simpleproviderdemo

//punspec.yaml
//add provider dependency
dependencies:
  flutter:
    sdk: flutter
  provider: ^4.1.3


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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      //notify povider
      home: ChangeNotifierProvider<MyCounter>(
        create: (_) => MyCounter(),
        child: MyHomePage(title: 'Flutter Demo Home Page'),
      ),
      //end notifying
    );
  }
}

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

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    final counterModel = Provider.of<MyCounter>(context);
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '${counterModel.getCounter()}',
              style: Theme.of(context).textTheme.headline1,
            ),
          ],
        ),
      ),
      floatingActionButton: Column(
        mainAxisAlignment: MainAxisAlignment.end,
        children: <Widget>[
          FloatingActionButton(
            onPressed: () => counterModel.incrementCounter(),
            tooltip: 'Increment',
            child: Icon(Icons.add),
          ),
          SizedBox(height: 10),
          FloatingActionButton(
            onPressed: () => counterModel.decrementCounter(),
            tooltip: 'Decrease',
            child: Icon(Icons.delete_outline),
          ),
        ],
      ),
    );
  }
}

//add a new class to listen
class MyCounter with ChangeNotifier {
  int _counter = 0;

  getCounter() => _counter;
  setCounter(int counter) => _counter = counter;

  void incrementCounter() {
    _counter++;
    notifyListeners();
  }

  void decrementCounter() {
    _counter--;
    notifyListeners();
  }
}
//end listener class