This is a very simple Dart package for adding observability to your objects. It does not have a dependency on Flutter, so can be used in Dart scripts or even server-side. It's a slight variation on Flutter's ChangeNotifier, bloc et cetera.

The gist of it is that our objects will be notifying of meaningful events that happen inside of them.

See the full example in example/dart_event_notifier_example.dart.

First define the events that can occur in your objects. Sealed classes and enums are great for this.

sealed class CounterEvent {
  const CounterEvent();
}

final class Incremented extends CounterEvent {
  const Incremented();
}

final class Decremented extends CounterEvent {
  const Decremented();
}

Then write your logic. Mix in the EventNotifer and use notify to post important events that users of your class might want to react to!

class Counter with EventNotifier<CounterEvent> {
 var _value = 0;
 int get value => _value;

 void increment() {
   _value++;
   notify(Incremented());
 }

 void decrement() {
   _value--;
   notify(Decremented());
 }
}

To receive the events, listen to the events stream. Simple as that.

void main() {
  final counter = Counter();

  counter.events.listen((event) {
    print('Received event: $event; current value: ${counter.value}');
  });
}