深入理解Flutter动画原理

一、概述[](http://gityuan.com/2019/07/13/flutter_animator/#一概述)

动画效果对于系统的用户体验非常重要，好的动画能让用户感觉界面更加顺畅，提升用户体验。

1.1 动画类型[](http://gityuan.com/2019/07/13/flutter_animator/#11-动画类型)

Flutter动画大的分类来说主要分为两大类：

• 补间动画：给定初值与终值，系统自动补齐中间帧的动画
• 物理动画：遵循物理学定律的动画，实现了弹簧、阻尼、重力三种物理效果

在应用使用过程中常见动画模式：

• 动画列表或者网格：例如元素的添加或者删除操作；
• 转场动画Shared element transition：例如从当前页面打开另一页面的过渡动画；
• 交错动画Staggered animations：比如部分或者完全交错的动画。

1.2 类图[](http://gityuan.com/2019/07/13/flutter_animator/#12-类图)

核心类：

• Animation对象是整个动画中非常核心的一个类；
• AnimationController用于管理Animation；
• CurvedAnimation过程是非线性曲线；
• Tween补间动画
• Listeners和StatusListeners用于监听动画状态改变。

AnimationStatus是枚举类型，有4个值；

取值	解释
dismissed	动画在开始时停止
forward	动画从头到尾绘制
reverse	动画反向绘制，从尾到头
completed	动画在结束时停止

1.3 动画实例[](http://gityuan.com/2019/07/13/flutter_animator/#13-动画实例)

 //[见小节2.1] AnimationController animationController = AnimationController( vsync: this, duration: Duration(milliseconds: 1000)); Animation animation = Tween(begin: 0.0,end: 10.0).animate(animationController); animationController.addListener(() { setState(() {}); }); //[见小节2.2] animationController.forward(); 

该过程说明：

• AnimationController作为Animation子类，在屏幕刷新时生成一系列值，默认情况下从0到1区间的取值。
• Tween的animate()方法来自于父类Animatable，该方法返回的对象类型为_AnimatedEvaluation，而该对象最核心的工作就是通过value来调用Tween的transform()；

调用链：

AnimationController.forward AnimationController.\_animateToInternal AnimationController.\_startSimulation Ticker.start() Ticker.scheduleTick() SchedulerBinding.scheduleFrameCallback() SchedulerBinding.scheduleFrame() ... Ticker.\_tick AnimationController.\_tick Ticker.scheduleTick 

二、原理分析[](http://gityuan.com/2019/07/13/flutter_animator/#二原理分析)

2.1 AnimationController初始化[](http://gityuan.com/2019/07/13/flutter_animator/#21-animationcontroller初始化)

[-> lib/src/animation/animation_controller.dart]

AnimationController({ double value, this.duration, this.debugLabel, this.lowerBound = 0.0, this.upperBound = 1.0, this.animationBehavior = AnimationBehavior.normal, @required TickerProvider vsync, }) : _direction = _AnimationDirection.forward { _ticker = vsync.createTicker(_tick); //[见小节2.1.1] _internalSetValue(value ?? lowerBound); //[见小节2.1.3] } 

该方法说明：

• AnimationController初始化过程，一般都设置duration和vsync初值；
• upperBound(上边界值)和lowerBound(下边界值)都不能为空，且upperBound必须大于等于lowerBound；
• 创建默认的动画方向为向前(_AnimationDirection.forward)；
• 调用类型为TickerProvider的vsync对象的createTicker()方法来创建Ticker对象；

TickerProvider作为抽象类，主要的子类有SingleTickerProviderStateMixin和TickerProviderStateMixin，这两个类的区别就是是否支持创建多个TickerProvider，这里SingleTickerProviderStateMixin为例展开。

2.1.1 createTicker[](http://gityuan.com/2019/07/13/flutter_animator/#211-createticker)

[-> lib/src/widgets/ticker_provider.dart]

mixin SingleTickerProviderStateMixin<T extends StatefulWidget> on State<T> implements TickerProvider { Ticker _ticker; Ticker createTicker(TickerCallback onTick) { //[见小节2.1.2] _ticker = Ticker(onTick, debugLabel: 'created by $this'); return _ticker; } 

