How to handle boolean observables with the async pipe in Angular

17
votes

I have the following code:

public obs$: Observable<boolean>

<div *ngIf="(obs$ | async) === true || (obs$ | async) === false">
  {{ (obs$ | async) ? 'yes' : 'no' }}
</div>

It works as intended, but the if looks a little verbose.

The problem is that I cannot simply do <div *ngIf="(obs$ | async)">. If I try that, it will work in the case when the observable did not emit a value yet or if the value is true, but it will not work if the value is false, because the if will evaluate to false and the div is not displayed.

I assume the same issue applies if a falsy value is returned, such as an empty string or 0.

Is there a better, easier way of doing that?

3
angularrxjsobservable
You can do (obs$ | async) !== null - Sachin Gupta
Can you please shed some light on when the Observable will not emit a value? - SiddAjmera
@SachinGupta Oh wow, it is actually that simple. I tried to compare it to undefined which didn't work and I just gave up. Thanks a lot! If you post an answer I will accept it. - Andreas Gassmann
@SiddAjmera I'm making an API call to get the value, so it sometimes takes a few seconds to emit a value. - Andreas Gassmann
You can also pipe the response and return a false in the catchError operator. That way you'd always be certain that a boolean value is emitted. And you won't have to use an *ngIf on the div - SiddAjmera

3 Answers

13
votes

You can do (obs$ | async) !== null

29
votes

Here's a way where you can share the subscription and not have to make any custom pipe:

<ng-container *ngIf="{value: obs$ | async} as context">
    {{context.value}}
</ng-container>

The *ngIf will evaluate to true no matter what (I believe) and then you can do your own tests on the value in the context object.

...would be better if they would just implement *ngLet I think!

The thing I like about this solution is that the resulting value is reusable with only the single use of an async pipe and its still pretty concise (even if it isn't perfect).

0
votes

I think the most elegant way should be creating a custom BoolPipe ， and chain it after the async pipe。

@Pipe({name: 'myBool'})
export class MyBoolPipe implements PipeTransform {
  transform(value: boolean, exponent: string): string {
    return !value ? 'yes' : 'no';
  }
}

and init the obj$ in the constructor of your component or service (don't need null check in your HTML template), then chain it with async pipe like this:

<div>{{ obs$ | async | myBool }}</div>