Simple stream-based state management in Flutter/Dart

state.dart

import 'dart:async';

import 'package:async/async.dart';
import 'package:flutter/widgets.dart';

/*
This class is a little bit different than the [ValueNotifier] class:
 - Setting the value ALWAYS notifies listeners, even if the value is the same.
 - It uses streams over [ValueListenableBuilder]'s setState approach to make it
   easier to use with other async code.
*/
class Observable<T> {
  T _value;
  T get value => _value;
  set value(T v) {
    _value = v;
    _controller.add(v);
  }

  void notify() {
    _controller.add(_value);
  }

  void setIfChanged(T v) {
    if (v != _value) {
      value = v;
    }
  }

  Observable(this._value);

  final _controller = StreamController<T>.broadcast();
  Stream<T> get stream => _controller.stream;
}

class Observer<T> extends StatelessWidget {
  final Observable<T> observable;
  final Widget Function(BuildContext, T) builder;
  const Observer({required this.observable, required this.builder, Key? key}) : super(key: key);
  @override
  Widget build(context) {
    return StreamBuilder<T>(
      stream: observable.stream,
      builder: (context, value) {
        return builder(context, observable.value);
      },
    );
  }
}

class MultiObserver extends StatelessWidget {
  final List<Observable> observables;
  final Widget Function(BuildContext, List) builder;
  const MultiObserver({required this.observables, required this.builder, Key? key})
      : super(key: key);
  @override
  Widget build(context) {
    return StreamBuilder(
      stream: StreamGroup.merge(observables.map((o) => o.stream)),
      builder: (context, value) {
        return builder(context, observables.map((o) => o.value).toList());
      },
    );
  }
}

/// A simplified [Observable] with "no" values
class Notifier extends Observable<bool> {
  Notifier() : super(true);
}

class NotifierBuilder extends StatelessWidget {
  final Notifier notifier;
  final Widget Function(BuildContext) builder;
  const NotifierBuilder({required this.notifier, required this.builder, Key? key})
      : super(key: key);
  @override
  Widget build(context) {
    return StreamBuilder(
      stream: notifier.stream,
      builder: (context, _) {
        return builder(context);
      },
    );
  }
}