將計數器Sample的StatefulWidget修改成只有數字的部份。方法二：使用ValueNotifier物件通知更新。

hello2.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatelessWidget {
  final String title;
  MyHomePage({Key key, this.title}) : super(key: key);

  //建立ValueNotifier物件
  final ValueNotifier<int> counter = ValueNotifier<int>(0);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            //將ValueNotifier傳遞給StatefulWidget物件
            CounterField(counter: counter),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          counter.value++;  //事件觸發時，直接更新ValueNotifier裡的值
        },
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

class CounterField extends StatefulWidget {
  //取得傳入的ValueNotifier物件
  final ValueNotifier<int> counter;
  CounterField({Key key, this.counter}) : super(key: key);

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

class _CounterFieldState extends State<CounterField> {
  @override
  void initState() {
    super.initState();
    //初始化時，訂閱ValueNotifier的更新狀態
    //當ValueNotifier的值異動時，直接刷新物件
    widget.counter?.addListener(() => setState(() {}));
  }

  @override
  Widget build(BuildContext context) {
    return Text(
      '${widget.counter.value}',  //顯示ValueNotifier的值
      style: Theme.of(context).textTheme.display1,
    );
  }
}