检索运行时更改的ThemeData问题

import 'package:flutter/material.dart';

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

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  ThemeData _themeData = ThemeData.light();

  void _changeTheme() {
    setState(() {
      _themeData = _themeData == ThemeData.light()
          ? ThemeData.dark()
          : ThemeData.light();
    });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: _themeData,
      home: Scaffold(
        appBar: AppBar(
          title: Text('Dynamic Theme'),
        ),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              Text(
                'Hello, World!',
                style: TextStyle(fontSize: 24),
              ),
              RaisedButton(
                child: Text('Change Theme'),
                onPressed: _changeTheme,
              ),
            ],
          ),
        ),
      ),
    );
  }
}