Understanding AfterViewChecked in Angular

AfterViewChecked is a lifecycle hook in Angular that is called after the change detection phase has completed and the view has been checked for any changes. This hook is called each time a component’s view is checked, which can result in multiple calls during the lifecycle of a component.

In this article, we will dive deep into AfterViewChecked and see how it can be used to perform different actions in an Angular application.

When is AfterViewChecked called?

AfterViewChecked is called after every change detection cycle. In Angular, the change detection cycle is triggered by various events such as user input, HTTP requests, timers, and so on. When a change detection cycle is triggered, Angular checks each component’s view for any changes, and if there are any changes, it updates the view accordingly. AfterViewChecked hook is called once the change detection cycle is completed and the view has been updated.

What can AfterViewChecked be used for?

AfterViewChecked can be used for various purposes in an Angular application. Some of the most common use cases are:

  • Updating the state of child components

  • Updating the UI based on the state of the component

  • Calling external APIs and updating the state of the component

  • Performing any required cleanup after the view has been updated

How to use AfterViewChecked?

Using AfterViewChecked is fairly straightforward. To use this hook, you need to implement the AfterViewChecked interface and define the ngAfterViewChecked() method in your component. This method will be called each time the view is checked.

Here’s an example of how to use AfterViewChecked in an Angular component:





import { Component, AfterViewChecked } from '@angular/core';



@Component({

    selector: 'app-example',

    templateUrl: './example.component.html',

    styleUrls: ['./example.component.scss']

})

export class ExampleComponent implements AfterViewChecked {



    ngAfterViewChecked(): void {

        // Perform actions after the view has been checked

    }

}

In this example, we have defined the ExampleComponent, which implements the AfterViewChecked interface. We have also defined the ngAfterViewChecked() method, which will be called after the view has been checked.

AfterViewChecked vs ngAfterViewInit

AfterViewChecked and ngAfterViewInit are two lifecycle hooks in Angular that are called at different stages of the component lifecycle. ngAfterViewInit is called once the view and child views have been initialized, but before change detection has been run. On the other hand, AfterViewChecked is called after each change detection cycle has completed.

While ngAfterViewInit is useful for initializing child components and accessing the DOM elements, AfterViewChecked can be used to perform actions after each change detection cycle.

FAQs

What is the difference between AfterViewChecked and ngAfterViewChecked?

There is no difference between AfterViewChecked and ngAfterViewChecked. AfterViewChecked is the name of the lifecycle hook, while ngAfterViewChecked is the method that needs to be implemented for this hook.

Can AfterViewChecked be called multiple times in a component?

Yes, AfterViewChecked can be called multiple times in a component. This hook is called each time the view is checked for any changes, which can result in multiple calls during the lifecycle of a component.

What happens if I modify the component’s state inside AfterViewChecked?

If you modify the component’s state inside AfterViewChecked, it will trigger another round of change detection, which can cause a possible infinite loop. Therefore, it is recommended to avoid modifying the component’s state inside this hook.

Can I use AfterViewChecked to access the DOM elements?

Yes, you can use AfterViewChecked to access the DOM elements. However, it is recommended to use ngAfterViewInit for this purpose, as it is called once the view and child views have been initialized.

How can I prevent multiple calls to AfterViewChecked?

You can prevent multiple calls to AfterViewChecked by using the ChangeDetectorRef.detectChanges() method to manually trigger change detection only when required. This can help in reducing unnecessary calls to AfterViewChecked.

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