AnimatedWidget

直接上代码

//...

class AnimState extends State<AnimScreen> with SingleTickerProviderStateMixin {

  Animation<double> animation;
  AnimationController controller;

  @override
  initState() {
    super.initState();
    controller = AnimationController(
        duration: const Duration(milliseconds: 2000), vsync: this);
    // 区别在这里，不需要主动 addListener 中去 setState
    animation = Tween(begin: 0.0, end: 300.0).animate(controller);
    controller.forward();
  }
  
  Widget build(BuildContext context) {
    return AnimatedLogo(animation: animation);
  }
  
  // ...
}

在 build 方法中返回了一个 AnimatedLogo，定义好的 Animation 作为参数。

class AnimatedLogo extends AnimatedWidget {
  AnimatedLogo({Key key, Animation<double> animation})
      : super(key: key, listenable: animation);

  Widget build(BuildContext context) {
    // 外部传递过来的 Animation 对象
    final Animation<double> animation = listenable;
    return Center(
      child: Container(
        margin: EdgeInsets.symmetric(vertical: 10.0),
        height: animation.value,
        width: animation.value,
        child: FlutterLogo(),
      ),
    );
  }
}

看起来 AnimatedLogo 的 build 其实和原来的写法区别不大，但这个 Widget 继承的是 AnimatedWidget，它本身继承自 StatefulWidget。看它的源码

class _AnimatedState extends State<AnimatedWidget> {
  @override
  void initState() {
    super.initState();
    // 对传进来的 Animation 对象 listenable 添加监听，监听里调用 setState
    widget.listenable.addListener(_handleChange);
  }

  @override
  void dispose() {
    // 还做了 removeListener 的操作
    widget.listenable.removeListener(_handleChange);
    super.dispose();
  }
  
  void _handleChange() {
    setState(() {
      // The listenable's state is our build state, and it changed already.
    });
  }
  // ...
}

即自动加了监听，value 变化时调用 setState 修改 UI。同时 dispose 方法中还移除了监听。

状态监听

前面通过 addListener() 监听动画值的变化，还可以通过 addStatusListener() 来监听动画的状态，如动画开始、结束、向前移动或向后移动。

状态由四个值

enum AnimationStatus {
  /// The animation is stopped at the beginning
  dismissed,

  /// The animation is running from beginning to end
  forward,

  /// The animation is running backwards, from end to beginning
  reverse,

  /// The animation is stopped at the end
  completed,
}

修改代码监听状态：

@override
initState() {
  super.initState();
  controller = AnimationController(
      duration: const Duration(milliseconds: 2000), vsync: this);
  animation = Tween(begin: 0.0, end: 300.0).animate(controller);
  animation.addStatusListener((status) {
    print("$status"); // 打印状态
    if (status == AnimationStatus.completed) {
      controller.reverse(); // 动画结束时，反转从尾到头播放，结束的状态是 dismissed
    } else if (status == AnimationStatus.dismissed) {
      controller.forward(); // 重新从头到尾播放
    }
  });
  controller.forward();
}

动画将无限循环

I/flutter (14046): AnimationStatus.forward
I/flutter (14046): AnimationStatus.completed
I/flutter (14046): AnimationStatus.reverse
I/flutter (14046): AnimationStatus.dismissed
I/flutter (14046): AnimationStatus.forward
I/flutter (14046): AnimationStatus.completed
I/flutter (14046): AnimationStatus.reverse
I/flutter (14046): AnimationStatus.dismissed
...

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