OnPush component in dev mode will only trigger change detection once.

[JiaLiPassion]

Consider the following case.

      @Component({
        template: `
          <button (click)="noop()">Trigger change detection</button>
          <div>{{increment('componentView')}}</div>
        `,
        changeDetection: ChangeDetectionStrategy.OnPush
      })
      class App {
        increment() {
          counters++;
        }
        noop() {}
      }
      const fixture = TestBed.createComponent(App);
      fixture.detectChanges();
      expect(counters).toEqual(2); // current behavior is 1 

The reason is App is a OnPush component with a hostView, calling fixture.detectChanges() will run a detectChanges() and checkNoChanges() .
And the App component is onPush, so the checkNoChanges() run will ignore App component, so the counters will be 1
Which is not correct.

Another case is:

    it('works', () => {
      @Component({
        template: '{{message}}',
        changeDetection: ChangeDetectionStrategy.OnPush
      })
      class OnPushCmp {
        @Input() message = 'initial';
        ngAfterViewInit() {
          this.message = 'updated';
        }
      }
      const comp = TestBed.configureTestingModule({declarations: [OnPushCmp]}).createComponent(OnPushCmp);
      expect(() => comp.detectChanges()).toThrow(); // current behavior is not throw.
    });

The reason is the same, the checkNoChanges() will skip the OnPushCmp, so the ExpressionChanged error is not thrown.

[pkozlowski-opensource]

This issues is raising 2 separate problems for which we've got tracking issues already:

• the first issue is a tracked under TestBed interaction with the OnPush components: TestBed: rethink interactions with the @Input / @output of the root component #12313
• the second issue is about the problem of the OnPush components not throwing ExpressionChangedAfterItHasBeenCheckedError: ExpressionChangedAfterItHasBeenCheckedError is not thrown if ChangeDetection.OnPush is used #45612

I'm going to close this issue in favour of the existing ones as those are more specific.

Duplicate of #12313
Duplicate of #45612

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