implicit_animations.dart阅读

这个文件定义了Flutter很多的基础动画,主要是基于Container的各个属性的动画封装,它的实现原理根据定义的时间曲线获取对应的补间值, 然后不断的更新widget的配置信息(如果有变动), 在vsync扫描时就会不断的更新每一帧的界面
连在一起看就成了动画效果, 实现它需要有时间函数曲线, 补间值, vsync同步信号

1.视图更新的同步信号(Vsync)

• Vsync信息由flutter engine提供, 当前的widget只需要with一个 TickerProviderStateMixin 类, 就能监听到系统的vsync信号, 以下面的 SingleTickerProvider 为栗子, 它直接控制和监听 SchedulerBinding.instance 来实现vsync的回调事件转发给AnimationController

  mixin SingleTickerProviderStateMixin<T extends StatefulWidget> on State<T> implements TickerProvider {
    //创建ticker,这里创建了一个tiker,而 `TickerProvider` 是个抽象类，所以vsync肯定和它有关
  
    @override
  
    Ticker createTicker(TickerCallback onTick) { ...
  
      _ticker = Ticker(onTick, debugLabel: kDebugMode ? 'created by $this' : null);
  
    @override
  
    void didChangeDependencies() { ...
  
        _ticker.muted = !TickerMode.of(context);
  
  }
  
  class Ticker {
  
    //Ticker的初始化只有一个参数,没有继承其他的内
  
    Ticker(this._onTick, { this.debugLabel }) {
  
    //控制ticker的事件队列,确保次序正确
  
    TickerFuture _future;
  
    //决定当前ticker是否只是监听vsync不执行callback,避免视图不现实的内存开销
  
    set muted(bool value) {
  
        isMuted ...
  
        unscheduleTick();
  
        shouldScheduleTick ...
  
        scheduleTick();
  
    //开启一个定时刷新,如果当前能执行动画(!isMetued)则执行它的callback事件
  
    TickerFuture start() {
  
      ...
  
      if (shouldScheduleTick) {
  
        scheduleTick();
  
      ...
  
      return _future;
  
    }
  
    void stop({ bool canceled = false }) { ...
  
      unscheduleTick();
  
      if (canceled) {
  
        localFuture._cancel(this);
  
      } else {
  
        localFuture._complete();
  
      }
  
    }
  
    //记录animationId，避免重复schedule
  
    @protected
  
    bool get scheduled => _animationId != null;
  
    //执行tick的三要素
  
    @protected
  
    bool get shouldScheduleTick => !muted && isActive && !scheduled;
  
    //
  
    void _tick(Duration timeStamp) {..
  
      _onTick(timeStamp - _startTime);
  
      if (shouldScheduleTick)
  
        scheduleTick(rescheduling: true);
  
    }
  
    //最终将tick事件发送给 `SchedulerBinding.instance` ,持续定于window的每一帧
  
    @protected
  
    void scheduleTick({ bool rescheduling = false }) { ...
  
      _animationId = SchedulerBinding.instance.scheduleFrameCallback(_tick, rescheduling: rescheduling);
  
    }
  
    @protected
  
    void unscheduleTick() { ...
  
        SchedulerBinding.instance.cancelFrameCallbackWithId(_animationId);
  
    }
  
    //用于接管另外一个ticker,释放原来的ticker,使用自己的ticker事件
  
    void absorbTicker(Ticker originalTicker) {...
  
        if (shouldScheduleTick)
  
          scheduleTick();
  
        originalTicker._future = null; // so that it doesn't get disposed when we dispose of originalTicker
  
        originalTicker.unscheduleTick();
  
      }
  
      originalTicker.dispose();
  
    }
  

2. 动画的时间曲线

• 通过Widget初始化时构造一个AnimationController, 它接收一个总的时常, 同是提供了ticker构造工厂函数, 而ticker可以监听vsync信号,
• 通过监听vsync事件和frameCallback的duration, 再结合动画的时间区间, 计算出每一帧绘制时的时间, 这样就生成一个动画的时间曲线

• 除了生成时间曲线以外, 它还提供了动画控制的能力, 比如重复开始, 反转, 每执行一个操作它都会去从新生成一个当前的时间值, 确保动画的进度按照代码设定的逻辑执行

  class _AnimatedWidgetDemoState extends State<AnimatedWidgetDemo>
      with SingleTickerProviderStateMixin {
  
    AnimationController _animationController;
  
    @override
  
    void initState() {
  
      super.initState();
  
      _animationController = AnimationController(
  
        vsync: this,
  
        duration: Duration(seconds: 2),
  
      )..repeat();
  
    }
  
  //在AnimationController创建的时候,会创建ticker
  
  class AnimationController ...
  
