Flutter 组合动画

主要包含了一个拖拽效果，以及松开之后的组合动画。

拖拽效果使用了组件 Draggable，组合动画使用到了CurvedAnimation。

使用 GifImage 插件，来控制GIF动画的时间。

 DraggableAnimation(
        duration: 5,
        landTime: 0.3,
        landDuration: 0.04,
        hover: Container(
          width: 58,
          height: 84,
          child: GifImage( 
            controller: GifController(vsync: this)..repeat(min: 0, max: 5, period: Duration(milliseconds: 500)),
            image: AssetImage("images/await.gif"),
          ),
        ),
        drag: Container(
          width: 54,
          height: 107,
          child: Image.asset("images/drag.gif"),
        ),
        fall: Container(
          width: 65,
          height: 108,
          child: Image.asset("images/fall.gif"),
        ),
        run: Container(
          width: 66,
          height: 120,
          child: GifImage(
            controller: GifController(vsync: this)..repeat(min: 0, max: 3, period: Duration(milliseconds: 300)),
            image: AssetImage("images/run.gif"),
          ),
        ),
        land: Container(
          width: 68,
          height: 90,
          child: GifImage(
            controller: GifController(vsync: this)..repeat(min: 0, max: 2, period: Duration(milliseconds: 200)),
            image: AssetImage("images/land.gif"),
          ),
        ),
      ),

class DraggableAnimation extends StatefulWidget {
  final Widget hover;
  final Widget drag;
  final Widget fall;
  final Widget land;
  final Widget run;
  final int duration;
  final double landTime;
  final double landDuration;
  const DraggableAnimation({
    Key key,
    this.hover,  // 静止样式
    this.drag,  // 拖拽样式
    this.fall,  // 下落样式
    this.land, // 落地样式
    this.run,  // 消失样式
    this.duration = 5, // 动画总时长
    this.landTime = 0.3,  // 落地开始时间
    this.landDuration = 0.1,  // 落地持续时间
  }) : super(key: key);
  @override
  State createState() {
    return DraggableAnimationState();
  }
}

enum AnimationState { drag, fall, land, run }

class DraggableAnimationState extends State with SingleTickerProviderStateMixin {
  AnimationState _state;
  bool _isAnimation = false;
  double _startLeft = 0;
  double _startBottom = 0;
  Size _containerSize = Size.zero;

  // 用于获取容器的位置和大小，处理偏移问题
  GlobalKey _keyContainer = GlobalKey(debugLabel: 'key_ontainer');
  // 用于拖拽组件的位置和大小，获取动画开始的位置
  GlobalKey _keyDragContainer = GlobalKey(debugLabel: 'key_drag_ontainer');

  Map _contentMap = Map();
  AnimationController _animationController;
  Animation _dropAnimation;
  Animation _runAnimation;
  @override
  void initState() {
    super.initState();
    _contentMap = {
      AnimationState.drag: widget.drag,
      AnimationState.fall: widget.fall,
      AnimationState.land: widget.land,
      AnimationState.run: widget.run,
    };
    _animationController = AnimationController(duration: Duration(seconds: widget.duration), vsync: this)
      ..addListener(() {
        setState(() {
          _isAnimation = _animationController.isAnimating;
          if (_animationController.value <= widget.landTime) {
            _state = AnimationState.fall;
          } else if (_animationController.value >= widget.landTime + widget.landDuration) {
            _state = AnimationState.run;
          } else {
            _state = AnimationState.land;
          }
        });
      });
  }

  @override
  Widget build(BuildContext context) {
    return SafeArea(
      child: Stack(
        key: _keyContainer,
        children: [
          Positioned(
            child: Visibility(
              child: _draggableWidget(),
              visible: !_isAnimation,
            ),
            right: 0,
            top: 120,
          ),
          Positioned(
            left: _runAnimation?.value ?? 0,
            bottom: _dropAnimation?.value ?? 0,
            child: Visibility(
              visible: _isAnimation,
              child: _contentMap[_state] ?? Container(),
            ),
          )
        ],
      ),
    );
  }

  Widget _draggableWidget() {
    return Draggable(
      child: widget.hover ?? Container(),
      onDragStarted: () {
        if (_state != AnimationState.drag) {
          setState(() {
            _state = AnimationState.drag;
          });
        }
      },
      onDraggableCanceled: (Velocity velocity, Offset offset) {
        RenderBox renderContainer = _keyContainer.currentContext.findRenderObject();
        _containerSize = renderContainer.size;
        var containerPosition = renderContainer.localToGlobal(Offset.zero);

        final RenderBox renderBoxRed = _keyDragContainer.currentContext.findRenderObject();
        var position = renderBoxRed.localToGlobal(Offset.zero);
        var size = renderBoxRed.size;

        _startLeft = position.dx;
        _startBottom = _containerSize.height - position.dy - size.height + containerPosition.dy;
        print('${_containerSize.width - _startLeft}');
        if (_containerSize.width - _startLeft < 2 * size.width) {
          return;
        }
        _startAnimation();
      },
      childWhenDragging: Container(),
      feedback: Container(
        child: widget.drag ?? Container(),
        key: _keyDragContainer,
      ),
    );
  }

  void _startAnimation() {
    _dropAnimation = Tween(begin: _startBottom, end: 0).animate(CurvedAnimation(
      parent: _animationController,
      curve: Interval(0.0, widget.landTime, curve: Curves.easeInCubic),
    ));
    _runAnimation = Tween(begin: _startLeft, end: _containerSize.width).animate(CurvedAnimation(
      parent: _animationController,
      curve: Interval(widget.landTime + widget.landDuration, 1.0, curve: Curves.easeOut),
    ));
    _animationController.reset();
    _animationController.forward();
  }
}