Flutter笔记——帧绘制系列之一(源码学习)

Flutter系列学习笔记

前言

前两篇文章Flutter笔记——runApp发生了什么(源码学习)Flutter笔记——State.setState发生了什么学习了Flutter中runApp()、修改UI元素State.setState()过程。
这篇文章主要学习的是Flutter中实际渲染UI的过程。

1 BaseBinding

BaseBinding系列是FlutterFramework的核心类,学习Flutter的UI渲染过程会涉及到WidgetsBindingRenderBindingSchedulerBinding等。由于Dart的mixIn菱形继承语法,该部分比较难搞明白,只能从局部入手,抽丝剥茧般的去学习理解整体流程。

1.1 handleDrawFrame

在我的Flutter笔记——runApp发生了什么(源码学习)文章中,了解到WidgetsFlutterBinding.scheduleWarmUpFrame()函数用于调度展示一个预热帧。而WidgetsFlutterBinding.scheduleAttachRootWidget(Widget rootWidget)函数使用Timer包裹,作为一个异步执行函数,在它执行完毕之时最终会调用WidgetsBinding.handleDrawFrame()函数绘制帧。
那么handleDrawFrame()函数到底发生了什么?

mixin WidgetsBinding on BindingBase, ServicesBinding, SchedulerBinding, GestureBinding, RendererBinding, SemanticsBinding {
  void handleDrawFrame() {
    assert(_schedulerPhase == SchedulerPhase.midFrameMicrotasks);
    Timeline.finishSync(); // end the "Animate" phase
    try {
      // PERSISTENT FRAME CALLBACKS
      _schedulerPhase = SchedulerPhase.persistentCallbacks;
      for (FrameCallback callback in _persistentCallbacks)
        _invokeFrameCallback(callback, _currentFrameTimeStamp);

      // POST-FRAME CALLBACKS
      _schedulerPhase = SchedulerPhase.postFrameCallbacks;
      final List localPostFrameCallbacks =
          List.from(_postFrameCallbacks);
      _postFrameCallbacks.clear();
      for (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;
    }
  }
}

首先学习WidgetsBinding类,见注释

Scheduler for running the following:
- Transient callbacks, triggered by the system's [Window.onBeginFrame] callback, for synchronizing the application's behavior to the system's display. For example, [Ticker]s and [AnimationController]s trigger from these.  
- Persistent callbacks, triggered by the system's [Window.onDrawFrame] callback, for updating the system's display after transient callbacks have executed. For example, the rendering layer uses this to drive its rendering pipeline.  
- Post-frame callbacks, which are run after persistent callbacks, just before returning from the [Window.onDrawFrame] callback.  
- Non-rendering tasks, to be run between frames. These are given a priority and are executed in priority order according to a [schedulingStrategy]  

简单理解下,该类主要作用就是调度帧渲染任务,当然也可以运行非渲染任务。主要是瞬间渲染、持久渲染与渲染回调任务等,例如持久的帧渲染监听注册WidgetsBinding.instance.addPersistentFrameCallback(callback)就是该类的作用了。
回到handleDrawFrame()函数,这里面循环执行SchedulerBinding._persistentCallbacksSchedulerBinding._postFrameCallbacks的注册回调之外,好像没做其他事情哦?那么线索断了吗?
Flutter笔记——帧绘制系列之一(源码学习)_第1张图片

1.2 initInstances

这里吐槽下mixIn菱形继承,这个语法特性真的香吗?

这里把眼光回到BaseBinding系列的初始化函数中,我们可以在RendererBinding.initInstances()函数中,找到SchedulerBinding.addPersistentFrameCallback(FrameCallback callback)函数的调用,这意味着在RendererBinding.initInstances()初始化阶段,已经注册了一个关键函数,噔噔瞪,见下面源码

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);
    //重点
    addPersistentFrameCallback(_handlePersistentFrameCallback);
    initMouseTracker();
  }
  //重点
  void _handlePersistentFrameCallback(Duration timeStamp) {
    drawFrame();
  }
}

我们可以看到,在SchedulerBinding._persistentCallbacks已经注册了drawFrame函数回调,到了这里handleDrawFrame渲染帧的线索又接上了,接着往下看。

1.3 drawFrame

drawFrame()函数有2处实现(有一处Test环境,忽略),并且都被WidgetsFlutterBinding继承,这个mixIn真的香吗?
Flutter笔记——帧绘制系列之一(源码学习)_第2张图片

mixin RendererBinding on BindingBase, ServicesBinding, SchedulerBinding, GestureBinding, SemanticsBinding, HitTestable {
  void drawFrame() {
    assert(renderView != null);
    pipelineOwner.flushLayout();
    pipelineOwner.flushCompositingBits();
    pipelineOwner.flushPaint();
    renderView.compositeFrame(); // this sends the bits to the GPU
    pipelineOwner.flushSemantics(); // this also sends the semantics to the OS.
  }
}
mixin WidgetsBinding on BindingBase, ServicesBinding, SchedulerBinding, GestureBinding, RendererBinding, SemanticsBinding {
  @override
  void drawFrame() {
    assert(!debugBuildingDirtyElements);
    assert(() {
      debugBuildingDirtyElements = true;
      return true;
    }());

    if (_needToReportFirstFrame && _reportFirstFrame) {
      assert(!_firstFrameCompleter.isCompleted);

