动画的基本使用代码示例：

class AnimationDemo extends StatefulWidget {
  @override
  _AnimationDemoState createState() => _AnimationDemoState();
}

class _AnimationDemoState extends State<AnimationDemo>
    with SingleTickerProviderStateMixin {
  AnimationController _controller;
  Animation _curvedAnimation;
  Animation _sizeAnimation;

  @override
  void initState() {
    super.initState();
    // 创建AnimationController
    _controller = AnimationController(
      vsync: this,
      duration: Duration(seconds: 2),
    );

    // 设置Curve的值
    _curvedAnimation = CurvedAnimation(
      parent: _controller,
      curve: Curves.linear,
    );

    // Tween
    _sizeAnimation = Tween(begin: 50.0, end: 150.0).animate(_curvedAnimation);

    // 监听动画值改变
    _controller.addListener(() {
      setState(() {});
    });

    // 监听动画状态改变
    _controller.addStatusListener((status) {
      if (status == AnimationStatus.completed) {
        _controller.reverse();
      } else if (status == AnimationStatus.dismissed) {
        _controller.forward();
      }
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Animation Demo'),
      ),
      body: Center(
        child: Icon(
          Icons.favorite,
          color: Colors.red,
          size: _sizeAnimation.value,
        ),
      ),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.play_arrow),
        onPressed: () {
          if (_controller.isAnimating) {
            _controller.stop();
            print(_controller.status);
          } else if (_controller.status == AnimationStatus.forward) {
            _controller.forward();
          } else if (_controller.status == AnimationStatus.reverse) {
            _controller.reverse();
          } else {
            _controller.forward();
          }
        },
      ),
    );
  }

  @override
  void dispose() {
    // 释放动画资源
    _controller.dispose();
    super.dispose();
  }
}

使用AnimatedWidget简化

上面的示例通过addListener()和setState() 来更新UI，这一步其实是通用的，如果每个动画中都加这么一句是比较繁琐的，还会造成不必要的widget重建。AnimatedWidget类封装了调用setState()的细节，并允许我们将widget分离出来。优化后的代码：

class _AnimatedIcon extends AnimatedWidget {
  _AnimatedIcon(Animation animation) : super(listenable: animation);

  @override
  Widget build(BuildContext context) {
    final Animation animation = listenable;
    return Icon(
      Icons.favorite,
      color: Colors.red,
      size: animation.value,
    );
  }
}

class _AnimationDemoState extends State<AnimationDemo>
    with SingleTickerProviderStateMixin {
  AnimationController _controller;
  Animation _curvedAnimation;
  Animation _sizeAnimation;

  @override
  void initState() {
    super.initState();
    // 创建AnimationController
    _controller = AnimationController(
      vsync: this,
      duration: Duration(seconds: 2),
    );

    // 设置Curve的值
    _curvedAnimation = CurvedAnimation(
      parent: _controller,
      curve: Curves.linear,
    );

    // Tween
    _sizeAnimation = Tween(begin: 50.0, end: 150.0).animate(_curvedAnimation);

    // 监听动画状态改变
    _controller.addStatusListener((status) {
      if (status == AnimationStatus.completed) {
        _controller.reverse();
      } else if (status == AnimationStatus.dismissed) {
        _controller.forward();
      }
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Animation Demo'),
      ),
      body: Center(
        child: _AnimatedIcon(_sizeAnimation),
      ),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.play_arrow),
        onPressed: () {
          if (_controller.isAnimating) {
            _controller.stop();
            print(_controller.status);
          } else if (_controller.status == AnimationStatus.forward) {
            _controller.forward();
          } else if (_controller.status == AnimationStatus.reverse) {
            _controller.reverse();
          } else {
            _controller.forward();
          }
        },
      ),
    );
  }

  @override
  void dispose() {
    // 释放动画资源
    _controller.dispose();
    super.dispose();
  }
}

用AnimatedBuilder重构

用AnimatedWidget可以从动画中分离出widget，而动画的渲染过程仍然在AnimatedWidget中，如果再添加一个widget另一个动画效果，那么需要再实现一个AnimatedWidget，这样不是很优雅，如果能把渲染过程也抽象出来，那就会好很多，而AnimatedBuilder正是将渲染逻辑分离出来，代码示例：

  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Animation Demo'),
      ),
      body: Center(
        child: AnimatedBuilder(
          animation: _controller,
          builder: (context, child) {
            return Icon(
              Icons.favorite,
              color: Colors.red,
              size: _sizeAnimation.value,
            );
          },
        ),
      ),

  }

通过AnimatedBuilder可以封装常见的过渡效果来复用动画，代码示例：

class GrowTransition extends StatelessWidget {
  GrowTransition({this.child, this.animation});

  final Widget child;
  final Animation animation;

  Widget build(BuildContext context) {
    return Center(
      child: new AnimatedBuilder(
        animation: animation,
        builder: (BuildContext context, Widget child) {
          return Container(
            height: animation.value,
            width: animation.value,
            child: child,
          );
        },
        child: child,
      ),
    );
  }
}

Flutter中正是通过这种方式封装了很多动画，如：FadeTransition、ScaleTransition、SizeTransition等。

代码传送门