false;
  }

16.object.dartflushPaint:绘制所有的renderobj

void flushPaint() {
    if (!kReleaseMode) {
      Timeline.startSync('Paint', arguments: timelineArgumentsIndicatingLandmarkEvent);
    }
    assert(() {
      _debugDoingPaint = true;
      return true;
    }());
    try {
      final List<RenderObject> dirtyNodes = _nodesNeedingPaint;
      _nodesNeedingPaint = <RenderObject>[];
      // 排序脏节点，深度最大的节点排在第一位
      for (final RenderObject node in dirtyNodes..sort((RenderObject a, RenderObject b) => b.depth - a.depth)) {
        assert(node._layer != null);
        if (node._needsPaint && node.owner == this) {
          //判断当前的节点是否在链接树上，在：重绘；不在：跳过
          if (node._layer.attached) {
            PaintingContext.repaintCompositedChild(node);
          } else {
            node._skippedPaintingOnLayer();
          }
        }
      }
      assert(_nodesNeedingPaint.isEmpty);
    } finally {
      assert(() {
        _debugDoingPaint = false;
        return true;
      }());
      if (!kReleaseMode) {
        Timeline.finishSync();
      }
    }
  }

17.object.dart repaintCompositedChild

  static void repaintCompositedChild(RenderObject child, { bool debugAlsoPaintedParent = false }) {
    assert(child._needsPaint);
    _repaintCompositedChild(
      child,
      debugAlsoPaintedParent: debugAlsoPaintedParent,
    );
  }

 static void _repaintCompositedChild(
    RenderObject child, {
    bool debugAlsoPaintedParent = false,
    PaintingContext childContext,
  }) {
    assert(child.isRepaintBoundary);
    assert(() {
      // register the call for RepaintBoundary metrics
      child.debugRegisterRepaintBoundaryPaint(
        includedParent: debugAlsoPaintedParent,
        includedChild: true,
      );
      return true;
    }());
    OffsetLayer childLayer = child._layer as OffsetLayer;
    if (childLayer == null) {
      assert(debugAlsoPaintedParent);
      child._layer = childLayer = OffsetLayer();
    } else {
      assert(childLayer is OffsetLayer);
      assert(debugAlsoPaintedParent || childLayer.attached);
      childLayer.removeAllChildren();
    }
    assert(identical(childLayer, child._layer));
    assert(child._layer is OffsetLayer);
    assert(() {
      child._layer.debugCreator = child.debugCreator ?? child.runtimeType;
      return true;
    }());
    childContext ??= PaintingContext(child._layer, child.paintBounds);
    //执行绘制操作
    child._paintWithContext(childContext, Offset.zero);

    assert(identical(childLayer, child._layer));
    childContext.stopRecordingIfNeeded();
  }

void _paintWithContext(PaintingContext context, Offset offset) {
    assert(() {
      if (_debugDoingThisPaint) {
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('Tried to paint a RenderObject reentrantly.'),
          describeForError(
            'The following RenderObject was already being painted when it was '
            'painted again'
          ),
          ErrorDescription(
            'Since this typically indicates an infinite recursion, it is '
            'disallowed.'
          ),
        ]);
      }
      return true;
    }());
    if (_needsLayout)
      return;
    assert(() {
      if (_needsCompositingBitsUpdate) {
        if (parent is RenderObject) {
          final RenderObject parent = this.parent as RenderObject;
          bool visitedByParent = false;
          parent.visitChildren((RenderObject child) {
            if (child == this) {
              visitedByParent = true;
            }
          });
          if (!visitedByParent) {
            throw FlutterError.fromParts(<DiagnosticsNode>[
              ErrorSummary(
                "A RenderObject was not visited by the parent's visitChildren "
                'during paint.',
              ),
              parent.describeForError(
                'The parent was',
              ),
              describeForError(
                'The child that was not visited was'
              ),
              ErrorDescription(
                'A RenderObject with children must implement visitChildren and '
                'call the visitor exactly once for each child; it also should not '
                'paint children that were removed with dropChild.'
              ),
              ErrorHint(
                'This usually indicates an error in the Flutter framework itself.'
              ),
            ]);
          }
        }
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary(
            'Tried to paint a RenderObject before its compositing bits were '
            'updated.'
          ),
          describeForError(
            'The following RenderObject was marked as having dirty compositing '
            'bits at the time that it was painted',
          ),
          ErrorDescription(
            'A RenderObject that still has dirty compositing bits cannot be '
            'painted because this indicates that the tree has not yet been '
            'properly configured for creating the layer tree.'
          ),
          ErrorHint(
            'This usually indicates an error in the Flutter framework itself.'
          ),
        ]);
      }
      return true;
    }());
    RenderObject debugLastActivePaint;
    assert(() {
      _debugDoingThisPaint = true;
      debugLastActivePaint = _debugActivePaint;
      _debugActivePaint = this;
      assert(!isRepaintBoundary || _layer != null);
      return true;
    }());
    _needsPaint = false;
    try {
    //执行paint的操作
      paint(context, offset);
      assert(!_needsLayout); // check that the paint() method didn't mark us dirty again
      assert(!_needsPaint); // check that the paint() method didn't mark us dirty again
    } catch (e, stack) {
      _debugReportException('paint', e, stack);
    }
    assert(() {
      debugPaint(context, offset);
      _debugActivePaint = debugLastActivePaint;
      _debugDoingThisPaint = false;
      return true;
    }());
  }

