Flutter AnimationController回调原理

每个动画帧会回调一次。

• 创建时，ticker的初始状态是禁用的。调用[start]启动。

• A [Ticker]可以通过将[muted]设置为true来进行沉默。在沉默状态下，时间仍然在流逝，[start]和[stop]仍然可以被调用，但是不会回调到Ticker中。

• 按照约定，[start]和[stop]方法由[ticker]的使用者使用，[muted]属性由[TickerProvider] 控制。

• Ticker回调是由[SchedulerBinding]驱动的。[SchedulerBinding.scheduleFrameCallback]。

**总结: **每次frame绘制之前会回调到ticker，由SchedulerBinding进行驱动

ScheduleBinding

/// 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].

调度运行时的这些回调

• Transient callbacks，由系统的[Window.onBeginFrame]回调，用于同步应用程序的行为到系统的展示。例如，[Ticker]s和[AnimationController]s触发器来自与它。

• Persistent callbacks 由系统的[Window.onDrawFrame]方法触发回调，用于在TransientCallback执行后更新系统的展示。例如，渲染层使用他来驱动渲染管道进行build，layout，paint

• _Post-frame callbacks_在下一帧绘制前回调，主要做一些清理和准备工作

• Non-rendering tasks 非渲染的任务，在帧构造之间，他们具有优先级，通过[schedulingStrategy]的优先级进行执行，例如用户的输入

**总结：**FrameCallback：SchedulerBinding 类中有三个FrameCallback回调队列， 在一次绘制过程中，这三个回调队列会放在不同时机被执行：

1. transientCallbacks：用于存放一些临时回调，一般存放动画回调。可以通过SchedulerBinding.instance.scheduleFrameCallback 添加回调。

2. persistentCallbacks：用于存放一些持久的回调，不能在此类回调中再请求新的绘制帧，持久回调一经注册则不能移除。SchedulerBinding.instance.addPersitentFrameCallback()，这个回调中处理了布局与绘制工作。

3. postFrameCallbacks：在Frame结束时只会被调用一次，调用后会被系统移除，可由 SchedulerBinding.instance.addPostFrameCallback() 注册，注意，不要在此类回调中再触发新的Frame，这可以会导致循环刷新。

/// Schedules the given transient frame callback.

///

/// Adds the given callback to the list of frame callbacks and ensures that a

/// frame is scheduled.

///

/// If this is a one-off registration, ignore the rescheduling argument.

///

/// If this is a callback that will be re-registered each time it fires, then

/// when you re-register the callback, set the rescheduling argument to

/// true. This has no effect in release builds, but in debug builds, it

/// ensures that the stack trace that is stored for this callback is the

/// original stack trace for when the callback was first registered, rather

/// than the stack trace for when the callback is re-registered. This makes it

/// easier to track down the original reason that a particular callback was

/// called. If rescheduling is true, the call must be in the context of a

/// frame callback.

///

/// Callbacks registered with this method can be canceled using

/// [cancelFrameCallbackWithId].

int scheduleFrameCallback(FrameCallback callback, { bool rescheduling = false }) {

scheduleFrame();

_nextFrameCallbackId += 1;

_transientCallbacks[_nextFrameCallbackId] = _FrameCallbackEntry(callback, rescheduling: rescheduling);

return _nextFrameCallbackId;

}

调度 transient frame callback队列

添加一个callBack，确保fram绘制之前能够回调到他

void scheduleFrame() {

if (_hasScheduledFrame || !_framesEnabled)

return;

assert(() {

if (debugPrintScheduleFrameStacks)

debugPrintStack(label: ‘scheduleFrame() called. Current phase is $schedulerPhase.’);

return true;

}());

ensureFrameCallbacksRegistered();

window.scheduleFrame();

_hasScheduledFrame = true;

}

@protected

void ensureFrameCallbacksRegistered() {

window.onBeginFrame ??= _handleBeginFrame;

window.onDrawFrame ??= _handleDrawFrame;

}

赋给了window.onBeginFram方法

/// Called by the engine to prepare the framework to produce a new frame.

///

/// This function calls all the transient frame callbacks registered by

/// [scheduleFrameCallback]. It then returns, any scheduled microtasks are run

/// (e.g. handlers for any [Future]s resolved by transient frame callbacks),

/// and [handleDrawFrame] is called to continue the frame.

///

/// If the given time stamp is null, the time stamp from the last frame is

/// reused.

///

/// To have a banner shown at the start of every frame in debug mode, set

/// [debugPrintBeginFrameBanner] to true. The banner will be printed to the

/// console using [debugPrint] and will contain the frame number (which

/// increments by one for each frame), and the time stamp of the frame. If the

/// given time stamp was null, then the string “warm-up frame” is shown

/// instead of the time stamp. This allows frames eagerly pushed by the

/// framework to be distinguished from those requested by the engine in

/// response to the “Vsync” signal from the operating system.

///

/// You can also show a banner at the end of every frame by setting

/// [debugPrintEndFrameBanner] to true. This allows you to distinguish log

/// statements printed during a frame from those printed between frames (e.g.

