基于Flutter 1.22.3的源碼刨析，分析Flutter的StatefulWidget的UI更新機制，相關源碼:

  widgets/framework.dart
  widgets/binding.dart
  scheduler/binding.dart
  lib/ui/window.dart
  flutter/runtime/runtime_controller.cc

一、概述

對于Flutter來說，萬物皆為Widget，常見的Widget子類為StatelessWidget(無狀態)和StatefulWidget(有狀態)；

• StatelessWidget：內部沒有保存狀態，界面創建后不會發生改變；
• StatefulWidget：內部有保存狀態，當狀態發生改變，調用setState()方法會觸發StatefulWidget的UI發生更新，對于自定義繼承自StatefulWidget的子類，必須要重寫createState()方法。

接下來看看setState()究竟干了哪些操作。

二、Widget更新流程

2.1 setState

[-> framework.dart::state]

abstract class State<T extends StatefulWidget> with Diagnosticable {
StatefulElement _element;
  /// It is an error to call this method after the framework calls [dispose].
  /// You can determine whether it is legal to call this method by checking
  /// whether the [mounted] property is true.
  @protected
  void setState(VoidCallback fn) {
   ...
    _element.markNeedsBuild();
  }
}

這里需要注意setState()方法要在dispose()方法調用前調用,可以通過mounted屬性值來判斷父Widget是否還包含該Widget.

2.2 markNeedsBuild

[->framework.dart::Element]

abstract class Element extends DiagnosticableTree implements BuildContext {
  /// Marks the element as dirty and adds it to the global list of widgets to
  /// rebuild in the next frame.
  ///標記元素為臟元素，然后添加到list集合里 等下一幀刷新
    void markNeedsBuild() {
      if (!_active)
        return;
      if (dirty)
        return;
      _dirty = true;
      owner.scheduleBuildFor(this);
    }
}

設置 Element的 _dirty 為 true

2.3 scheduleBuildFor

[->framework.dart::BuildOwner]

abstract class Element extends DiagnosticableTree implements BuildContext {
  /// Adds an element to the dirty elements list so that it will be rebuilt
  /// when [WidgetsBinding.drawFrame] calls [buildScope].
 void scheduleBuildFor(Element element) {  
  ...
   if (element._inDirtyList) { //是否在集合里面
        _dirtyElementsNeedsResorting = true;
        return;
      }
      if (!_scheduledFlushDirtyElements && onBuildScheduled != null) {
        _scheduledFlushDirtyElements = true;
        onBuildScheduled();
      }
      _dirtyElements.add(element);//記錄所有臟元素
      element._inDirtyList = true;
  }
}

將element添加到臟element集合，之后會被重建

2.4 _handleBuildScheduled

[-> binding.dart:: WidgetsBinding]

/// The glue between the widgets layer and the Flutter engine.
mixin WidgetsBinding on BindingBase, ServicesBinding, SchedulerBinding, GestureBinding, RendererBinding, SemanticsBinding {
  @override
  void initInstances() {
    super.initInstances();
    _instance = this;
    // Initialization of [_buildOwner] has to be done after
    // [super.initInstances] is called, as it requires [ServicesBinding] to
    // properly setup the [defaultBinaryMessenger] instance.
    _buildOwner = BuildOwner();
    buildOwner.onBuildScheduled = _handleBuildScheduled;
    window.onLocaleChanged = handleLocaleChanged;
    window.onAccessibilityFeaturesChanged = handleAccessibilityFeaturesChanged;
    SystemChannels.navigation.setMethodCallHandler(_handleNavigationInvocation);
    FlutterErrorDetails.propertiesTransformers.add(transformDebugCreator);
  }

  void _handleBuildScheduled() {
    ensureVisualUpdate();
 }

在Flutter應用啟動過程初始化WidgetsBinding時，賦值onBuildScheduled等于_handleBuildScheduled()。

2.5 ensureVisualUpdate

[ -> binding.dart::SchedulerBinding]

mixin SchedulerBinding on BindingBase {
  @override
  void initInstances() {
    super.initInstances();
    _instance = this;
  }

  void ensureVisualUpdate() {
    switch (schedulerPhase) {
      case SchedulerPhase.idle:
      case SchedulerPhase.postFrameCallbacks:
        scheduleFrame();
        return;
      case SchedulerPhase.transientCallbacks:
      case SchedulerPhase.midFrameMicrotasks:
      case SchedulerPhase.persistentCallbacks:
        return;
    }
  }

schedulerPhase的初始值為SchedulerPhase.idle。SchedulerPhase是一個enum枚舉類型，有以下5個可取值：

2.6 scheduleFrame

   void scheduleFrame() {
    //只有當APP處于用戶可見狀態才會準備調度下一幀方法
    if (_hasScheduledFrame || !framesEnabled)
      return;
    ensureFrameCallbacksRegistered();
    window.scheduleFrame();
    _hasScheduledFrame = true;
  }