  AnimationController({ ...
  
      _ticker = vsync.createTicker(_tick);
  
      ...
  
    }
  
    void _tick(Duration elapsed) { ...
  
      //事件比例计算,也就是当前的动画进度
  
      _value = _simulation.x(elapsedInSeconds).clamp(lowerBound, upperBound) as double;
  
      notifyListeners();
  
      _checkStatusChanged();
  
    }
  
  //重复执行
  
  TickerFuture repeat({ double min, double max, bool reverse = false, Duration period }) {
  
      ...
  
      stop();  
  
      return _startSimulation(_RepeatingSimulation(_value, min, max, reverse, period, _directionSetter));
  
    }
  
    //开始执行动画
  
  TickerFuture forward({ double from }) { ...  
  
      _direction = _AnimationDirection.forward;
  
      return _animateToInternal(upperBound);
  
    }
  
    TickerFuture _animateToInternal(double target, { Duration duration, Curve curve = Curves.linear }) {...
  
      stop(); ...
  
      return _startSimulation(_InterpolationSimulation(_value, target, simulationDuration, curve, scale));
  
    }
  
   TickerFuture _startSimulation(Simulation simulation) { ...
  
      _value = simulation.x(0.0).clamp(lowerBound, upperBound) as double;
  
      final TickerFuture result = _ticker.start();
  
      _checkStatusChanged();
  
      return result;
  
    }
  
    //可以看到AnimationController对动画的控制最终都会传递给 `Ticker` ,它主要负责时间进度的计算,计算完毕后根据代码设定的逻辑通知ticker是否需要监听下一帧的回调事件。
  

• 以下为Flutter定义的Curve曲线,用于适配X,Y之间的比例关系

  ParametricCurve (curves.dart)
      _BottomSheetSuspendedCurve (scaffold.dart)
  
      _BottomSheetSuspendedCurve (bottom_sheet.dart)
  
      Curve2D (curves.dart)
  
          CatmullRomSpline (curves.dart)
  
      Curve (curves.dart)
  
          FlippedCurve (curves.dart)
  
          _Linear (curves.dart)
  
          ElasticInOutCurve (curves.dart)
  
          SawTooth (curves.dart)
  
          Cubic (curves.dart)
  
          ElasticOutCurve (curves.dart)
  
          Interval (curves.dart)
  
          ElasticInCurve (curves.dart)
  
          _BounceInOutCurve (curves.dart)
  
          CatmullRomCurve (curves.dart)
  
          _BounceOutCurve (curves.dart)
  
          Threshold (curves.dart)
  
          _DecelerateCurve (curves.dart)
  
          _BounceInCurve (curves.dart)
  

3. 补间值

• 它代表的是动画在每一帧渲染时, Widget(RenderObject的最终提交渲染信息)的属性的值, 简单的动画一般只有开始和结束2个值,需要由时间曲线函数和动画的起始值共同作用来完成
• 动画过程也是线性的, 随着时间的推移, 按线性规律逐步完成的, 为了动画过渡的更加完美, 需要额外对补间值进行加工, 在保证时间因子是线性移动的前提下, 通过引入Curve函数, 每个时间节点的 属性值按照重新计算, 这样我们的补间值就变得丰富多样。

• 按照这个规律我们可以把线性的补间值变成各种变化规律的补间值
  在flutter仓库的源码中定义了很多的动画

• 补间值转换, 在flutter的动画设计中可以把它看作是一个以时间为 X 轴, 动画进度为 Y 的函数, 函数的具体实现都封装在 transform方法中

• 基于此规律我们可以定义自己Curve, 生成自己的曲线函数, 这样就能实时的调整动画的补间值, 以下为Flutter目前现有的一些Curve,基本上满足了绝大部分应用场景。

• 补间值的具体类, 基本上满足了绝大部分的场景, 直接就能使用

  Animatable (tween.dart)
      Tween (tween.dart)
  
          AlignmentTween (tweens.dart)
  
          FractionalOffsetTween (tweens.dart)
  
          AlignmentGeometryTween (tweens.dart)
  
          RelativeRectTween (transitions.dart)
  
          Matrix4Tween (implicit_animations.dart)
  
          BoxConstraintsTween (implicit_animations.dart)
  
          EdgeInsetsTween (implicit_animations.dart)
  
          DecorationTween (implicit_animations.dart)
  
          BorderTween (implicit_animations.dart)
  
          BorderRadiusTween (implicit_animations.dart)
  
          TextStyleTween (implicit_animations.dart)
  
          EdgeInsetsGeometryTween (implicit_animations.dart)
  
          ThemeDataTween (theme.dart)
  
          _FontWeightTween (sliding_segmented_control.dart)
  
