如何在flutter中创建子菜单下拉菜单

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Submenu Dropdown Menu'),
        ),
        body: Center(
          child: PopupMenuButton<String>(
            onSelected: (value) {
              // 处理菜单项的选择
              print('Selected: $value');
            },
            itemBuilder: (BuildContext context) => <PopupMenuEntry<String>>[
              PopupMenuItem<String>(
                value: 'item1',
                child: Text('Item 1'),
              ),
              PopupMenuItem<String>(
                value: 'item2',
                child: Text('Item 2'),
              ),
              PopupMenuDivider(),
              PopupMenuItem<String>(
                value: 'submenu',
                child: PopupMenuButton<String>(
                  itemBuilder: (BuildContext context) => <PopupMenuEntry<String>>[
                    PopupMenuItem<String>(
                      value: 'subitem1',
                      child: Text('Subitem 1'),
                    ),
                    PopupMenuItem<String>(
                      value: 'subitem2',
                      child: Text('Subitem 2'),
                    ),
                  ],
                  child: Text('Submenu'),
                ),
              ),
            ],
            child: Text('Open Menu'),
          ),
        ),
      ),
    );
  }
}