AngularDart: How do you pass an event from a Child

Question

I am using StreamControllers with Events, and essentially I have a 3 level component heirarchy lets call them, A,B,C. The heirarchy is A -> B -> C.

The origin of the event is in c and i want the event to be processed by A.

I know this is rather easy to do with a direct parent -> child relationship using @Output but not sure how to properly handle multiple levels upwards.

Answer 1

Thanks for asking.

There are a couple ways to do this.

(1) Create an event handler in B that forwards from C

@Component(
  selector: 'b',
  directives: const [C],
  template: '<c (event)="cDidEvent()"></c>',
)
class B {
  final _onEvent = new StreamController();
  Stream get onEvent => _onEvent.stream;

  void cDidEvent() {
    _onEvent.add(null);
  }
}

(2) Use dependency injection.

This requires deeper coupling between components, so it won't be appropriate for all designs, but it could make sense in some scenarios.

abstract class OnEvent {
  /// Called when an event happens.
  void onEvent();
}

@Component(
  selector: 'a',
  directives: const [B],
  template: '<b></b>',
  providers: const [
    const Provider(OnEvent, useExisting: A),
  ],
)
class A implements OnEvent {
  @override
  void onEvent() {
    print('>>> An event was triggered!');
  }
}

class C {
  final OnEvent _eventHandler;

  C(this._eventHandler);

  void onSomeAction() {
    _eventHandler.onEvent();
  }
}

Answer 2

I think it is easier just to create an "event bus" singleton service that is injected in Component A and Component C.

Here is the code:

class Event {
     // your data
}

@Injectable()
class EventBus {

    final StreamController<Event> _onEventStream = new StreamController<Event>();
    Stream<Selection> onEventStream = null;

    static final EventBus _singleton = new EventBus._internal(); 

    factory EventBus() {
         return _singleton;
    }

    EventBus._internal() {
         onEventStream = _onEventStream.stream;
    }

    onEvent(Event event) {
         _onEventStream.add(selection);
    }
}


@Component(
   selector: 'C',
   templateUrl: 'C.html',
   providers: const [
       EventBus
   ]
 )
 class C {

     final EventBus _eventBus;

     C(this._eventBus);

     onAction() {
         _eventBus.onEvent(new Event());
    }
 }


@Component(
    selector: 'A',
    templateUrl: 'A.html',
    providers: const [
        EventBus
    ]
)
class A {

    final EventBus _eventBus;

    A(this._eventBus) {
        _eventBus.onEventStream.listen((Event e) => /* do something with the event */)
    }
}