          ShapeBorderTween (material.dart)
  
          _InputBorderTween (input_decorator.dart)
  
          MaterialPointArcTween (arc.dart)
  
          ReverseTween (tween.dart)
  
          StepTween (tween.dart)
  
          SizeTween (tween.dart)
  
          ColorTween (tween.dart)
  
          ConstantTween (tween.dart)
  
          IntTween (tween.dart)
  
          RectTween (tween.dart)
  
              MaterialRectCenterArcTween (arc.dart)
  
              MaterialRectArcTween (arc.dart)
  

示例Demo

• 下面以最常用的 AnimatedContainer 为例, 通过它来构造一个动画, 下面是一个简易的Demo,
  通过点击手势来触发ValueListenableBuilder重新构建, 切换动画的的初始值

AnimatedContainer示例

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';

class AnimatedContainerDemo extends StatelessWidget {
  final CheckStatusListener statusListener = CheckStatusListener(true);

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: () {
        statusListener.value = !statusListener.value;
      },
      child: ValueListenableBuilder<bool>(
          valueListenable: statusListener,
          builder: (BuildContext context, bool selected, Widget child) {
            return Center(
              child: AnimatedContainer(
                 selected ? 200.0 : 100.0,
                height: selected ? 100.0 : 200.0,
                color: selected ? Colors.red : Colors.blue,
                alignment: selected
                    ? Alignment.center
                    : AlignmentDirectional.topCenter,
                duration: Duration(seconds: 2),
                curve: Curves.fastOutSlowIn,
                child: FlutterLogo(size: 75),
              ),
            );
          }),
    );
  }
}

class CheckStatusListener extends ValueNotifier<bool>
    implements ValueListenable<bool> {
  CheckStatusListener(bool isSelected) : super(isSelected);
}

AnimatedContainer代码实现

• 从构造方法就可以看出它支持设置多种不同的补间值

  class AnimatedContainer extends ImplicitlyAnimatedWidget {
  AnimatedContainer({
  
      Key key,
  
      this.alignment,
  
      this.padding,
  
      Color color,
  
      Decoration decoration,
  
      this.foregroundDecoration,
  
      double width,
  
      double height,
  
      BoxConstraints constraints,
  
      this.margin,
  
      this.transform,
  
      this.child,
  
      Curve curve = Curves.linear,
  
      @required Duration duration,
  
      VoidCallback onEnd,
  
    }) 
  
    ...
  
  class _AnimatedContainerState extends AnimatedWidgetBaseState<AnimatedContainer> {
  
    AlignmentGeometryTween _alignment;
  
    EdgeInsetsGeometryTween _padding;
  
    DecorationTween _decoration;
  
    DecorationTween _foregroundDecoration;
  
    BoxConstraintsTween _constraints;
  
    EdgeInsetsGeometryTween _margin;
  
    Matrix4Tween _transform;
  
  //它的父类也是一个空壳,只是包装了setState方法,这个方法会触发这个widget及children重建,慎用,不要将复杂过多的逻辑用这种方案时间,一般使用与小部件的widget,继续向上查找它的Ancestor
  
  abstract class AnimatedWidgetBaseState<T extends ImplicitlyAnimatedWidget> extends ImplicitlyAnimatedWidgetState<T> {
  
    @override
  
    void initState() {
  
      super.initState();
  
      controller.addListener(_handleAnimationChanged);
  
    }
  
    void _handleAnimationChanged() {
  
      setState(() { /* The animation ticked. Rebuild with new animation value */ });
  
    }
  
  }
  
  //父类实现了tickerProvider协议,可以通过Tiker发送监听屏幕刷新事件，同时也持有了`AnimationController`用于控制基本的动画行为
  
  abstract class ImplicitlyAnimatedWidgetState<T extends ImplicitlyAnimatedWidget> extends State<T> with SingleTickerProviderStateMixin<T> {
  
    @protected
  
    AnimationController get controller => _controller;
  
    Animation<double> _animation;
  
    @override
  
    void initState() {
  
      super.initState();
  
      _controller = AnimationController( ...
  
      _controller.addStatusListener((AnimationStatus status) { ...
  
      _updateCurve(); //这里将 Curve函数和时间关联在一起,它和Curve将Tween关联在一起是一样的，因为最终都是通过3个数的乘积的到最终补间值
  
      _constructTweens(); //初始化设置子类的Tween
  
      didUpdateTweens(); //供子类在视图更新时刷新Tween
  
    }
  
    @override
  
    void didUpdateWidget(T oldWidget) {
  
      super.didUpdateWidget(oldWidget);
  
        ...
  
