Inherited Widget + Notifications

flutter_model_notification.dart

import 'package:flutter/material.dart';

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

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

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  final model = CounterModel();
  @override
  Widget build(BuildContext context) {
    return Model<CounterModel>(
      value: model,
      child: NotificationListener<CounterUpdate>(
        onNotification: (notification) {
          if (mounted)
            setState(() {
              model.value = notification.value;
            });
          return true;
        },
        child: Scaffold(
          appBar: AppBar(
            title: Text(widget.title),
          ),
          body: CounterDisplay(),
          floatingActionButton: CounterActions(),
        ),
      ),
    );
  }
}

class CounterActions extends StatelessWidget {
  const CounterActions({
    Key key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final model = Model.of<CounterModel>(context);
    return FloatingActionButton(
      onPressed: () => CounterUpdate(model.value + 1)..dispatch(context),
      tooltip: 'Increment',
      child: Icon(Icons.add),
    );
  }
}

class CounterDisplay extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final model = Model.of<CounterModel>(context);
    return Center(
      child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          Text(
            'You have pushed the button this many times:',
          ),
          Text(
            '${model.value}',
            style: Theme.of(context).textTheme.headline4,
          ),
        ],
      ),
    );
  }
}

class CounterUpdate extends Notification {
  final int value;

  CounterUpdate(this.value);
}

class CounterModel {
  int value = 0;
}

class Model<T> extends InheritedWidget {
  Model({
    Key key,
    @required this.value,
    @required this.child,
  }) : super(key: key, child: child);

  final Widget child;

  final T value;

  static T of<T>(BuildContext context) {
    return context.dependOnInheritedWidgetOfExactType<Model<T>>()?.value;
  }

  @override
  bool updateShouldNotify(Model oldWidget) {
    return oldWidget != this;
  }

  @override
  bool operator ==(Object o) {
    if (identical(this, o)) return true;

    return o is Model && o.child == child && o.value == value;
  }

  @override
  int get hashCode => child.hashCode ^ value.hashCode;
}