最近写了一些flutter的小动画，在这里也写几个篇章介绍下flutter的动画的实现，先实现个简单的帧动画，举例美团的加载动画就是帧动画的实现，那换flutter的怎么实现呢，一起看看

主要的类实现FrameAnimationImage

class FrameAnimationImage extends StatefulWidget {
  final List<String> _assetList;
  final double width;
  final double height;
  bool start = true;
  int interval = 200;

  FrameAnimationImage(Key key, this._assetList,
      {this.width, this.height, this.interval, this.start})
      : super(key: key);

  @override
  State<StatefulWidget> createState() {
    return FrameAnimationImageState();
  }
}

class FrameAnimationImageState extends State<FrameAnimationImage>
    with SingleTickerProviderStateMixin {
  // 动画控制
  Animation<double> _animation;
  AnimationController _controller;
  int interval = 200;

  @override
  void initState() {
    super.initState();

    if (widget.interval != null) {
      interval = widget.interval;
    }
    final int imageCount = widget._assetList.length;
    final int maxTime = interval * imageCount;

    // 启动动画controller
    _controller = new AnimationController(
        duration: Duration(milliseconds: maxTime), vsync: this);
    _controller.addStatusListener((AnimationStatus status) {
      if (status == AnimationStatus.completed) {
        _controller.forward(from: 0.0); // 完成后重新开始
      }
    });

    _animation = new Tween<double>(begin: 0, end: imageCount.toDouble())
        .animate(_controller)
      ..addListener(() {
        setState(() {
          // the state that has changed here is the animation object’s value
        });
      });
    if (widget.start) {
      _controller.forward();
    }
  }

  void startAnimation() => _controller.forward();
  void stopAnimation() => _controller.stop();
  void reStartAnimation(){
    _controller.reset();
    _controller.forward();
  }

  @override
  void didUpdateWidget(FrameAnimationImage oldWidget) {
    // TODO: implement didUpdateWidget
    super.didUpdateWidget(oldWidget);
    print("didUpdateWidget called");
    if (widget.start) {
      _controller.forward();
    }
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    int ix = _animation.value.floor() % widget._assetList.length;

    List<Widget> images = [];
    // 把所有图片都加载进内容，否则每一帧加载时会卡顿
    for (int i = 0; i < widget._assetList.length; ++i) {
      if (i != ix) {
        images.add(Image.asset(
          ImageUtils.getImagePath( widget._assetList[i]),///ImageUtils只是一个简单出来图片格式的方法
          width: 0,
          height: 0,
        ));
      }
    }

    images.add(Image.asset(
      ImageUtils.getImagePath(widget._assetList[ix]),//

      width: widget.width,
      height: widget.height,
    ));

    return Stack(alignment: AlignmentDirectional.center, children: images);
  }
}
//class ImageUtils {
//  static String getImagePath(String name, {String format: 'png'}) {
//    return 'assets/images/$name.$format';
//  }
//
//  static ImageProvider getImageProvider(String imageUrl,
//      {String holderImg: "none"}) {
//    if (TextUtil.isEmpty(imageUrl)) {
//      return AssetImage(getImagePath(holderImg));
//    }
//    return CachedNetworkImageProvider(imageUrl);
//  }
//} 

这个类的实现也比较好理解的，通过监听AnimationStatus 动画的状态，对动画实现控制。下面看下实现示例.
准备好图片：

go1.png

go2.png

go3.png

go4.png

class Frame extends StatelessWidget {

  bool b = false;
 
  final List<String> list = [
    'go1',
    'go2',
    'go3',
    'go4',

  ];
  final GlobalKey<FrameAnimationImageState> _cotroller = new GlobalKey<FrameAnimationImageState>();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text('帧动画'),
          centerTitle: true,
        ),
        body: Column(
          children: <Widget>[
            Center(
              child: GestureDetector(
                onTap: () {
                  if (b) {
                    _cotroller.currentState.reStartAnimation();
                  } else {
                    _cotroller.currentState.startAnimation();

                  }

                  b = !b;
                },
                child: FrameAnimationImage(_cotroller, list,
                    width: 220, height: 200, interval: 50, start: false),
              ),
            ),
            RaisedButton(
              onPressed: (){
                _cotroller.currentState.stopAnimation();
              },
              child: Text("结束"),
            )
          ],
        )

 );
  }

}

将图片上去，运行、就可以看见小鹿跑动的动画啦。
很简单是不是，帧动画就这样吧，要是控制时间什么的自己添加修改就好了，还有注意帧动画图片的大小喔。