/// in response to events or timers).

void handleBeginFrame(Duration rawTimeStamp) {

Timeline.startSync(‘Frame’, arguments: timelineWhitelistArguments);

_firstRawTimeStampInEpoch ??= rawTimeStamp;

_currentFrameTimeStamp = _adjustForEpoch(rawTimeStamp ?? _lastRawTimeStamp);

if (rawTimeStamp != null)

_lastRawTimeStamp = rawTimeStamp;

_hasScheduledFrame = false;

try {

// TRANSIENT FRAME CALLBACKS

Timeline.startSync(‘Animate’, arguments: timelineWhitelistArguments);

_schedulerPhase = SchedulerPhase.transientCallbacks;

final Map<int, _FrameCallbackEntry> callbacks = _transientCallbacks;

//调用callBack******

_transientCallbacks = <int, _FrameCallbackEntry>{};

callbacks.forEach((int id, _FrameCallbackEntry callbackEntry) {

if (!_removedIds.contains(id))

_invokeFrameCallback(callbackEntry.callback, _currentFrameTimeStamp, callbackEntry.debugStack);

//**************************callback(timeStamp);

});

_removedIds.clear();

} finally {

_schedulerPhase = SchedulerPhase.midFrameMicrotasks;

}

}

• 当框架准备构建一帧的时候由engine调用，可以猜想endgine调用的实际上是window的onBegainFrame

• 它会回调[scheduleFrameCallback]中的所有方法（包括一些future的方法），当执行完成后，系统会调用[handleDrawFrame]

/// Signature for frame-related callbacks from the scheduler.

///

/// The timeStamp is the number of milliseconds since the beginning of the

/// scheduler’s epoch. Use timeStamp to determine how far to advance animation

/// timelines so that all the animations in the system are synchronized to a

/// common time base.

typedef FrameCallback = void Function(Duration timeStamp);

callback(timeStamp);

• “时间戳”是自scheduler开始以来的毫秒数。使用时间戳来确定动画时间线要前进多远，以便系统中的所有动画都同步到一通用的时间线上。

**总结：**当每次系统绘制的之前，会回调到ui.windows的onBegainFrame，而这个onBegainFrame执行的是handleBeginFrame，将时间值回调给了每一个callback。这里注意的是，是在在绘制之前，因为我们一般在绘制之前去通过改变控件的属性值完成动画，而这个动作必须在绘制前完成。

Ticker

反向搜索谁调用了scheduleFrameCallback，发现是在Ticker中的scheduleTick，而scheduleTick有几个地方调用后面来看

/// Schedules a tick for the next frame.

///

/// This should only be called if [shouldScheduleTick] is true.

@protected

void scheduleTick({ bool rescheduling = false }) {

assert(!scheduled);

assert(shouldScheduleTick);

_animationId = SchedulerBinding.instance.scheduleFrameCallback(_tick, rescheduling: rescheduling);

}

这里发现ticker传入了一个_tick对象到scheduleFrameCallback中

void _tick(Duration timeStamp) {

assert(isTicking);

assert(scheduled);

_animationId = null;

_startTime ??= timeStamp;

_onTick(timeStamp - _startTime);

// The onTick callback may have scheduled another tick already, for

// example by calling stop then start again.

if (shouldScheduleTick)

scheduleTick(rescheduling: true);

}

final TickerCallback _onTick;

Ticker(this._onTick, { this.debugLabel }) {

assert(() {

_debugCreationStack = StackTrace.current;

return true;

}());

}

这里发现，_ticke方法，将时间戳，转换为一个相对时间，回调的到onTick中。onTicker是Ticker构造时候创建的。而Ticker的创建其实主要只在TickProvider中

@override

Ticker createTicker(TickerCallback onTick) {

_ticker = Ticker(onTick, debugLabel: kDebugMode ? ‘created by $this’ : null);

return _ticker;

}

那谁调用了这个createTicker呢，其实自然能联想到是AnimationController

AnimationController({

double value,

this.duration,

this.reverseDuration,

this.debugLabel,

this.lowerBound = 0.0,

this.upperBound = 1.0,

this.animationBehavior = AnimationBehavior.normal,

@required TickerProvider vsync,

}) : assert(lowerBound != null),

assert(upperBound != null),

assert(upperBound >= lowerBound),

assert(vsync != null),

_direction = _AnimationDirection.forward {

_ticker = vsync.createTicker(_tick);

_internalSetValue(value ?? lowerBound);

}

void _tick(Duration elapsed) {

_lastElapsedDuration = elapsed;

final double elapsedInSeconds = elapsed.inMicroseconds.toDouble() / Duration.microsecondsPerSecond;

assert(elapsedInSeconds >= 0.0);

_value = _simulation.x(elapsedInSeconds).clamp(lowerBound, upperBound);

if (_simulation.isDone(elapsedInSeconds)) {

_status = (_direction == _AnimationDirection.forward) ?

AnimationStatus.completed :

AnimationStatus.dismissed;

stop(canceled: false);

}

///注意到了 notifyListeners()方法，果然AnimationController继承自Animation继承自Listenable。

notifyListeners();

_checkStatusChanged();

}
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