最终的调用跟layout一样都会执行到obejct的performLayout paint方法中，这些方法都会被子类进行重写，子类实现自己的具体layout以及paint

18.view.dart compositeFrame

void compositeFrame() {
    Timeline.startSync('Compositing', arguments: timelineArgumentsIndicatingLandmarkEvent);
    try {
      final ui.SceneBuilder builder = ui.SceneBuilder();
      final ui.Scene scene = layer.buildScene(builder);
      if (automaticSystemUiAdjustment)
        _updateSystemChrome();
      _window.render(scene);
      scene.dispose();
      assert(() {
        if (debugRepaintRainbowEnabled || debugRepaintTextRainbowEnabled)
          debugCurrentRepaintColor = debugCurrentRepaintColor.withHue((debugCurrentRepaintColor.hue + 2.0) % 360.0);
        return true;
      }());
    } finally {
      Timeline.finishSync();
    }
  }

分别创建Flutter框架(dart)和引擎层(C++)的两个SceneBuilder；
分别创建Flutter框架(dart)和引擎层(C++)的两个Scene；
执行render()将layer树发送给GPU线程；

19.SceneBuilder 的初始化

  @pragma('vm:entry-point')
  SceneBuilder() {
    _constructor();
  }
  void _constructor() native 'SceneBuilder_constructor';

此方法会被调用到C++SceneBuilder中即创建的是一个C++层的对象

20.layer.buildScene:

ui.Scene buildScene(ui.SceneBuilder builder) {
    List<PictureLayer> temporaryLayers;
    assert(() {
      if (debugCheckElevationsEnabled) {
        temporaryLayers = _debugCheckElevations();
      }
      return true;
    }());
  updateSubtreeNeedsAddToScene();  //遍历layer树，将需要子树加入到scene
  addToScene(builder); //将layer添加到SceneBuilder
    _needsAddToScene = false;
    final ui.Scene scene = builder.build();//调用C++层的build来构建Scene对象。
    assert(() {
      if (temporaryLayers != null) {
        for (final PictureLayer temporaryLayer in temporaryLayers) {
          temporaryLayer.remove();
        }
      }
      return true;
    }());
    return scene;
  }

遍历layer树，将需要更新的全部都加入到SceneBuilder。再调用build()，同样也是native方法，执行SceneBuilder::build()来构建Scene对象。

21.window.dartrender

void render(Scene scene) native 'PlatformConfiguration_render';

window.dart的render会调用到C++层window.cc中

22.window.cc ``

void Render(Dart_NativeArguments args) {
  UIDartState::ThrowIfUIOperationsProhibited();
  Dart_Handle exception = nullptr;
  //获得之前创建的scene
  Scene* scene =
      tonic::DartConverter<Scene*>::FromArguments(args, 1, exception);
  if (exception) {
    Dart_ThrowException(exception);
    return;
  }
  UIDartState::Current()->window()->client()->Render(scene);
}

23.runtime_controller.cc RuntimeController::Render

// |WindowClient|
void RuntimeController::Render(Scene* scene) {
  client_.Render(scene->takeLayerTree());
}

24.engine.cc Engine::Render

void Engine::Render(std::unique_ptr<flutter::LayerTree> layer_tree) {
  if (!layer_tree)
    return;

