如何在flutter中实现这种效果/布局？

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        appBar: AppBar(
          title: Text('Flutter Demo'),
        ),
        body: Center(
          child: Container(
            width: 200,
            height: 200,
            color: Colors.red,
            child: Text(
              'Hello, Flutter!',
              style: TextStyle(fontSize: 24, color: Colors.white),
            ),
          ),
        ),
      ),
    );
  }
}