Immutable State and StatelessWidgets Without Library

main.dart

import 'package:flutter/material.dart';

@immutable
class Counter {
  const Counter(this.count);

  final int count;

  Counter copyWith({int? count}) => Counter(count ?? this.count);
}

class CounterNotifier extends ValueNotifier<Counter> {
  CounterNotifier(super.value);

  void increment() => value = value.copyWith(count: value.count + 1);
}

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  MyApp({super.key});

  final CounterNotifier counter = CounterNotifier(const Counter(0));

  @override
  Widget build(BuildContext context) => MaterialApp(
        title: 'Flutter Demo',
        theme: ThemeData(
          primarySwatch: Colors.blue,
        ),
        home: MyHomePage(
          title: 'Flutter Demo Home Page',
          counter: counter,
        ),
      );
}

class MyHomePage extends StatelessWidget {
  const MyHomePage({
    required this.title,
    required this.counter,
    super.key,
  });

  final String title;
  final CounterNotifier counter;

  @override
  Widget build(BuildContext context) => Scaffold(
        appBar: AppBar(
          title: Text(title),
        ),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              const Text(
                'You have pushed the button this many times:',
              ),
              AnimatedBuilder(
                animation: counter,
                builder: (context, widget) => Text(
                  '${counter.value.count}',
                  style: Theme.of(context).textTheme.headline4,
                ),
              ),
            ],
          ),
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: counter.increment,
          tooltip: 'Increment',
          child: const Icon(Icons.add),
        ),
      );
}