將計數器Sample的StatefulWidget修改成只有數字的部份。方法三：使用StreamController與Provider注入。

hello3.dart

import 'dart:async';
import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

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

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  //建立StreamController，並宣告注入值的型態
  final _controller = StreamController<int>();

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: StreamProvider.value(
          initialData: 0, //設定注入的初始值
          stream: _controller.stream,
          child: MyHomePage(
              title: 'Flutter Demo Home Page', controller: _controller)),
    );
  }

  @override
  void dispose() {
    //StreamController寫成區域變數要close()
    _controller.close();
    super.dispose();
  }
}

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

  @override
  Widget build(BuildContext context) {
    //取得上層注入的值
    int _counter = Provider.of<int>(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:',
            ),
            CounterField(),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          controller.add(++_counter); //更新注入的值
        },
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

class CounterField extends StatefulWidget {
  @override
  _CounterFieldState createState() => _CounterFieldState();
}

class _CounterFieldState extends State<CounterField> {
  @override
  Widget build(BuildContext context) {
    //取得上層注入的值
    int _counter = Provider.of<int>(context);

    return Text(
      '$_counter', //顯示注入的值
      style: Theme.of(context).textTheme.display1,
    );
  }
}