app_model.dart

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

class AppModel extends InheritedWidget {
  const AppModel({
    Key? key,
    required Widget child,
  }) : super(key: key, child: child);

  static V get<K, V>(BuildContext context, K key) {
    final el = context.getElementForInheritedWidgetOfExactType<AppModel>()
        as AppModelElement;

    el.subscribe(context as Element, key);
    return el.get<K, V>(key);
  }

  static void set<K, V>(BuildContext context, K key, V value) {
    final el = context.getElementForInheritedWidgetOfExactType<AppModel>()
        as AppModelElement;

    el.set<K, V>(key, value);
  }

  @override
  bool updateShouldNotify(covariant InheritedWidget oldWidget) {
    return false;
  }

  @override
  InheritedElement createElement() {
    return AppModelElement(this);
  }
}

class AppModelElement extends InheritedElement {
  AppModelElement(InheritedWidget widget) : super(widget);

  final Map<Object?, Set<Element>> _subscribers = {};
  final Map _storage = {};

  V get<K, V>(K key) {
    final value = _storage[key];

    if (null is V && value == null) return value;
    return value as V;
  }

  void set<K, V>(K key, V value) {
    _storage[key] = value;
    _notify(key);
  }

  void subscribe(Element dependent, Object? aspect) {
    _subscribers[aspect] ??= {};
    _subscribers[aspect]!.add(dependent);

    setDependencies(dependent, this);
  }

  void unsubscribe(Element element, Object? aspect) {
    _subscribers[aspect]?.remove(element);
  }

  @override
  void updateDependencies(Element dependent, Object? aspect) {
    subscribe(dependent, aspect);
  }

  _notify(Object? key) {
    final subscribers = _subscribers[key];
    final _inactive = [];

    subscribers?.forEach((element) {
      // subscriber element was deactivated and removed itself from dependants
      if (getDependencies(element) != this) {
        _inactive.add(element);
      } else {
        element.markNeedsBuild();
      }
    });

    for (var element in _inactive) {
      subscribers?.remove(element);
    }
  }

  @override
  void unmount() {
    super.unmount();
    _subscribers.clear();
  }
}

// Displays the value of AppModel.get(context, appModelKey) and the number of times
// that this widget has been built.

class ShowAppModelValue<K, V> extends StatefulWidget {
  const ShowAppModelValue({
    Key? key,
    required this.modelKey,
  }) : super(key: key);

  final K modelKey;

  @override
  _ShowAppModelValueState createState() => _ShowAppModelValueState<K, V>();
}

class _ShowAppModelValueState<K, V> extends State<ShowAppModelValue> {
  int _buildCount = 0;

  @override
  Widget build(BuildContext context) {
    _buildCount += 1;
    return Padding(
      padding: const EdgeInsets.only(bottom: 16),
      child: Text(
        '${widget.modelKey}: ${AppModel.get<K, V>(context, widget.modelKey)} [$_buildCount]',
      ),
    );
  }
}

// Demonstrates that changes to the AppModel _only_ cause the dependent widgets
// to be rebuilt.

class Home extends StatefulWidget {
  const Home({Key? key}) : super(key: key);

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

class _HomeState extends State<Home> {
  int _fooCount = 0;
  int _barCount = 0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: <Widget>[
            const ShowAppModelValue<String, String?>(modelKey: 'Home'),
            const ShowAppModelValue<String, String?>(modelKey: 'foo'),
            const ShowAppModelValue<String, String?>(modelKey: 'bar'),
            ElevatedButton(
              child: const Text('foo = FOO'),
              onPressed: () {
                _fooCount += 1;
                AppModel.set<String, String>(
                  context,
                  'foo',
                  'FOO $_fooCount',
                ); // note: no setState()
              },
            ),
            const SizedBox(height: 16),
            ElevatedButton(
              child: const Text('bar = BAR'),
              onPressed: () {
                _barCount += 1;
                AppModel.set<String, String>(
                  context,
                  'bar',
                  'BAR $_barCount',
                ); // note: no setState()
              },
            ),
          ],
        ),
      ),
    );
  }
}

void main() {
  runApp(
    AppModel(child: const MaterialApp(home: Home())),
  );
}