Counter Notifier

main.dart

import 'package:flutter/material.dart';

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

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

class HomePage extends StatelessWidget {
  const HomePage({
    Key? key,
    required this.title,
  }) : super(key: key);

  final String title;

  @override
  Widget build(BuildContext context) {
    return CounterScope(
      notifier: CounterController(),
      child: Scaffold(
        appBar: AppBar(
          title: Text(title),
        ),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: const [
              Text(
                'You have pushed the button this many times:',
              ),
              CounterDisplay(),
            ],
          ),
        ),
        floatingActionButton: const IncrementButton(),
      ),
    );
  }
}

class CounterDisplay extends StatelessWidget {
  const CounterDisplay({super.key});

  @override
  Widget build(BuildContext context) {
    return Text(
      '${CounterScope.of(context).value}',
      style: Theme.of(context).textTheme.headlineMedium,
    );
  }
}

class IncrementButton extends StatelessWidget {
  const IncrementButton({super.key});

  @override
  Widget build(BuildContext context) {
    return Builder(
      builder: (context) => FloatingActionButton(
        onPressed: CounterScope.of(context).increment,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ),
    );
  }
}

class CounterScope extends InheritedNotifier<CounterController> {
  const CounterScope({
    super.key,
    required super.child,
    super.notifier,
  });

  static CounterController of(BuildContext context) {
    final CounterScope? result =
        context.dependOnInheritedWidgetOfExactType<CounterScope>();

    assert(result != null, 'No CounterScope found in context');
    assert(result!.notifier != null, 'No CounterController found in context');

    return result!.notifier!;
  }
}

class CounterController extends ValueNotifier<int> {
  CounterController([int initialValue = 0]) : super(initialValue);

  void increment() {
    value = value + 1;
  }

  void decrement() {
    value = value - 1;
  }
}