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)和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WidgetsBinding、RenderBinding、SchedulerBinding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._persistentCallbacks与SchedulerBinding._postFrameCallbacks的注册回调之外，好像没做其他事情哦？那么线索断了吗？

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？

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RendererBinding和WidgetsBinding，见下图的顺序看看drawFrame到底发生了什么，再进行源码追踪

过程比较复杂，源码学习按照序列图中的顺序来

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;
  }
}

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();
    }
  }

_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Element的depth值都大于ParentElement的depth值

  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;
  }

_dirtyElements[index].rebuild()：遍历_dirtyElements容器中的元素，调用它们的rebuild()函数。

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();
  }

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State的build():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);
    }
  }

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child与newWidget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);
  }

Element inflateWidget(Widget newWidget, dynamic newSlot)：根据给定的Widget和newSlot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。
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RenderObject、Rect、PipelineOwner等知识点。
buildOwner.finalizeTree():调用该函数来完成元素构建。

2 小结

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。
本文从Element的create与update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。
BaseBinding中的WidgetsBinding、RenderBinding、SchedulerBinding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。
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！