draggable.dart

class MyDraggable extends StatefulWidget {
  final Widget child;
  final Widget childWhenDragging;
  final Widget feedback;
  final VoidCallback onDragEnd;
  MyDraggable({
    @required this.child,
    @required this.childWhenDragging,
    @required this.feedback,
    this.onDragEnd
  });

  @override
  State createState() {
    return MyDraggableState();
  }
}

class MyDraggableState extends State<MyDraggable> {
  OverlayEntry _dragOoverlayEntry;
  Offset _dragPosition;
  bool _isDragging = false;

  void _setIsDragging(bool isDragging) {
    setState(() {
      _isDragging = isDragging;
    });
  }

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      child: _isDragging ? this.widget.childWhenDragging : this.widget.child,
      onPanStart: (e) {
        _setIsDragging(true);

        _dragPosition = e.globalPosition;

        final overlayState = Overlay.of(context);
        _dragOoverlayEntry = OverlayEntry(builder: _buildOverlayEntry);

        overlayState.insert(_dragOoverlayEntry);
      },
      onPanUpdate: (e) {
        _dragPosition += e.delta;
        _dragOoverlayEntry.markNeedsBuild();
      },
      onPanCancel: () {
        this._stopDragging();
      },
      onPanEnd: (e) {
        this._stopDragging();
      },
    );
  }

  Widget _buildOverlayEntry(BuildContext context) {
    return Positioned(
        left: _dragPosition.dx,
        top: _dragPosition.dy,
        child: this.widget.feedback);
  }

  void _stopDragging() {
    if (this.widget.onDragEnd != null) {
      this.widget.onDragEnd();
    }
    
    _setIsDragging(false);
    if (_dragOoverlayEntry != null) {
      _dragOoverlayEntry.remove();
      _dragOoverlayEntry = null;
    }
  }
}