flutter创建可移动的stack小部件

import 'package:flutter/material.dart';

class MoveableStack extends StatefulWidget {
  @override
  _MoveableStackState createState() => _MoveableStackState();
}

class _MoveableStackState extends State<MoveableStack> {
  Offset _offset = Offset(0, 0);

  void _updatePosition(Offset offset) {
    setState(() {
      _offset += offset;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Stack(
      children: <Widget>[
        Positioned(
          left: _offset.dx,
          top: _offset.dy,
          child: GestureDetector(
            onPanUpdate: (PanUpdateDetails details) => _updatePosition(details.globalPosition),
            child: Container(
              color: Colors.blue,
              width: 100,
              height: 100,
            ),
          ),
        ),
      ],
    );
  }
}