Flutter StatefulWidget 有状态组件、页面上绑定数据、改变页面数据

在 Flutter 中自定义组件其实就是一个类，这个类需要继承 StatelessWidget/StatefulWidget。

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

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('Flutter Demo')
        ),
        body: HomePage(),
      )
    );
  }
}

class HomePage extends StatelessWidget {
  int countNum=1;
  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        SizedBox(height: 200),
        Text("${this.countNum}"),
        SizedBox(height: 20),
        RaisedButton(
          child: Text("按钮"),
          onPressed: (){
              // setState()   //错误写法       没法改变页面里面的数据
          this.countNum++;
              print(this.countNum);
          },
        )
      ],
    );
  }
}

---

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('Flutter Demo')
        ),
        body: HomePage(),
      )
    );
  }
}

//自定义有状态组件
class HomePage extends StatefulWidget {
  HomePage({Key key}) : super(key: key);
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  int countNum=0;
  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        SizedBox(height: 200),
        Chip(
          label:Text('${this.countNum}') ,
        ),
        SizedBox(height: 20),
        RaisedButton(
          child: Text('按钮'),
          onPressed: (){
             setState(() {   //只有有状态组件里面才有
                  this.countNum++;
             });
          },
        )
      ],
    );
  }
}

---

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('Flutter Demo')
        ),
        body: HomePage(),
      )
    );
  }
}

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

  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {

  List list=new List();
  @override
  Widget build(BuildContext context) {
    return ListView(

      children: <Widget>[
        Column(
          children: this.list.map((value){
            return ListTile(
              title: Text(value),
            );
          }).toList()
        ),
        SizedBox(height: 20),
        RaisedButton(
          child: Text("添加数据"),
          onPressed: (){
            setState(() {
                this.list.add('新增数据1');
                this.list.add('新增数据2');
            });
          },
        )
      ],
    );
  }
}