FakeAsync/tick (Async/whenStable) vs detectChanges

Question

Could you please help me differentiate those two things.

According to my understanding, if you only work with observable, you can use detectChanges(). So you can change a component property directly or spy on a service call and return an observable, then you call detectChanges(), the changes will be available on html elements.

But for [(ngModel)] on input fields you need to call tick() for the changes to be render on html element.

So I'd it would be great if I know what they do and when to use what.

Thanks in advance.

Answer 1

detectChanges

The method detectChanges is available on the ViewRef.

class ViewRef extends ChangeDetectorRef {
  // inherited from core/ChangeDetectorRef
  markForCheck(): void   <-----------------------------
  detach(): void
  detectChanges(): void
  checkNoChanges(): void
  reattach(): void
}

ViewRef is an underlying representation of a component. When writing tests instead of ViewRef another abstraction is introduced which is fixture:

fixture = TestBed.createComponent(BannerComponent);

It wraps the component similar to ViewRef.

detectChanges method runs change detection for the underlying component and performs the following operations:

• update input bindings properties
• update DOM

and many others.

To learn more you can read Everything you need to know about change detection in Angular. So in order to validate changes in the DOM or validate input bindings you need to run detectChanges.

tick

Angular docs describe it pretty well:

The tick function is one of the Angular testing utilities and a companion to fakeAsync. You can only call it within a fakeAsync body.

Calling tick() simulates the passage of time until all pending asynchronous activities finish, including the resolution of the getQuote promise in this test case.

With ngModel you need to call it because the control that is created inside ngModel is registered asynchonously. Here is the quote from the article by Victor Savkin on forms:

To make it work, NgModel doesn’t add a form control synchronously — it does it in a microtask. In the example above, the three ngModels will schedule three microtasks to add the speaker, title, and highRating controls.