2.1.2 Ticker初始化[](http://gityuan.com/2019/07/13/flutter_animator/#212-ticker初始化)

[-> lib/src/scheduler/ticker.dart]

class Ticker { Ticker(this._onTick, { this.debugLabel }) { } final TickerCallback _onTick; } 

将AnimationControllerd对象中的_tick()方法，赋值给Ticker对象的_onTick成员变量，再来看看该_tick方法。

2.1.3 _internalSetValue[](http://gityuan.com/2019/07/13/flutter_animator/#213-_internalsetvalue)

[-> lib/src/animation/animation_controller.dart ::AnimationController]

void _internalSetValue(double newValue) { _value = newValue.clamp(lowerBound, upperBound); if (_value == lowerBound) { _status = AnimationStatus.dismissed; } else if (_value == upperBound) { _status = AnimationStatus.completed; } else { _status = (_direction == _AnimationDirection.forward) ? AnimationStatus.forward : AnimationStatus.reverse; } } 

根据当前的value值来初始化动画状态_status

2.2 forward[](http://gityuan.com/2019/07/13/flutter_animator/#22-forward)

[-> lib/src/animation/animation_controller.dart ::AnimationController]

TickerFuture forward({ double from }) { //默认采用向前的动画方向 _direction = _AnimationDirection.forward; if (from != null) value = from; return _animateToInternal(upperBound); //[见小节2.3] } 

_AnimationDirection是枚举类型，有forward(向前)和reverse(向后)两个值，也就是说该方法的功能是指从from开始向前滑动，

2.3 _animateToInternal[](http://gityuan.com/2019/07/13/flutter_animator/#23-_animatetointernal)

[-> lib/src/animation/animation_controller.dart ::AnimationController]

TickerFuture _animateToInternal(double target, { Duration duration, Curve curve = Curves.linear }) { double scale = 1.0; if (SemanticsBinding.instance.disableAnimations) { switch (animationBehavior) { case AnimationBehavior.normal: scale = 0.05; break; case AnimationBehavior.preserve: break; } } Duration simulationDuration = duration; if (simulationDuration == null) { final double range = upperBound - lowerBound; final double remainingFraction = range.isFinite ? (target - _value).abs() / range : 1.0; //根据剩余动画的百分比来评估仿真动画剩余时长 simulationDuration = this.duration * remainingFraction; } else if (target == value) { //已到达动画终点，不再执行动画 simulationDuration = Duration.zero; } //停止老的动画[见小节2.3.1] stop(); if (simulationDuration == Duration.zero) { if (value != target) { _value = target.clamp(lowerBound, upperBound); notifyListeners(); } _status = (_direction == _AnimationDirection.forward) ? AnimationStatus.completed : AnimationStatus.dismissed; _checkStatusChanged(); //当动画执行时间已到，则直接结束 return TickerFuture.complete(); } //[见小节2.4] return _startSimulation(_InterpolationSimulation(_value, target, simulationDuration, curve, scale)); } 

默认采用的是线性动画曲线Curves.linear。

2.3.1 AnimationController.stop[](http://gityuan.com/2019/07/13/flutter_animator/#231-animationcontrollerstop)

void stop({ bool canceled = true }) { _simulation = null; _lastElapsedDuration = null; //[见小节2.3.2] _ticker.stop(canceled: canceled); } 

2.3.2 Ticker.stop[](http://gityuan.com/2019/07/13/flutter_animator/#232-tickerstop)

[-> lib/src/scheduler/ticker.dart]

void stop({ bool canceled = false }) { if (!isActive) //已经不活跃，则直接返回 return; final TickerFuture localFuture = _future; _future = null; _startTime = null; //[见小节2.3.3] unscheduleTick(); if (canceled) { localFuture._cancel(this); } else { localFuture._complete(); } } 

