Angular HTTP Requests with Async Pipe

According to Angular AsyncPipe documentation:

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.

The most important part here is "when the component gets destroyed, the async pipe unsubscribes automatically to avoid potential memory leaks."

That said, we don't have to worry about unsubscribing the Observables. The Async Pipe will take care of it - good practice (and the most important).

Check out this in-depth article about Async Pipe in Angular.

The Observables

Let's create some observables and get started:

private onDestroy$ = new Subject<boolean>();
user$ = new BehaviorSubject<{ id: number, name?: string }>(null);

As you can see, I defined two Observables, onDestroy$ and user$. It's a good practice to have an observable call when the component gets destroyed, but I'll talk about it later.

Based on user$ BehaviorSubject, we'll create a new Observable getUserInfo$ which sends a server request - each time the user$ gets emitted:

  getUserInfo$: Observable<IUser> = this.user$.pipe(
    exhaustMap(user => user && this.http.get(user.id).pipe(
      map(...),
      tap(...)
    ) || of(null)),
    takeUntil(this.onDestroy$),
  );

If you don't understand what's going on in this code and the .pipe function is unclear for you, maybe you should first take a look at this article and then continue reading!

There are two Operators to pay attention, exhaustMap and takeUntil. Of course, you can add more operators, but these two are the most important.

Using an exhaustMap operator, we can be sure that getUserInfo$ will always wait for the server's response.

Even if the user$ emits a new value, it's the opposite of the switchMap operator. See the picture below:

You can read more about RxJs Mapping here, to have a better idea of why we used exhaustMap and not other mapping operators.

takeUntil subscribes and begins mirroring the user$ Observable.

It also monitors a second Observable, this.onDestroy$ that we provided.

If the this.onDestroy$ emits a value, the getUserInfo$ Observable stops mirroring the user$ Observable and complete.

For more info about takeUntil, see the official documentation.

Subscribing via Angular AsyncPipe

Now as we created the getUserInfo$ Observable, let's see how to subscribe to it using AsyncPipe:

<ng-container *ngIf="getUserInfo$ | async as user">
	{{ user | json }}
</ng-container>

And then just emit a new value for user$ in order to trigger the server request:

<button mat-icon-button (click)="user$.next({ id: 2, name: 'John Doe' })">
    Get user with id 2
</button>

If we click the button, even if we double-click, the getUserInfo$ will send a server request and emit new user info, which will be handled by Angular AsyncPipe and insert further user info into the page.

As we use the takeUntil operator, we have to emit some value for this.onDestroy and complete it.

takeUntil is optional here, we subscribed to getUserInfo$ via Angular AsyncPipe which takes care of unsubscribing when the element gets destroyed.

Still, anyway, it's a good practice, and it's good to know about it.

  ngOnDestroy() {
    this.onDestroy$.next(true);
    this.onDestroy$.complete();
  }

Now we don't have to worry about memory leaks, as we did the best practices for subscribing and unsubscribing from Observables.

We can be sure that our code will work faster as AsyncPipe allows us to change the changeDetection of our component to ChangeDetectionStrategy.Push, which (after me) is the best way to optimize a component and also a good practice.

Haven't you tried yet? Try it and let me know how it works for you 💻.