        _updateCurve();
  
      _controller.duration = widget.duration;
  
      if (_constructTweens()) {
  
        forEachTween((Tween<dynamic> tween, dynamic targetValue, TweenConstructor<dynamic> constructor) {
  
          _updateTween(tween, targetValue);
  
          return tween;
  
        });
  
        _controller
  
          ..value = 0.0
  
          ..forward();
  
        didUpdateTweens();
  
      }
  
    }
  
    //更新widget自带的curve函数,更新动画的时间进度,间接修改补间值
  
    void _updateCurve() { ...
  
        _animation = CurvedAnimation(parent: _controller, curve: widget.curve); 
  
    //自动更新补间动画值
  
    void _updateTween(Tween<dynamic> tween, dynamic targetValue) {
  
        ..begin = tween.evaluate(_animation)
  
        ..end = targetValue;
  
    }
  
    //充分利多态的特性,实现子类Tween的构造,并执行
  
    bool _constructTweens() {
  
      bool shouldStartAnimation = false;
  
      forEachTween((Tween<dynamic> tween, dynamic targetValue, TweenConstructor<dynamic> constructor) {
  
        if (targetValue != null) {
  
          tween ??= constructor(targetValue);
  
          if (_shouldAnimateTween(tween, targetValue))
  
            shouldStartAnimation = true;
  
        } else {
  
          tween = null;
  
        }
  
        return tween;
  
      });
  
      return shouldStartAnimation;
  
    }
  
    //子类通过重写这个方法
  
    //1. 子类传递 `Tween<T> tween, T targetValue,`给父类,举个栗子,Tween<T> tween就是子类的 AlginmentTween,targetValue就是Aliginment.topLeft
  
    //2. 父类调用`constructor`方法,子类生成对应的AlginmentTween,这样子类就持有了AlginmentTween,在子类的build方法中获取父类的`animation`,就能实时的改变Contianer相关的属性了(此处的tween回传给父类主要是为了检测动画进度)
  
    //typedef TweenVisitor<T> = Tween<T> Function(Tween<T> tween, T targetValue, TweenConstructor<T> constructor);
  
    @protected
  
    void forEachTween(TweenVisitor<dynamic> visitor);
  
    //用于hook子视图在widgetupdate时更新补间值
  
    @protected
  
    void didUpdateTweens() { }
  
  }
  

• 子类_AnimatedContainerState在每次构建中更新当前的补间值

  class _AnimatedContainerState extends AnimatedWidgetBaseState<AnimatedContainer> {...
    @override
  
    Widget build(BuildContext context) {
  
      return Container(
  
        child: widget.child,
  
        alignment: _alignment?.evaluate(animation),
  
        padding: _padding?.evaluate(animation),
  
        decoration: _decoration?.evaluate(animation),
  
        foregroundDecoration: _foregroundDecoration?.evaluate(animation),
  
        constraints: _constraints?.evaluate(animation),
  
        margin: _margin?.evaluate(animation),
  
        transform: _transform?.evaluate(animation),
  
      );
  
    }
  

• 创建Tween的具体实现在父类初始化是触发forEachTween来创建子类的Tween

  ``` dart
  abstract class ImplicitlyAnimatedWidgetState<T extends ImplicitlyAnimatedWidget> extends State<T> with SingleTickerProviderStateMixin<T> { ...
  
  void initState() { ...
  
       _constructTweens(); -> forEachTween((Tween<dynamic> tween, dynamic targetValue, TweenConstructor<dynamic> constructor) {
  
  void didUpdateWidget(T oldWidget) {...
  
    forEachTween((Tween<dynamic> tween, dynamic targetValue, TweenConstructor<dynamic> constructor) {
  
  子类重写`forEachTween`,传入当前的`_alignmentTween`(父类动画进度检查用)和`widget.alignmentValue`
  
  class _AnimatedContainerState extends AnimatedWidgetBaseState<AnimatedContainer> { ...
  
  @override
  
    void forEachTween(TweenVisitor<dynamic> visitor) {
  
      _alignment = visitor(_alignment, widget.alignment, (dynamic value) => 
  
      AlignmentGeometryTween(begin: value as AlignmentGeometry)) as AlignmentGeometryTween;//用于父类构造通过Tween开启动画用
  
      ...
  
    }
  
  ```
  

小结

• 理解AnimatedContainer动画的封装主要是要了解forEachTween的执行过程,它是一个嵌套函数,同时实现了父子协同工作的方式, AnimationController通过持有Ticker,可以很方便的注册和取消系统下一帧的回调,实时的计算当前的动画进度

• 动画的三要素:

  • Vsync信号,视图的刷新周期
  • Tween: 动画的补间值
  • TimeFunction: 时间函数,动画在某个时刻内它的动画进度值

• 复杂的动画都是基于动画的 补间值和动画时间函数计算得来的.