最近项目中有个需求，widget要能够移动、旋转、点击，代码实现如下。

class GestureDetectorDemo extends StatefulWidget {
  @override
  _GestureDetectorDemoState createState() => _GestureDetectorDemoState();
}

class _GestureDetectorDemoState extends State<GestureDetectorDemo> {
  bool _show = true;
  @override
  Widget build(BuildContext context) {
    // final control = TransformationController();
    // control.
    return _buildPanel();
  }

  //静止状态下的offset
  Offset _idleOffset = Offset(0, 0);
  //本次移动的offset
  Offset _moveOffset = Offset(0, 0);
  //最后一次down事件的offset
  Offset _lastStartOffset = Offset(0, 0);
  double _previousScale = 1;
  double _scale = 1;

  double _itemWidth = 150;
  double _itemHeight = 150;
  Widget _buildPanel() {
    return Transform.scale(
      scale: _scale,
      alignment: Alignment(0, 0),
      child: Transform.translate(
        offset: _moveOffset,
        child: GestureDetector(
            onScaleStart: _onScaleStart,
            onScaleUpdate: _onScaleUpdate,
            onScaleEnd: _onScaleEnd,
            onTap: _onTap,
            child: Container(
              color: Colors.red,
              width: _itemWidth,
              height: _itemHeight,

            )),
      ),
    );
  }

  void _onTap(){
    print('_onTap');
  }
  void _onScaleStart(ScaleStartDetails details) {
    setState(() {
      _lastStartOffset = details.focalPoint;
      print('onScaleStart $_lastStartOffset');
      Offset local = details.localFocalPoint;
    });
  }

  void _onScaleUpdate(ScaleUpdateDetails details) {
    setState(() {
      // 缩放
      _scale = _previousScale * details.scale;
      // 移动
      _moveOffset =
          (details.focalPoint - _lastStartOffset + _idleOffset) / _scale;

      // 控制移动边界
      double left =
          (_itemWidth * _scale - _itemWidth) / 2.0 / _scale - _itemWidth / 2;
      double top = ((_itemHeight * _scale - _itemHeight) / 2.0) / _scale -
          _itemHeight / 2;
      _moveOffset = Offset(max(left, _moveOffset.dx), max(top, _moveOffset.dy));

      double right = (SizeFit.screenWidth -
                  _itemWidth -
                  (_itemWidth * _scale - _itemWidth) / 2.0) /
              _scale +
          _itemWidth / 2;
      double bottom = (SizeFit.screenHeight -
                  _itemHeight -
                  (_itemHeight * _scale - _itemHeight) / 2.0 -
                  LayoutUtil.getAppBarHeight(context)) /
              _scale +
          _itemHeight / 4;
      _moveOffset =
          Offset(min(right, _moveOffset.dx), min(bottom, _moveOffset.dy));
    });
  }

  void _onScaleEnd(ScaleEndDetails details) {
    print('onScaleEnd');

    setState(() {
      _idleOffset = _moveOffset * _scale;
      _previousScale = _scale;
    });
  }
}

基本功能都使用没什么问题，但是测试中发现在进行缩放的时候总是延迟，手势执行一定时间后widget才开始缩放。
造成这种情况的原因应该是手势冲突，解决的思路是用Listener处理tap事件，GestureDetector只处理缩放和移动事件。代码如下

class GestureDetectorDemo extends StatefulWidget {
 @override
 _GestureDetectorDemoState createState() => _GestureDetectorDemoState();
}

class _GestureDetectorDemoState extends State<GestureDetectorDemo> {
 bool _show = true;
 @override
 Widget build(BuildContext context) {
   // final control = TransformationController();
   // control.
   return _buildPanel();
 }

 //静止状态下的offset
 Offset _idleOffset = Offset(0, 0);
 //本次移动的offset
 Offset _moveOffset = Offset(0, 0);
 //最后一次down事件的offset
 Offset _lastStartOffset = Offset(0, 0);
 double _previousScale = 1;
 double _scale = 1;

 double _itemWidth = 150;
 double _itemHeight = 150;

 Duration _tapDownTimeStamp;

 Widget _buildPanel() {
   return Transform.scale(
     scale: _scale,
     alignment: Alignment(0, 0),
     child: Transform.translate(
       offset: _moveOffset,
       child: Listener(
         onPointerDown: (PointerDownEvent event) {
           // 记录点击时间
           _tapDownTimeStamp = event.timeStamp;
         },
         onPointerUp: (PointerUpEvent event) {
           // 手指抬起时，计算时间差，100ms以内算点击事件
           int interval = event.timeStamp.inMilliseconds -
               _tapDownTimeStamp.inMilliseconds;

           if (interval <= 100) {
             // 处理tap事件
             _onTap();
           }
         },
         child: GestureDetector(
             onScaleStart: _onScaleStart,
             onScaleUpdate: _onScaleUpdate,
             onScaleEnd: _onScaleEnd,
             child: Container(
               color: Colors.red,
               width: _itemWidth,
               height: _itemHeight,
             )),
       ),
     ),
   );
 }

 void _onTap() {
   print('_onTap');
 }

 void _onScaleStart(ScaleStartDetails details) {
   setState(() {
     _lastStartOffset = details.focalPoint;
     print('onScaleStart $_lastStartOffset');
     Offset local = details.localFocalPoint;
   });
 }

 void _onScaleUpdate(ScaleUpdateDetails details) {
   setState(() {
     // 缩放
     _scale = _previousScale * details.scale;
     // 移动
     _moveOffset =
         (details.focalPoint - _lastStartOffset + _idleOffset) / _scale;

     // 控制移动边界
     double left =
         (_itemWidth * _scale - _itemWidth) / 2.0 / _scale - _itemWidth / 2;
     double top = ((_itemHeight * _scale - _itemHeight) / 2.0) / _scale -
         _itemHeight / 2;
     _moveOffset = Offset(max(left, _moveOffset.dx), max(top, _moveOffset.dy));

     double right = (SizeFit.screenWidth -
                 _itemWidth -
                 (_itemWidth * _scale - _itemWidth) / 2.0) /
             _scale +
         _itemWidth / 2;
     double bottom = (SizeFit.screenHeight -
                 _itemHeight -
                 (_itemHeight * _scale - _itemHeight) / 2.0 -
                 LayoutUtil.getAppBarHeight(context)) /
             _scale +
         _itemHeight / 4;
     _moveOffset =
         Offset(min(right, _moveOffset.dx), min(bottom, _moveOffset.dy));
   });
 }

 void _onScaleEnd(ScaleEndDetails details) {
   print('onScaleEnd');

   setState(() {
     _idleOffset = _moveOffset * _scale;
     _previousScale = _scale;
   });
 }
}