九九久久视屏,国产麻豆一区二区精选

Flutter系列學(xué)習(xí)筆記

前言

前兩篇文章Flutter筆記——runApp發(fā)生了什么(源碼學(xué)習(xí))和Flutter筆記——State.setState發(fā)生了什么學(xué)習(xí)了Flutter中runApp()、修改UI元素State.setState()過程。
這篇文章主要學(xué)習(xí)的是Flutter中實(shí)際渲染UI的過程。

1 BaseBinding

BaseBinding系列是FlutterFramework的核心類，學(xué)習(xí)Flutter的UI渲染過程會(huì)涉及到WidgetsBinding、RenderBinding、SchedulerBinding等。由于Dart的mixIn菱形繼承語法，該部分比較難搞明白，只能從局部入手，抽絲剝繭般的去學(xué)習(xí)理解整體流程。

1.1 handleDrawFrame

在我的Flutter筆記——runApp發(fā)生了什么(源碼學(xué)習(xí))文章中，了解到WidgetsFlutterBinding.scheduleWarmUpFrame()函數(shù)用于調(diào)度展示一個(gè)預(yù)熱幀。而WidgetsFlutterBinding.scheduleAttachRootWidget(Widget rootWidget)函數(shù)使用Timer包裹，作為一個(gè)異步執(zhí)行函數(shù)，在它執(zhí)行完畢之時(shí)最終會(huì)調(diào)用WidgetsBinding.handleDrawFrame()函數(shù)繪制幀。
那么handleDrawFrame()函數(shù)到底發(fā)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<FrameCallback> localPostFrameCallbacks =
          List<FrameCallback>.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;
    }
  }
}

首先學(xué)習(xí)WidgetsBinding類，見注釋

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]

簡單理解下，該類主要作用就是調(diào)度幀渲染任務(wù)，當(dāng)然也可以運(yùn)行非渲染任務(wù)。主要是瞬間渲染、持久渲染與渲染回調(diào)任務(wù)等，例如持久的幀渲染監(jiān)聽注冊(cè)WidgetsBinding.instance.addPersistentFrameCallback(callback)就是該類的作用了。
回到handleDrawFrame()函數(shù)，這里面循環(huán)執(zhí)行SchedulerBinding._persistentCallbacks與SchedulerBinding._postFrameCallbacks的注冊(cè)回調(diào)之外，好像沒做其他事情哦？那么線索斷了嗎？

事情并不簡單.jpg

1.2 initInstances

這里吐槽下mixIn菱形繼承，這個(gè)語法特性真的香嗎？

這里把眼光回到BaseBinding系列的初始化函數(shù)中，我們可以在RendererBinding.initInstances()函數(shù)中，找到SchedulerBinding.addPersistentFrameCallback(FrameCallback callback)函數(shù)的調(diào)用，這意味著在RendererBinding.initInstances()初始化階段，已經(jīng)注冊(cè)了一個(gè)關(guān)鍵函數(shù)，噔噔瞪，見下面源碼

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

我們可以看到，在SchedulerBinding._persistentCallbacks已經(jīng)注冊(cè)了drawFrame函數(shù)回調(diào)，到了這里handleDrawFrame渲染幀的線索又接上了，接著往下看。

1.3 drawFrame

drawFrame()函數(shù)有2處實(shí)現(xiàn)(有一處Test環(huán)境，忽略)，并且都被WidgetsFlutterBinding繼承，這個(gè)mixIn真的香嗎？

香嗎.jpg

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<FrameTiming> timings) {
        if (!kReleaseMode) {
          developer.Timeline.instantSync('Rasterized first useful frame');
          developer.postEvent('Flutter.FirstFrame', <String, dynamic>{});
        }
        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，我們知道drawFrame在每個(gè)handleDrawFrame函數(shù)中都會(huì)被調(diào)用，我們的WidgetsFlutterBinding繼承自RendererBinding和WidgetsBinding，見下圖的順序看看drawFrame到底發(fā)生了什么，再進(jìn)行源碼追蹤

drawFrame.png

過程比較復(fù)雜，源碼學(xué)習(xí)按照序列圖中的順序來

WidgetsBinding.drawFrame()：該函數(shù)在每一次handleDrawFrame都會(huì)被調(diào)用，并且還會(huì)調(diào)用super.drawFrame函數(shù)

///偽代碼
mixin WidgetsBinding ...{
  ///忽略斷言和調(diào)試部分代碼
  @override
  void drawFrame() {
    try {
      ///如果renderViewElement不為空，調(diào)用BuildOwner.buildScope函數(shù)，生成WidgetTree更新域
      if (renderViewElement != null){
        buildOwner.buildScope(renderViewElement);
      }
      //調(diào)用RenderBinding.drawFrame函數(shù)
      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)：這里的renderViewElement是一個(gè)RenderObjectToWidgetElement<RenderBox>對(duì)象，在runApp(Widget app)函數(shù)中被初始化，不了解的請(qǐng)看我的這篇文章Flutter筆記——runApp發(fā)生了什么(源碼學(xué)習(xí))。
buildOwner.buildScope(renderViewElement)函數(shù)的作用是建立WidgetTree構(gòu)建的域。

