ngIf 内的异步管道仍然有价值

Question

I want to write a collapsible component which has an "Expand" button that opens a list of items. This list is created through a web request, and I'm wondering when this request happens - since to my understanding the async pipe is like a subscription and the ngIf would cause the component subscribing to be recreated, I would expect the value of the request to only be available once, however it is created each time I 'show' the expander.

StackBlitz minimal example

Inside my AppComponent I have this logic:

  data$ = of('expensive data from server').pipe(
    take(1),
    tap(() => console.log('data from server emit!')),
    // shareReplay(1),
    finalize(() => console.log('complete!'))
  );
  showSon = true;

The template is like this:

<button (click)="showSon=!showSon">Click me!</button>
<p *ngIf="showSon">
  <ng-container *ngIf="data$ | async as data">
    <div>{{data}}</div>
  </ng-container>
</p>

My understanding is that since the observable is created via of , that it should fire its value once and then complete, confirmed by the console logs. However I'm surprised that every time I show the element inside the ngIf , I still get a value even though async is now subscribing to a completed observable that is not a ReplaySubject .

I think using shareReplay(1) would solve the issue of making multiple web requests, but I still don't understand how the single-use observable is repeatedly available whenever the template inside the ngIf is recreated.

Answer 1

MikeOne's comment is correct. Everytime the If is true, the element gets recreated.. so the async pipe re-subscribes to the data$ observable causing it to emit again..

Your solution is also correct, to use shareReplay(1) as you've mentioned.