Change Detection Strategy: OnPush

To inform Angular that we are going to comply with the conditions mentioned before to improve performance, we will use the OnPush change detection strategy on the MovieComponent.

app/movie.component.ts

@Component({
// ...
changeDetection: ChangeDetectionStrategy.OnPush
})
export class MovieComponent {
// ...
}

View Example

This will inform Angular that our component only depends on its inputs and that any object that is passed to it should be considered immutable. This time when we click the "Change Actor Properties" button nothing changes in the view.

Let's follow the logic behind it again. When the user clicks the button, the method changeActorProperties is called and the properties of the actor object get updated.

When the change detection analyzes the properties bound to the AppComponent's template, it will see the same picture as before:

  • Is slogan !== previousSlogan No, it's the same.

  • Is title !== previousTitle? No, it's the same.

  • Is actor !== previousActor? No, it's the same.

But this time, we explicitly told Angular that our component only depends on its inputs and all of them are immutable. Angular then assumes that the MovieComponent hasn't changed and will skip the check for that component. Because we didn't force the actor object to be immutable, we end up with our model out of sync with the view.

Let's rerun the app but this time we will click the "Change Actor Object" button. This time, we are creating a new instance of the Actor class and assigning it to the this.actor object. When change detection analyzes the properties bound to the AppComponent's template it will find:

  • Is slogan !== previousSlogan No, it's the same.

  • Is title !== previousTitle? No, it's the same.

  • Is actor !== previousActor? Yes, it has changed.

Because change detection now knows that the actor object changed (it's a new instance) it will go ahead and continue checking the template for MovieComponent to update its view. At the end, our templates and models are in sync.