      TimingsCallback firstFrameCallback;
      firstFrameCallback = (List timings) {
        if (!kReleaseMode) {
          developer.Timeline.instantSync('Rasterized first useful frame');
          developer.postEvent('Flutter.FirstFrame', {});
        }
        SchedulerBinding.instance.removeTimingsCallback(firstFrameCallback);
        _firstFrameCompleter.complete();
      };
      SchedulerBinding.instance.addTimingsCallback(firstFrameCallback);
    }

    try {
      if (renderViewElement != null)
        buildOwner.buildScope(renderViewElement);
      super.drawFrame();
      buildOwner.finalizeTree();
    } finally {
      assert(() {
        debugBuildingDirtyElements = false;
        return true;
      }());
    }
    if (!kReleaseMode) {
      if (_needToReportFirstFrame && _reportFirstFrame) {
        developer.Timeline.instantSync('Widgets built first useful frame');
      }
    }
    _needToReportFirstFrame = false;
  }
}

在1.2中,我们知道drawFrame在每个handleDrawFrame函数中都会被调用,我们的WidgetsFlutterBinding继承自RendererBindingWidgetsBinding,见下图的顺序看看drawFrame到底发生了什么,再进行源码追踪
Flutter笔记——帧绘制系列之一(源码学习)_第3张图片

过程比较复杂,源码学习按照序列图中的顺序来

  1. WidgetsBinding.drawFrame()该函数在每一次handleDrawFrame都会被调用,并且还会调用super.drawFrame函数

    ///伪代码
    mixin WidgetsBinding ...{
      ///忽略断言和调试部分代码
      @override
      void drawFrame() {
        try {
          ///如果renderViewElement不为空,调用BuildOwner.buildScope函数,生成WidgetTree更新域
          if (renderViewElement != null){
            buildOwner.buildScope(renderViewElement);
          }
          //调用RenderBinding.drawFrame函数
          super.drawFrame();
          //
          buildOwner.finalizeTree();
        } finally {
          assert(() {
            debugBuildingDirtyElements = false;
            return true;
          }());
        }
        if (!kReleaseMode) {
          if (_needToReportFirstFrame && _reportFirstFrame) {
            developer.Timeline.instantSync('Widgets built first useful frame');
          }
        }
        _needToReportFirstFrame = false;
      }
    }
  2. buildOwner.buildScope(renderViewElement):这里的renderViewElement是一个RenderObjectToWidgetElement对象,在runApp(Widget app)函数中被初始化,不了解的请看我的这篇文章Flutter笔记——runApp发生了什么(源码学习)
    buildOwner.buildScope(renderViewElement)函数的作用是建立WidgetTree构建的域。

      ///删除断言和callback相关代码
      void buildScope(Element context, [ VoidCallback callback ]) {
        Timeline.startSync('Build', arguments: timelineWhitelistArguments);
        try{
          _dirtyElements.sort(Element._sort);
          _dirtyElementsNeedsResorting = false;
          int dirtyCount = _dirtyElements.length;
          int index = 0;
          while (index < dirtyCount) {
            try {
              _dirtyElements[index].rebuild();
            } catch (e, stack) {
              _debugReportException(
                ErrorDescription('while rebuilding dirty elements'),
                e,
                stack,
                informationCollector: () sync* {
                  yield DiagnosticsDebugCreator(DebugCreator(_dirtyElements[index]));
                  yield _dirtyElements[index].describeElement('The element being rebuilt at the time was index $index of $dirtyCount');
                },
              );
            }
            index += 1;
            if (dirtyCount < _dirtyElements.length || _dirtyElementsNeedsResorting) {
              _dirtyElements.sort(Element._sort);
              _dirtyElementsNeedsResorting = false;
              dirtyCount = _dirtyElements.length;
              while (index > 0 && _dirtyElements[index - 1].dirty) {
                index -= 1;
              }
            }
          }
        } finally {
          for (Element element in _dirtyElements) {
            element._inDirtyList = false;
          }
          _dirtyElements.clear();
          _scheduledFlushDirtyElements = false;
          _dirtyElementsNeedsResorting = null;
          Timeline.finishSync();
        }
      }
  3. _dirtyElements.sort(Element._sort):排列Element,根据Element中的depth值,depth值是当期Element所在树的层次整数。每个Elementdepth值都大于ParentElementdepth

