Example of state management without Libraries - by Simon Lightfoot - Humpday Q&A :: 13th December 2023 #Flutter #Dart - https://www.youtube.com/live/7k_tMJF1B-0?si=l5u-AN9YrpN_il9i&t=4402

humpday_131223.dart

// MIT License
//
// Copyright (c) 2023 Simon Lightfoot
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
//
import 'package:flutter/material.dart';

void main() {
  runApp(const ExampleApp());
}

@immutable
class AppState {
  const AppState({
    required this.counter,
  });

  final int counter;

  static const initial = AppState(counter: 0);

  AppState copyWith({
    int? counter,
  }) {
    return AppState(
      counter: counter ?? this.counter,
    );
  }
}

class AppStateRepo extends ChangeNotifier {
  AppStateRepo() : _state = AppState.initial;

  AppState _state;

  AppState get state => _state;

  void increaseCount(int increase) {
    _state = _state.copyWith(
      counter: _state.counter + increase,
    );
    print('inc: ${_state.counter}');
    notifyListeners();
  }
}

class ExampleApp extends StatefulWidget {
  const ExampleApp({super.key});

  static AppStateRepo appStateRepo(BuildContext context) {
    return context.findAncestorStateOfType<_ExampleAppState>()!.appStateRepo;
  }

  @override
  State<ExampleApp> createState() => _ExampleAppState();
}

class _ExampleAppState extends State<ExampleApp> {
  final appStateRepo = AppStateRepo();

  @override
  Widget build(BuildContext context) {
    return ListenableBuilder(
      listenable: appStateRepo,
      builder: (BuildContext context, Widget? child) {
        return AppStateScope(
          appState: appStateRepo.state,
          child: child!,
        );
      },
      child: const MaterialApp(
        debugShowCheckedModeBanner: false,
        home: HomeScreen(),
      ),
    );
  }
}

class AppStateScope extends InheritedWidget {
  const AppStateScope({
    super.key,
    required this.appState,
    required super.child,
  });

  final AppState appState;

  static AppState of(BuildContext context) {
    return context
        .dependOnInheritedWidgetOfExactType<AppStateScope>()!
        .appState;
  }

  @override
  bool updateShouldNotify(AppStateScope oldWidget) {
    return appState != oldWidget.appState;
  }
}

mixin AppStateRepoMixin<T extends StatefulWidget> on State<T> {
  late final AppStateRepo appStateRepo = context.appStateRepo;
  late final AppState appState = context.appStateRepo.state;
}

extension AppStateRepoExtension on BuildContext {
  AppStateRepo get appStateRepo => ExampleApp.appStateRepo(this);

  AppState get appState => AppStateScope.of(this);
}

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Example')),
      body: const PageContent(),
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return SizedBox.expand(
      child: DefaultTextStyle.merge(
        style: const TextStyle(fontSize: 32.0),
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text('Count: ${context.appState.counter}'),
            const SizedBox(height: 32.0),
            if (context.appState.counter < 5) //
              const IncreaseCounterButton(increase: 1, label: 'First'),
            const SizedBox(height: 32.0),
            const IncreaseCounterButton(increase: 2, label: 'Second'),
          ],
        ),
      ),
    );
  }
}

class IncreaseCounterButton extends StatelessWidget {
  const IncreaseCounterButton({
    super.key,
    required this.increase,
    required this.label,
  });

  final int increase;
  final String label;

  @override
  Widget build(BuildContext context) {
    return InkWell(
      onTap: () => context.appStateRepo.increaseCount(increase),
      child: Padding(
        padding: const EdgeInsets.all(12.0),
        child: Text('Button $label'),
      ),
    );
  }
}