2.7 scheduleFrame

[ -> lib/ui/window.dart::Window]

  void scheduleFrame() native 'PlatformConfiguration_scheduleFrame';

window是Flutter引擎中跟圖形相關接口打交道的核心類，這里是一個native方法

2.7.1 ScheduleFrame(C++)

[->引擎庫 lib/ui/window/platform_configuration.cc]

  void ScheduleFrame(Dart_NativeArguments args) {
  UIDartState::ThrowIfUIOperationsProhibited();
  UIDartState::Current()->platform_configuration()->client()->ScheduleFrame();
}

通過RegisterNatives()完成native方法的注冊，“PlatformConfiguration_scheduleFrame”所對應的native方法如上所示。

2.7.2 RuntimeController::ScheduleFrame

[->runtime/runtime_controller.cc]

  // |PlatformConfigurationClient|
void RuntimeController::ScheduleFrame() {
  client_.ScheduleFrame();
}

2.7.3 Engine::ScheduleFrame

[->flutter/shell/common/engine.cc]

  void Engine::ScheduleFrame(bool regenerate_layer_tree) {
  animator_->RequestFrame(regenerate_layer_tree);
}

Engine::ScheduleFrame()經過層層調用，最終會注冊Vsync回調。 等待下一次vsync信號的到來，然后再經過層層調用最終會調用到Window::BeginFrame()。這里不展開解釋了。

2.8 Window::BeginFrame

[-> flutter/lib/ui/window/window.cc]

  void Window::BeginFrame(fml::TimePoint frameTime) {
  std::shared_ptr<tonic::DartState> dart_state = library_.dart_state().lock();
  if (!dart_state)
    return;
  tonic::DartState::Scope scope(dart_state);

  int64_t microseconds = (frameTime - fml::TimePoint()).ToMicroseconds();

  DartInvokeField(library_.value(), "_beginFrame",
                  {
                      Dart_NewInteger(microseconds),
                  });

  //執行MicroTask
  UIDartState::Current()->FlushMicrotasksNow();

  DartInvokeField(library_.value(), "_drawFrame", {});
}

Window::BeginFrame()過程主要工作：

• 執行_beginFrame
• 執行FlushMicrotasksNow
• 執行_drawFrame

可見，Microtask位于beginFrame和drawFrame之間，那么Microtask的耗時會影響ui繪制過程。

2.9handleBeginFrame

[-> lib/src/scheduler/binding.dart:: SchedulerBinding]

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

  profile(() {
    _profileFrameNumber += 1;
    _profileFrameStopwatch.reset();
    _profileFrameStopwatch.start();
  });

  //此時階段等于SchedulerPhase.idle;
  _hasScheduledFrame = false;
  try {
    Timeline.startSync('Animate', arguments: timelineWhitelistArguments);
    _schedulerPhase = SchedulerPhase.transientCallbacks;
    //執行動畫的回調方法
    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;
  }
}

該方法主要功能是遍歷_transientCallbacks，執行相應的Animate操作，可通過scheduleFrameCallback()/cancelFrameCallbackWithId()來完成添加和刪除成員，再來簡單看看這兩個方法。

2.10 handleDrawFrame

[-> lib/src/scheduler/binding.dart:: SchedulerBinding]

  void handleDrawFrame() {
  assert(_schedulerPhase == SchedulerPhase.midFrameMicrotasks);
  Timeline.finishSync(); // 標識結束"Animate"階段
  try {
    _schedulerPhase = SchedulerPhase.persistentCallbacks;
    //執行PERSISTENT FRAME回調
    for (FrameCallback callback in _persistentCallbacks)
      _invokeFrameCallback(callback, _currentFrameTimeStamp);

    _schedulerPhase = SchedulerPhase.postFrameCallbacks;
    // 執行POST-FRAME回調
    final List<FrameCallback> localPostFrameCallbacks = List<FrameCallback>.from(_postFrameCallbacks);
    _postFrameCallbacks.clear();
    for (FrameCallback callback in localPostFrameCallbacks)
      _invokeFrameCallback(callback, _currentFrameTimeStamp);
  } finally {
    _schedulerPhase = SchedulerPhase.idle;
    Timeline.finishSync(); //標識結束”Frame“階段
    profile(() {
      _profileFrameStopwatch.stop();
      _profileFramePostEvent();
    });
    _currentFrameTimeStamp = null;
  }
}

該方法主要功能：

• 遍歷_persistentCallbacks，執行相應的回調方法，可通過addPersistentFrameCallback()注冊，一旦注冊后不可移除，后續每一次frame回調都會執行；
• 遍歷_postFrameCallbacks，執行相應的回調方法，可通過addPostFrameCallback()注冊，handleDrawFrame()執行完成后會清空_postFrameCallbacks內容。

三、小結

可見，setState()過程主要工作是記錄所有的臟元素，添加到BuildOwner對象的_dirtyElements成員變量，然后調用scheduleFrame來注冊Vsync回調。 當下一次vsync信號的到來時會執行handleBeginFrame()和handleDrawFrame()來更新UI。

本文參考了gityan大佬的博客，感謝大佬分享！