2.3.3 Ticker.unscheduleTick[](http://gityuan.com/2019/07/13/flutter_animator/#233-tickerunscheduletick)

[-> lib/src/scheduler/ticker.dart]

void unscheduleTick() { if (scheduled) { SchedulerBinding.instance.cancelFrameCallbackWithId(_animationId); _animationId = null; } } 

2.3.4 _InterpolationSimulation初始化[](http://gityuan.com/2019/07/13/flutter_animator/#234-_interpolationsimulation初始化)

[-> lib/src/animation/animation_controller.dart ::_InterpolationSimulation]

class _InterpolationSimulation extends Simulation { _InterpolationSimulation(this._begin, this._end, Duration duration, this._curve, double scale) : _durationInSeconds = (duration.inMicroseconds * scale) / Duration.microsecondsPerSecond; final double _durationInSeconds; final double _begin; final double _end; final Curve _curve; } 

该方法创建插值模拟器对象，并初始化起点、终点、动画曲线以及时长。这里用的Curve是线性模型，也就是说采用的是匀速运动。

2.4 _startSimulation[](http://gityuan.com/2019/07/13/flutter_animator/#24-_startsimulation)

[-> lib/src/animation/animation_controller.dart]

TickerFuture _startSimulation(Simulation simulation) { _simulation = simulation; _lastElapsedDuration = Duration.zero; _value = simulation.x(0.0).clamp(lowerBound, upperBound); //[见小节2.5] final TickerFuture result = _ticker.start(); _status = (_direction == _AnimationDirection.forward) ? AnimationStatus.forward : AnimationStatus.reverse; //[见小节2.4.1] _checkStatusChanged(); return result; } 

2.4.1 _checkStatusChanged[](http://gityuan.com/2019/07/13/flutter_animator/#241-_checkstatuschanged)

[-> lib/src/animation/animation_controller.dart]

void _checkStatusChanged() { final AnimationStatus newStatus = status; if (_lastReportedStatus != newStatus) { _lastReportedStatus = newStatus; notifyStatusListeners(newStatus); //通知状态改变 } } 

这里会回调_statusListeners中的所有状态监听器，这里的状态就是指AnimationStatus的dismissed、forward、reverse以及completed。

2.5 Ticker.start[](http://gityuan.com/2019/07/13/flutter_animator/#25-tickerstart)

[-> lib/src/scheduler/ticker.dart]

TickerFuture start() { _future = TickerFuture._(); if (shouldScheduleTick) { scheduleTick(); //[见小节2.6] } if (SchedulerBinding.instance.schedulerPhase.index > SchedulerPhase.idle.index && SchedulerBinding.instance.schedulerPhase.index < SchedulerPhase.postFrameCallbacks.index) _startTime = SchedulerBinding.instance.currentFrameTimeStamp; return _future; } 

此处的shouldScheduleTick等于!muted && isActive && !scheduled，也就是没有调度过的活跃状态才会调用Tick。

2.6 Ticker.scheduleTick[](http://gityuan.com/2019/07/13/flutter_animator/#26-tickerscheduletick)

[-> lib/src/scheduler/ticker.dart]

void scheduleTick({ bool rescheduling = false }) { //[见小节2.7] _animationId = SchedulerBinding.instance.scheduleFrameCallback(_tick, rescheduling: rescheduling); } 

此处的_tick会在下一次vysnc触发时回调执行，见小节2.10。

2.7 scheduleFrameCallback[](http://gityuan.com/2019/07/13/flutter_animator/#27-scheduleframecallback)

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

int scheduleFrameCallback(FrameCallback callback, { bool rescheduling = false }) { //[见小节2.8] scheduleFrame(); _nextFrameCallbackId += 1; _transientCallbacks[_nextFrameCallbackId] = _FrameCallbackEntry(callback, rescheduling: rescheduling); return _nextFrameCallbackId; } 

将前面传递过来的Ticker._tick()方法保存在_FrameCallbackEntry的callback中，然后将_FrameCallbackEntry记录在Map类型的_transientCallbacks，

2.8 scheduleFrame[](http://gityuan.com/2019/07/13/flutter_animator/#28-scheduleframe)