  // Ensure frame dimensions are sane.
  if (layer_tree->frame_size().isEmpty() ||
      layer_tree->frame_physical_depth() <= 0.0f ||
      layer_tree->frame_device_pixel_ratio() <= 0.0f)
    return;

  animator_->Render(std::move(layer_tree));
}

25.animator.cc

void Animator::Render(std::unique_ptr<flutter::LayerTree> layer_tree) {
  if (dimension_change_pending_ &&
      layer_tree->frame_size() != last_layer_tree_size_) {
    dimension_change_pending_ = false;
  }
  last_layer_tree_size_ = layer_tree->frame_size();
  if (layer_tree) {
    //将本次的绘制时间进行记录
    layer_tree->RecordBuildTime(last_frame_begin_time_,
                                last_frame_target_time_);
  }
  //提交continuation，此次pipeline produce 完成
  bool result = producer_continuation_.Complete(std::move(layer_tree));
  if (!result) {
    FML_DLOG(INFO) << "No pending continuation to commit";
  }
  delegate_.OnAnimatorDraw(layer_tree_pipeline_, last_frame_target_time_);
}

UI线程的耗时从doFrame(frameTimeNanos)中的frameTimeNanos为起点，以Animator::Render()方法结束为终点，并将结果保存到LayerTree的成员变量construction_time_，这便是UI线程的耗时时长。

26.shell.cc Shell::OnAnimatorDraw

// |Animator::Delegate|
void Shell::OnAnimatorDraw(fml::RefPtr<Pipeline<flutter::LayerTree>> pipeline,
                           fml::TimePoint frame_target_time) {
  FML_DCHECK(is_setup_);

  // record the target time for use by rasterizer.
  {
    std::scoped_lock time_recorder_lock(time_recorder_mutex_);
    if (!latest_frame_target_time_) {
      latest_frame_target_time_ = frame_target_time;
    } else if (latest_frame_target_time_ < frame_target_time) {
      latest_frame_target_time_ = frame_target_time;
    }
  }

  task_runners_.GetRasterTaskRunner()->PostTask(
      [&waiting_for_first_frame = waiting_for_first_frame_,
       &waiting_for_first_frame_condition = waiting_for_first_frame_condition_,
       rasterizer = rasterizer_->GetWeakPtr(),
       pipeline = std::move(pipeline)]() {
        if (rasterizer) {
          rasterizer->Draw(pipeline);

          if (waiting_for_first_frame.load()) {
            waiting_for_first_frame.store(false);
            waiting_for_first_frame_condition.notify_all();
          }
        }
      });
}

此方法主要是将当前的绘制内容交给Raster线程（GPU线程），绘制相关内容

27.object.cc flushSemantics

void flushSemantics() {
    if (_semanticsOwner == null)
      return;
    if (!kReleaseMode) {
      Timeline.startSync('Semantics');
    }
    assert(_semanticsOwner != null);
    assert(() {
      _debugDoingSemantics = true;
      return true;
    }());
    try {
      final List<RenderObject> nodesToProcess = _nodesNeedingSemantics.toList()
        ..sort((RenderObject a, RenderObject b) => a.depth - b.depth);
      _nodesNeedingSemantics.clear();
      //遍历_nodesNeedingSemantics，更新需要更新node的渲染对象
      for (final RenderObject node in nodesToProcess) {
        if (node._needsSemanticsUpdate && node.owner == this)
          node._updateSemantics();
      }
      _semanticsOwner.sendSemanticsUpdate();
    } finally {
      assert(_nodesNeedingSemantics.isEmpty);
      assert(() {
        _debugDoingSemantics = false;
        return true;
      }());
      if (!kReleaseMode) {
        Timeline.finishSync();
      }
    }
  }
}

28.object.dart _updateSemantics

 void _updateSemantics() {
    assert(_semanticsConfiguration.isSemanticBoundary || parent is! RenderObject);
    if (_needsLayout) {
     //此子树中没有足够的信息来计算语义，子树可能被视图窗口保持活着但没有布局
      return;
    }
    final _SemanticsFragment fragment = _getSemanticsForParent(
      mergeIntoParent: _semantics?.parent?.isPartOfNodeMerging ?? false,
    );
    assert(fragment is _InterestingSemanticsFragment);
    final _InterestingSemanticsFragment interestingFragment = fragment as _InterestingSemanticsFragment;
    final SemanticsNode node = interestingFragment.compileChildren(
      parentSemanticsClipRect: _semantics?.parentSemanticsClipRect,
      parentPaintClipRect: _semantics?.parentPaintClipRect,
      elevationAdjustment: _semantics?.elevationAdjustment ?? 0.0,
    ).single;
    // Fragment only wants to add this node's SemanticsNode to the parent.
    assert(interestingFragment.config == null && node == _semantics);
  }

