如何在屏幕中间显示文本，并在Flutter中显示列？

dependencies:
  flutter:
    sdk: flutter

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('Center and Column Example'),
        ),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              Text(
                'Hello, World!',
                style: TextStyle(fontSize: 24),
              ),
              SizedBox(height: 16),
              Text(
                'This is a Flutter app.',
                style: TextStyle(fontSize: 18),
              ),
            ],
          ),
        ),
      ),
    );
  }
}