  ///刪除斷言和callback相關(guān)代碼
  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，根據(jù)Element中的depth值，depth值是當(dāng)期Element所在樹的層次整數(shù)。每個(gè)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()：遍歷_dirtyElements容器中的元素，調(diào)用它們的rebuild()函數(shù)。
element.rebuild()：這里以ComponentElement作為示例，rebuild()函數(shù)源碼如下
```
  void rebuild() {
    ///刪除很多斷言和其他代碼
    performRebuild();
  }
```

ComponentElement.performRebuild()：在這里我們可以看到performRebuild()函數(shù)會(huì)調(diào)用Element中的build()函數(shù)，這對(duì)于我們應(yīng)該是最熟悉的Flutter代碼之一了。這里面的built = build()有幾個(gè)繼承，StatefulWidget通過createState()函數(shù)生成State，再通過State的build():Widget函數(shù)生成Widget。

  @override
  void performRebuild() {
    ///刪除很多斷言和其他代碼
    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對(duì)象，這里面有三個(gè)參數(shù)，第一個(gè)是之前的Widget對(duì)象，也就是類對(duì)象child。第二個(gè)是新生成的newWidget對(duì)象，由build()函數(shù)生成，第三個(gè)newSlot是父Element給與子Element的位置參數(shù)，如果slot位置發(fā)生了變化，即使child與newWidget相同，也會(huì)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)：根據(jù)給定的Widget和newSlot生成一個(gè)Element，該方法通常由updateChild()函數(shù)直接調(diào)用。如果該Widget生成Element已經(jīng)存在或者存在相同的GlobalKey將會(huì)復(fù)用。該函數(shù)還會(huì)調(diào)用Widget.canUpdate(Widget oldWidget, Widget newWidget)來比較Widget對(duì)象是否相同。
該部分源碼較長，在之后文章看是否記錄學(xué)習(xí)，這里知道其作用即可。
- 如果newWidget的key是GlobalKey，并且通過Element _retakeInactiveElement(GlobalKey key, Widget newWidget)能拿回來一個(gè)Element，那么在更新狀態(tài)與slot、配置之后便返回一個(gè)Element。
- 不能從key中拿回已有的Element，會(huì)調(diào)用Element newChild = newWidget.createElement()生成一個(gè)新的newChild，并掛載它newChild.mount(this, newSlot)并返回。
super.drawFrame()：也就是RenderBinding.drawFrame()函數(shù)，該函數(shù)涉及知識(shí)點(diǎn)較多，下篇文章學(xué)習(xí)。它主要涉及到了RenderObject、Rect、PipelineOwner等知識(shí)點(diǎn)。
buildOwner.finalizeTree():調(diào)用該函數(shù)來完成元素構(gòu)建。

2 小結(jié)

本篇文章從預(yù)熱幀WidgetsFlutterBinding.scheduleWarmUpFrame()函數(shù)入手，找到FlutterFramework渲染幀的過程函數(shù)handleDrawFrame()，再通過BaseBinding系列找到drawFrame()的持久監(jiān)聽與回調(diào)來學(xué)習(xí)幀繪制的部分內(nèi)容。
本文從Element的create與update中，也找到了State.setState時(shí)，有些UI元素沒有重繪的根本原因，也了解了key的作用。
BaseBinding中的WidgetsBinding、RenderBinding、SchedulerBinding等子類是FlutterFramework幀渲染的核心類。本文從drawFrame入手學(xué)習(xí)了部分內(nèi)容，另外BuildOwner全局管理類也要著重了解。
本文篇章有限，還有許多內(nèi)容沒有學(xué)習(xí)到，等下篇文章再著重學(xué)習(xí)RenderBinding.drawFrame()的作用，之后再做一個(gè)階段性總結(jié)。

謝謝閱讀，如有錯(cuò)誤勞煩指出糾正，十分感謝，新春快樂哦！

色偷偷精品伊人,欧洲久久精品,欧美综合婷婷骚逼,国产AV主播,国产最新探花在线,九色在线视频一区,伊人大交九欧美,1769亚洲,黄色成人av

Flutter筆記——幀繪制系列之一(源碼學(xué)習(xí))

Flutter筆記——幀繪制系列之一(源碼學(xué)習(xí))

Flutter系列學(xué)習(xí)筆記

前言

1 BaseBinding

1.1 handleDrawFrame

1.2 initInstances

1.3 drawFrame

2 小結(jié)

友情鏈接更多精彩內(nèi)容

色偷偷精品伊人,欧洲久久精品,欧美综合婷婷骚逼,国产AV主播,国产最新探花在线,九色在线视频一区,伊人大交九 欧美,1769亚洲,黄色成人av

Flutter筆記——幀繪制系列之一(源碼學(xué)習(xí))

Flutter系列學(xué)習(xí)筆記

前言

1 BaseBinding

1.1 handleDrawFrame

1.2 initInstances

1.3 drawFrame

2 小結(jié)

友情鏈接更多精彩內(nèi)容

色偷偷精品伊人,欧洲久久精品,欧美综合婷婷骚逼,国产AV主播,国产最新探花在线,九色在线视频一区,伊人大交九欧美,1769亚洲,黄色成人av