Async pipe within ngIf directive

Issue

When using the AsyncPipe inside of an *ngIf, if the Observable connected to the AsyncPipe pushes its values before the *ngIf becomes true, the value returned from the AsyncPipe will be incorrect.

For example, let's say I have:

<div *ngIf="showPipe">
    <div *ngFor="let item of arrObs | async">{{item}}</div>
</div>

Then say events happen in this order:

1. showPipe is false
2. arrObs pushes [1,2,3]
3. showPipe is set to true

From what I've seen, the *ngFor will act as if arrObs | async returned null.

One solution to this problem is to use [hidden] instead, but there are a lot of benefits to *ngIf, like performance and making null handling easier.

What's the proper way to do this? Should I just not use an observable at all for displaying content? I had assumed that using an observable was the most Angular-y way of doing things.

Edit: My observable is actually just a new Subject() which I call next() on.

Solution

There are some ways you can solve this. I would suggest adding a shareReplay operator on your Observable:

readonly arrObs = this.someDataFromSomewhere().pipe(
  shareReplay(1)
);

_{if your Observable is actually a Subject, you can also change it to a BehaviorSubject or ReplaySubject}

This way you will always receive the most up to date data on subscription.

There are also ways to handle this in the template and still maintain the *ngIf benefits:

<ng-container *ngIf="arrObs | async as arr">
  <div *ngIf="showPipe">
    <div *ngFor="let item of arr">{{item}}</div>
  </div>
</ng-container>

Or if you don't want the *ngIf to wait for the observable

<ng-container *ngIf="{ arr: arrObs | async } as data">
 <div *ngIf="showPipe">
   <div *ngFor="let item of data.arr">{{item}}</div>
 </div>
</ng-container>

Answered By - Poul Kruijt