29.semantics.dart sendSemanticsUpdate

void sendSemanticsUpdate() {
    if (_dirtyNodes.isEmpty)
      return;
    final Set<int> customSemanticsActionIds = <int>{};
    final List<SemanticsNode> visitedNodes = <SemanticsNode>[];
    while (_dirtyNodes.isNotEmpty) {
      final List<SemanticsNode> localDirtyNodes = _dirtyNodes.where((SemanticsNode node) => !_detachedNodes.contains(node)).toList();
      _dirtyNodes.clear();
      _detachedNodes.clear();
      localDirtyNodes.sort((SemanticsNode a, SemanticsNode b) => a.depth - b.depth);
      visitedNodes.addAll(localDirtyNodes);
      for (final SemanticsNode node in localDirtyNodes) {
        assert(node._dirty);
        assert(node.parent == null || !node.parent.isPartOfNodeMerging || node.isMergedIntoParent);
        if (node.isPartOfNodeMerging) {
          assert(node.mergeAllDescendantsIntoThisNode || node.parent != null);
         //如果合并到到父节点，确保父节点已被添加到脏列表
          if (node.parent != null && node.parent.isPartOfNodeMerging) {
            node.parent._markDirty(); // 将节点添加到脏列表
            node._dirty = false; // We don't want to send update for this node.
          }
        }
      }
    }
    visitedNodes.sort((SemanticsNode a, SemanticsNode b) => a.depth - b.depth);
    final ui.SemanticsUpdateBuilder builder = SemanticsBinding.instance.createSemanticsUpdateBuilder();
    for (final SemanticsNode node in visitedNodes) {
      assert(node.parent?._dirty != true); // could be null (no parent) or false (not dirty)
      // The _serialize() method marks the node as not dirty, and
      // recurses through the tree to do a deep serialization of all
      // contiguous dirty nodes. This means that when we return here,
      // it's quite possible that subsequent nodes are no longer
      // dirty. We skip these here.
      // We also skip any nodes that were reset and subsequently
      // dropped entirely (RenderObject.markNeedsSemanticsUpdate()
      // calls reset() on its SemanticsNode if onlyChanges isn't set,
      // which happens e.g. when the node is no longer contributing
      // semantics).
      if (node._dirty && node.attached)
        node._addToUpdate(builder, customSemanticsActionIds);
    }
    _dirtyNodes.clear();
    for (final int actionId in customSemanticsActionIds) {
      final CustomSemanticsAction action = CustomSemanticsAction.getAction(actionId);
      builder.updateCustomAction(id: actionId, label: action.label, hint: action.hint, overrideId: action.action?.index ?? -1);
    }
    SemanticsBinding.instance.window.updateSemantics(builder.build());
    notifyListeners();//通知已注册监听器
  }

30.updateSemantics这是window.dart中的一个native方法，调用到如下方法。void updateSemantics(SemanticsUpdate update) native 'PlatformConfiguration_updateSemantics';

void UpdateSemantics(Dart_NativeArguments args) {
  UIDartState::ThrowIfUIOperationsProhibited();
  Dart_Handle exception = nullptr;
  SemanticsUpdate* update =
      tonic::DartConverter<SemanticsUpdate*>::FromArguments(args, 1, exception);
  if (exception) {
    Dart_ThrowException(exception);
    return;
  }
  UIDartState::Current()->window()->client()->UpdateSemantics(update);
}

31.runtime_contorller.cc RuntimeController::UpdateSemantics

void RuntimeController::UpdateSemantics(SemanticsUpdate* update) {
  if (window_data_.semantics_enabled) {
    client_.UpdateSemantics(update->takeNodes(), update->takeActions());
  }
}

32.engine.cc Engine::UpdateSemantics

void Engine::UpdateSemantics(SemanticsNodeUpdates update,
                             CustomAccessibilityActionUpdates actions) {
  delegate_.OnEngineUpdateSemantics(std::move(update), std::move(actions));
}

32.shell.ccShell::OnEngineHandlePlatformMessage

void Shell::OnEngineHandlePlatformMessage(
    fml::RefPtr<PlatformMessage> message) {
  FML_DCHECK(is_setup_);
  FML_DCHECK(task_runners_.GetUITaskRunner()->RunsTasksOnCurrentThread());

  if (message->channel() == kSkiaChannel) {
    HandleEngineSkiaMessage(std::move(message));
    return;
  }

  task_runners_.GetPlatformTaskRunner()->PostTask(
      [view = platform_view_->GetWeakPtr(), message = std::move(message)]() {
        if (view) {
          view->HandlePlatformMessage(std::move(message));
        }
      });
}

该方法主要是向各自的平台线程中提交message。