@angular/common

AsyncPipe

pipe

Unwraps a value from an asynchronous primitive.

API

  
    class AsyncPipe implements OnDestroy ,PipeTransform {
}

ngOnDestroy

void

@returnsvoid

transform

3 overloads

@paramobjObservable<T> | Subscribable<T> | Promise<T>

@returnsT | null

@paramobjnull | undefined

@returnsnull

@paramobjObservable<T> | Subscribable<T> | Promise<T> | null | undefined

@returnsT | null

Description

Unwraps a value from an asynchronous primitive.

The async pipe subscribes to an Observable or Promise and returns the latest value it has emitted. When a new value is emitted, the async pipe marks the component to be checked for changes. When the component gets destroyed, the async pipe unsubscribes automatically to avoid potential memory leaks. When the reference of the expression changes, the async pipe automatically unsubscribes from the old Observable or Promise and subscribes to the new one.

Exported by

CommonModule

Usage Notes

Examples

This example binds a Promise to the view. Clicking the Resolve button resolves the promise.

          @Component({  selector: 'async-promise-pipe',  template: `<div>    <code>promise|async</code>:    <button (click)="clicked()">{{ arrived ? 'Reset' : 'Resolve' }}</button>    <span>Wait for it... {{ greeting | async }}</span>  </div>`,  standalone: false,})export class AsyncPromisePipeComponent {  greeting: Promise<string>| null = null;  arrived: boolean = false;  private resolve: Function | null = null;  constructor() {    this.reset();  }  reset() {    this.arrived = false;    this.greeting = new Promise<string>((resolve, reject) => {      this.resolve = resolve;    });  }  clicked() {    if (this.arrived) {      this.reset();    } else {      this.resolve!('hi there!');      this.arrived = true;    }  }}

It's also possible to use async with Observables. The example below binds the time Observable to the view. The Observable continuously updates the view with the current time.

          @Component({  selector: 'async-observable-pipe',  template: '<div><code>observable|async</code>: Time: {{ time | async }}</div>',  standalone: false,})export class AsyncObservablePipeComponent {  time = new Observable<string>((observer: Observer<string>) => {    setInterval(() => observer.next(new Date().toString()), 1000);  });}