How to write unit test for ActionsSubject subscription in Angular Component

I have an Angular component that uses ngrx ActionsSubject to listen to dispatched actions. How can I write a test that can check this subscription of ActionsSubject?

The component looks like this:

export class TestComponent {

constructor(public actionsSubject: ActionsSubject) { }

  public isUpdated = false;

  public ngOnInit() {
    this.actionSubscription = this.actionsSubject.subscribe(action => {
      if (action.type === "Update") {
        this.isUpdated = true; <----- I want to test this condition
      }
    });
  }
}

Currently, I am dispatching update action manually:

  it('isUpdated should be true, when update action dispatched', () => {
    component.store.dispatch(new UpdateAction());
    expect(component.isUpdated).toEqual(true);
  });

It works, but I want to mock this subscription instead of calling action manually.

For example:

  it('isUpdated should be true, when update action dispatched', () => {
    let actionSubject = TestBed.get(ActionsSubject);
    const action = {
      type: 'Update'
    };
    actionSubject = of({ action });
    component.ngOnInit();
    fixture.detectChanges()
    expect(component.isUpdated).toEqual(true);
  });

But it never triggers the subscription in the component. How can I test it efficiently?