如何在Flutter中使用flex实现嵌套的列/行

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('Flex布局示例'),
        ),
        body: Flex(
          direction: Axis.vertical,
          children: <Widget>[
            Expanded(
              child: Flex(
                direction: Axis.horizontal,
                children: <Widget>[
                  Expanded(
                    child: Text('子组件1'),
                    flex: 1,
                  ),
                  Expanded(
                    child: Text('子组件2'),
                    flex: 2,
                  ),
                ],
              ),
            ),
            Expanded(
              child: Text('子组件3'),
              flex: 1,
            ),
          ],
        ),
      ),
    );
  }
}