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

void scheduleFrame() { if (_hasScheduledFrame || !_framesEnabled) return; ui.window.scheduleFrame(); _hasScheduledFrame = true; } 

从文章Flutter之setState更新机制，可知此处调用的ui.window.scheduleFrame()，会注册vsync监听。当当下一次vsync信号的到来时会执行handleBeginFrame()。

2.9 handleBeginFrame[](http://gityuan.com/2019/07/13/flutter_animator/#29-handlebeginframe)

[-> 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; ... //此时阶段等于SchedulerPhase.idle; _hasScheduledFrame = false; try { Timeline.startSync('Animate', arguments: timelineWhitelistArguments); _schedulerPhase = SchedulerPhase.transientCallbacks; //执行动画的回调方法 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; } } 

该方法主要功能是遍历_transientCallbacks，从前面小节[2.7]，可知该过程会执行Ticker._tick()方法。

2.10 Ticker._tick[](http://gityuan.com/2019/07/13/flutter_animator/#210-ticker_tick)

[-> lib/src/scheduler/ticker.dart]

void _tick(Duration timeStamp) { _animationId = null; _startTime ??= timeStamp; //[见小节2.11] _onTick(timeStamp - _startTime); //根据活跃状态来决定是否再次调度 if (shouldScheduleTick) scheduleTick(rescheduling: true); } 

该方法主要功能：

• 小节[2.1.2]的Ticker初始化中，可知此处_onTick便是AnimationController的_tick()方法；
• 小节[2.5]已介绍当仍处于活跃状态，则会再次调度，回到小节[2.6]的scheduleTick()，从而形成动画的连续绘制过程。

2.11 AnimationController._tick[](http://gityuan.com/2019/07/13/flutter_animator/#211-animationcontroller_tick)

[-> lib/src/animation/animation_controller.dart]

void _tick(Duration elapsed) { _lastElapsedDuration = elapsed; //获取已过去的时长 final double elapsedInSeconds = elapsed.inMicroseconds.toDouble() / Duration.microsecondsPerSecond; _value = _simulation.x(elapsedInSeconds).clamp(lowerBound, upperBound); if (_simulation.isDone(elapsedInSeconds)) { _status = (_direction == _AnimationDirection.forward) ? AnimationStatus.completed : AnimationStatus.dismissed; stop(canceled: false); //当动画已完成，则停止 } notifyListeners(); //通知监听器[见小节2.11.1] _checkStatusChanged(); //通知状态监听器[见小节2.11.2] } 

2.11.1 notifyListeners[](http://gityuan.com/2019/07/13/flutter_animator/#2111-notifylisteners)

[-> lib/src/animation/listener_helpers.dart ::AnimationLocalListenersMixin]

void notifyListeners() { final List<VoidCallback> localListeners = List<VoidCallback>.from(_listeners); for (VoidCallback listener in localListeners) { try { if (_listeners.contains(listener)) listener(); } catch (exception, stack) { ... } } } 

AnimationLocalListenersMixin的addListener()会向_listeners中添加监听器

2.11.2 _checkStatusChanged[](http://gityuan.com/2019/07/13/flutter_animator/#2112-_checkstatuschanged)

[-> lib/src/animation/listener_helpers.dart ::AnimationLocalStatusListenersMixin]

void notifyStatusListeners(AnimationStatus status) { final List<AnimationStatusListener> localListeners = List<AnimationStatusListener>.from(_statusListeners); for (AnimationStatusListener listener in localListeners) { try { if (_statusListeners.contains(listener)) listener(status); } catch (exception, stack) { ... } } } 

从前面的小节[2.4.1]可知，当状态改变时会调用notifyStatusListeners方法。AnimationLocalStatusListenersMixin的addStatusListener()会向_statusListeners添加状态监听器。

三、总结[](http://gityuan.com/2019/07/13/flutter_animator/#三总结)

3.1 动画流程图[](http://gityuan.com/2019/07/13/flutter_animator/#31-动画流程图)

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原文作者：gityuan
原文链接：http://gityuan.com/2019/07/13/flutter_animator/