StatefulWidget StatelessWidget组

• StatelessWidget 是无状态组件，状态不可变的 Widget

• StatefulWidget 是有状态组件，持有的状态可能在 widget 生命周期改变。通俗的讲:如果我们想改变页面中的数据的话这个时候就需要用到 StatefulWidget

• StatelessWidget的例子

import 'package:flutter/material.dart';

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

//MaterialApp组件作为根组件使用
// Scaffold  有下面几个主要属性
// appBar-界面顶部导航栏 body-界面显示的主要内容Widget drawer-抽屉菜单控件

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // TODO: implement buildå
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('导航栏'),
        ),
        body: HomePage(),
      ),
    );
  }
}

class HomePage extends StatelessWidget {
  int countNum = 1;

  //自动生成的常量构造函数
  //变量必须是常量，否则就注释常量构造函数
  // const HomePage({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        SizedBox(height: 200),
        Text('${this.countNum}'),
        SizedBox(height: 50),
        RaisedButton(
          child: Text('刷新+1'),
          onPressed: () {
            this.countNum++;
            print(this.countNum);
          },
        ),
      ],
    );
  }
}

• StateflulWidget组件

import 'package:flutter/material.dart';

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

//MaterialApp组件作为根组件使用
// Scaffold  有下面几个主要属性
// appBar-界面顶部导航栏 body-界面显示的主要内容Widget drawer-抽屉菜单控件

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // TODO: implement buildå
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('导航栏'),
        ),
        body: HomePage(),
      ),
    );
  }
}

class HomePage extends StatefulWidget {
  HomePage({Key key}) : super(key: key);

  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  int countNum = 1;

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        SizedBox(height: 200),
        Text('${this.countNum}'),
        SizedBox(height: 50),
        RaisedButton(
          child: Text('刷新+1'),
          onPressed: () {
            setState(() {
              this.countNum++;
              print(this.countNum);
            });
          },
        ),
      ],
    );
  }
}