Flutter学习笔记-Widget

在Flutter中，Widget是构建用户界面的基本单位。每个Widget都是一个StatelessWidget或者StatefulWidget类的实例。

StatelessWidget是不可变的，它的状态不会改变。每次调用build方法都会返回相同的结果。

StatefulWidget可以在其生命周期内改变状态，通过使用State对象来保存状态。

以下是一个简单的Flutter应用程序的代码示例，它包含一个标题为"Hello, World!"的MaterialApp。

import 'package:flutter/material.dart';
 
void main() => runApp(MyApp());
 
class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Hello, World!'),
    );
  }
}
 
class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, 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.
    );
  }
}

在这个例子中，MyApp是一个StatelessWidget，它创建了一个带有标题和主题的MaterialApp。MyHomePage是一个StatefulWidget，它有一个状态_MyHomePageState，可以通过点击FloatingActionButton来增加计数。这个例子展示了如何在Flutter中创