      static int _sort(Element a, Element b) {
        if (a.depth < b.depth)
          return -1;
        if (b.depth < a.depth)
          return 1;
        if (b.dirty && !a.dirty)
          return -1;
        if (a.dirty && !b.dirty)
          return 1;
        return 0;
      }
  4. _dirtyElements[index].rebuild():遍历_dirtyElements容器中的元素,调用它们的rebuild()函数。
  5. element.rebuild():这里以ComponentElement作为示例,rebuild()函数源码如下

      void rebuild() {
        ///删除很多断言和其他代码
        performRebuild();
      }
  6. ComponentElement.performRebuild():在这里我们可以看到performRebuild()函数会调用Element中的build()函数,这对于我们应该是最熟悉的Flutter代码之一了。这里面的built = build()有几个继承,StatefulWidget通过createState()函数生成State,再通过Statebuild():Widget函数生成Widget。

      @override
      void performRebuild() {
        ///删除很多断言和其他代码
        Widget built;
        try {
          built = build();
          debugWidgetBuilderValue(widget, built);
        } catch (e, stack) {
          built = ErrorWidget.builder(
            _debugReportException(
              ErrorDescription('building $this'),
              e,
              stack,
              informationCollector: () sync* {
                yield DiagnosticsDebugCreator(DebugCreator(this));
              },
            ),
          );
        } finally {
          _dirty = false;
        }
        try {
          _child = updateChild(_child, built, slot);
          assert(_child != null);
        } catch (e, stack) {
          built = ErrorWidget.builder(
            _debugReportException(
              ErrorDescription('building $this'),
              e,
              stack,
              informationCollector: () sync* {
                yield DiagnosticsDebugCreator(DebugCreator(this));
              },
            ),
          );
          _child = updateChild(null, built, slot);
        }
      }
  7. updateChild(Element child, Widget newWidget, dynamic newSlot):更新Element中的Widget对象,这里面有三个参数,第一个是之前的Widget对象,也就是类对象child。第二个是新生成的newWidget对象,由build()函数生成,第三个newSlot是父Element给与子Element的位置参数,如果slot位置发生了变化,即使childnewWidget相同,也会重新渲染。

      @protected
      Element updateChild(Element child, Widget newWidget, dynamic newSlot) {
        if (newWidget == null) {
          if (child != null)
            deactivateChild(child);
          return null;
        }
        if (child != null) {
          if (child.widget == newWidget) {
            if (child.slot != newSlot)
              updateSlotForChild(child, newSlot);
            return child;
          }
          if (Widget.canUpdate(child.widget, newWidget)) {
            if (child.slot != newSlot)
              updateSlotForChild(child, newSlot);
            child.update(newWidget);
            assert(child.widget == newWidget);
            assert(() {
              child.owner._debugElementWasRebuilt(child);
              return true;
            }());
            return child;
          }
          deactivateChild(child);
          assert(child._parent == null);
        }
        return inflateWidget(newWidget, newSlot);
      }
  8. Element inflateWidget(Widget newWidget, dynamic newSlot)根据给定的WidgetnewSlot生成一个Element,该方法通常由updateChild()函数直接调用。如果该Widget生成Element已经存在或者存在相同的GlobalKey将会复用。该函数还会调用Widget.canUpdate(Widget oldWidget, Widget newWidget)来比较Widget对象是否相同。
    该部分源码较长,在之后文章看是否记录学习,这里知道其作用即可。

    • 如果newWidget的key是GlobalKey,并且通过Element _retakeInactiveElement(GlobalKey key, Widget newWidget)能拿回来一个Element,那么在更新状态与slot、配置之后便返回一个Element
    • 不能从key中拿回已有的Element,会调用Element newChild = newWidget.createElement()生成一个新的newChild,并挂载它newChild.mount(this, newSlot)并返回。
  9. super.drawFrame():也就是RenderBinding.drawFrame()函数,该函数涉及知识点较多,下篇文章学习。它主要涉及到了RenderObjectRectPipelineOwner等知识点。
  10. buildOwner.finalizeTree():调用该函数来完成元素构建。

2 小结

  1. 本篇文章从预热帧WidgetsFlutterBinding.scheduleWarmUpFrame()函数入手,找到FlutterFramework渲染帧的过程函数handleDrawFrame(),再通过BaseBinding系列找到drawFrame()的持久监听与回调来学习帧绘制的部分内容。
  2. 本文从Elementcreateupdate中,也找到了State.setState时,有些UI元素没有重绘的根本原因,也了解了key的作用。
  3. BaseBinding中的WidgetsBindingRenderBindingSchedulerBinding等子类是FlutterFramework帧渲染的核心类。本文从drawFrame入手学习了部分内容,另外BuildOwner全局管理类也要着重了解。
  4. 本文篇章有限,还有许多内容没有学习到,等下篇文章再着重学习RenderBinding.drawFrame()的作用,之后再做一个阶段性总结。

谢谢阅读,如有错误劳烦指出纠正,十分感谢,新春快乐哦!

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