Understanding ChangeDetectionStrategy.OnPush in Angular

Angular provides a powerful feature for performance optimization, known as ChangeDetectionStrategy.OnPush. This strategy helps in preventing unnecessary change detection, making your application faster and more efficient.

How does ChangeDetectionStrategy.OnPush Work?

Angular's change detection process goes through every component to check if any data-bound properties have changed and updates the view accordingly. This approach, however, can lead to performance issues, especially when your app has a large component tree.

With ChangeDetectionStrategy.OnPush, Angular will only run the change detection if:

• The Input reference changes
• An event originated from the component itself or one of its children
• The ChangeDetectorRef's markForCheck() method is applied
• An observable linked to the component via the async pipe emits a new value

If none of these conditions are met, Angular skips the component while running change detection, resulting in fewer checks and improved performance.

Practical Application

Consider an Angular application where you have a list of items and each item is a separate component. When an item is updated, without ChangeDetectionStrategy.OnPush, Angular would run change detection on all components, including those that didn't change.

However, by applying ChangeDetectionStrategy.OnPush, Angular would only trigger change detection on the component that was updated. This can considerably optimize your application performance.

@Component({
  selector: 'my-component',
  templateUrl: './my.component.html',
  changeDetection: ChangeDetectionStrategy.OnPush
})

Best Practices

While ChangeDetectionStrategy.OnPush is very beneficial for performance optimization, it should be used carefully.

• Be diligent about ensuring that your inputs are immutable. When mutations occur on an input object, change detection might not work as expected.
• If you find that change detection isn't being triggered when you think it should, consider using ChangeDetectorRef's markForCheck() method to manually trigger it. However, use this sparingly as overuse defeats the purpose of optimizing performance with OnPush.

In summary, ChangeDetectionStrategy.OnPush stands as a crucial feature in Angular for mitigating unnecessary change detection operations, thus contributing to the overall performance optimization of your Angular applications.