bug(core): EventEmitter#subscribe() should be properly typed

[jorroll]

🚀 feature request

Relevant Package

This feature request is for @angular/core

Description

Currently, subscribing to an EventEmitter (via EventEmitter#subscribe()) stripes the EventEmitter's type information and converts it to any. This is undesirable, as already pointed out in #6934.

That issue was closed with a nondescript explanation that ~ "you shouldn't use the EventEmitter's subscribe() method". However, EventEmitter#subscribe() is not marked as internal API.

The suggestion that someone should create a duplicate property, using an rxjs subject, solely for the purpose of improved typing, is unsatisfactory.

This is a request to do whatever needs to be done to allow subscribing to EventEmitter directly while preserving type information.

Describe the solution you'd like

The type information, which is already present on EventEmitter, should be retained.

Describe alternatives you've considered

Currently, the suggested workaround seems to be to create a duplicate property and update both the @Output() property and the duplicate Subject property whenever the associated data changes.

e.g.

class MyComponent {
  @Input() name: string;
  
  @Output()
  nameChange = new EventEmitter<string>();
  nameChange2 = new Subject<string>();

  @Input() weather: {prediction: string};
  
  @Output()
  weatherChange = new EventEmitter<{prediction: string}>();
  weatherChange2 = new Subject<{prediction: string}>();

  private updateName(name: string) {
    this.name = name;
    this.nameChange.emit(name);
    this.nameChange2.next(name);
  }

  private weatherName(value: {prediction: string}) {
    this.weather = value;
    this.weatherChange.emit(value)
    this.weatherChange2.next(value)
  }
}
``

[william-lohan]

What about this workaround

this.weatherChange.asObservable().subscribe(weather => {});

mouse over weather is { prediction: string; } not any

https://stackblitz.com/edit/angular-issues-29016?file=src%2Fapp%2Fmy%2Fmy.component.ts

[jorroll]

@william-lohan thanks! That's good enough for me.

[Airblader]

I still think this should be typed correctly right away, so I would vote to not close this.

[jorroll]

@Airblader ok, I'll leave open for now.

[michaeljota]

I made a simple implementation based on overloads to follow the current implementation. If you want, I can make a PR with this

playground link

In the Playground all three are showed, but with the overload, only the first two will, and also will allow to discriminate between the calls.