Flutter: Simple counter using ChangeNotifierProvider

main.dart

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

class Counter extends ChangeNotifier {
  Counter(int initialValue) : _value = initialValue;

  int _value;
  set value(int newValue) {
    _value = newValue;
    notifyListeners();
  }
  int get value => _value;
}

void main() {
  runApp(
    ChangeNotifierProvider<Counter>(
      builder: (_) => Counter(0),
      child: MyApp(),
    ),
  );
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'ChangeNotifierProvider Demo',
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  void _incrementCounter(BuildContext context) {
    final counter = Provider.of<Counter>(context);
    counter.value++;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text('You have pushed the button this many times:'),
            Consumer<Counter>(
              builder: (context, counter, _) => Text(
                    '${counter.value}',
                    style: Theme.of(context).textTheme.display1,
                  ),
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () => _incrementCounter(context),
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}