如何在Flutter中使用Draggable拖动堆栈中的元素？

import 'package:flutter/material.dart';

Widget draggableWidget() {
  return Draggable(
    child: Container(
      width: 100,
      height: 100,
      color: Colors.blue,
      child: Center(
        child: Text('Draggable'),
      ),
    ),
    feedback: Container(
      width: 100,
      height: 100,
      color: Colors.blue.withOpacity(0.5),
      child: Center(
        child: Text('Dragging'),
      ),
    ),
    childWhenDragging: Container(
      width: 100,
      height: 100,
      color: Colors.blue.withOpacity(0.2),
      child: Center(
        child: Text('Draggable'),
      ),
    ),
  );
}

Widget stackWidget() {
  return Stack(
    children: [
      Positioned(
        left: 0,
        top: 0,
        child: draggableWidget(),
      ),
    ],
  );
}

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Draggable Example'),
        ),
        body: Center(
          child: stackWidget(),
        ),
      ),
    );
  }
}