HostListener is still listening after ngOnDestroy started

[MarsiBarsi]

🐞 bug report

Affected Package

@angular/core

Is this a regression?

No, I caught this bug in a version 7 first

Description

There can be some situations in that we need to dispatch an event on component destroy.
If we have HostListener listening type of event we dispatch, it catches this.

So, in other words, HostListener continues listening even after ngOnDestroy has already started.

Is it a bug or by design?

🔬 Minimal Reproduction

https://stackblitz.com/edit/angular-hostlintener-bug?file=src%2Fapp%2Fhello.component.ts

🔥 Exception or Error

🌍 Your Environment

Angular Version:

10.0.1 and 
7.22.0

[crisbeto]

This might need some discussion, because I can see it going the other way. E.g. if you want to fire off one last listener before the component is destroyed. Also the behavior is consistent between ViewEngine and Ivy.

For future reference, here's a unit test that shows the behavior:

it('', () => {
  let counter = 0;

  @Directive({selector: '[hasHostListener]'})
  class DirectiveWithHostListener implements OnDestroy {
    constructor(private _elementRef: ElementRef<HTMLElement>) {}

    @HostListener('click')
    clicked() {
      counter++;
    }

    ngOnDestroy() {
      this._elementRef.nativeElement.click();
    }
  }

  @Component({template: `<button *ngIf="showButton" hasHostListener>Click me!</button>`})
  class App {
    showButton = true;
  }

  TestBed.configureTestingModule({declarations: [App, DirectiveWithHostListener]});
  const fixture = TestBed.createComponent(App);
  fixture.detectChanges();

  fixture.componentInstance.showButton = false;
  fixture.detectChanges();
  expect(counter).toBe(0);
});

[waterplea]

It might be unclear which behavior is better but to the argument of the last listener there's a counter argument that a lot of the time events are handled with RxJS and common pattern is takeUntil(this.destroyedStream). This pattern completes manually in ngOnDestroy so past ngOnDestroy no events would be handled. Seems reasonable to me that we have hook that guarantees to be the end of the lifecycle, while currently it seems not to be the case.