hello flutter

//导入包
import 'package:flutter/material.dart';
 
void main() {
  /**
   * 应用入口
   */
  runApp(const MyApp());
}
 
class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);
 
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      //应用名称
      title: 'Flutter Demo',
      theme: ThemeData(
        //应用主题
        primarySwatch: Colors.blue,
      ),
      //首页路由
      home: const MyHomePage(title: 'hello world'),
    );
  }
}
/*
    StatefulWidget类，表示它是一个有状态的组件 可以拥有状态
    Stateless widget 是不可变的
    Stateful widget 至少由两个类组成 一个StatefulWidget类。
    一个 State类； StatefulWidget类本身是不变的，但是State类中持有的状态在 widget 生命周期中可能会发生变化。
    _MyHomePageState类是MyHomePage类对应的状态类
    MyHomePage类中并没有build方法
    取而代之的是，build方法被挪到了_MyHomePageState
 */
 
class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);
  final String title;
 
  @override
  _MyHomePageState createState() => _MyHomePageState();
}
 
class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;
 
  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }
 
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text('You have pushed the button this many times:'),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}