Flutter StatefulWidget 有狀態組件、頁面上綁定數據、改變頁面數據

在 Flutter 中自定義組件其實就是一個類，這個類需要繼承 StatelessWidget/StatefulWidget。

StatelessWidget 是無狀態組件，狀態不可變的 widget
StatefulWidget 是有狀態組件，持有的狀態可能在 widget 生命周期改變。通俗的講:如果我 們想改變頁面中的數據的話這個時候就需要用到 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("按鈕"),
          onPressed: (){
              // setState()   //錯誤寫法       沒法改變頁面里面的數據
          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(),
      )
    );
  }
}


//自定義有狀態組件
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('按鈕'),
          onPressed: (){
             setState(() {   //只有有狀態組件里面才有
                  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("添加數據"),
          onPressed: (){
            setState(() {
                this.list.add('新增數據1');
                this.list.add('新增數據2');
            });
          },
        )
      ],
    );
  }
}