Async pipe subscription not work correctly when nested inside ngIf

6
votes

I have a quick search box which I would like to have a loading animation. I used ng-template with ngIf to show/hide this animation. And I have some li's nested inside the same div which subscribed to a search result Observable using async pipe to display results. This async pipe works great when there's no *ngIf on the parent div, but seems it is not subscribing anymore when I apply ngIf. Is this an expected behavior? Or am I doing anything wrong?

My markup looks like this.

<input #searchBox type="text" (keyup)="itemLoading=true;searchTerm$.next(searchBox.value)"/>
<div *ngIf="!itemLoading else loading">
<!--Remove ngIf then items will display correctly when search-->
<!-- <div> -->
  <ul>
    <li *ngFor="let item of result$ | async ">{{item}}</li>
  </ul>
</div>
<ng-template #loading>
  <p>LOADING...</p>
</ng-template>

And I am using switchMap to run the search:

private source = of(['apple', 'pear', 'banana']).pipe(delay(500));

  searchTerm$ = new Subject<string>();
  result$: Observable<string[]>;
  itemLoading = false;  

  constructor() {
    this.result$ = this.searchTerm$.pipe(
      tap(term => console.log('search term captured: ' + term)),
      debounceTime(300),
      distinctUntilChanged(),
      switchMap(() => this.source.pipe(
        tap(_ => {
            console.log('source reached');
            this.itemLoading = false;
          })
      ))
    );
  }

When ngIf is present in parent div, the 'source reached' message is never logged in console, as well as the loading template keeps hanging there.

Here is a full working example of what I am talking about: https://stackblitz.com/edit/angular-2ukcqu

3
javascriptangulartypescriptrxjsangular6

3 Answers

8
votes

Rewriting the *ngIf as hidden should solve the problem. The reason result$ isn't working is because those elements inside the *ngIf won't be added to the dom until itemLoading is false. At that point they'll subscribe to result$, but the event will have already occurred.

Alternatively shareReplay(1) might also do the trick without you having to rewrite anything else, as the reply will run when the Observable is subscribed too.

Solution A 

<div [hidden]="itemLoading">
<!-- ... -->
</div>
<div [hidden]="!itemLoading">
  <p>LOADING...</p>
</div>

Solution B

this.result$ = this.searchTerm$.pipe(
  //...
  shareReplay(1)
);
0
votes

Use async pipe within your *ngIf. You can find a great writeup at https://toddmotto.com/angular-ngif-async-pipe

In short:

<div *ngIf="(user$ | async) as user; else loading">
  <user-profile
    [user]="user">
  </user-profile>
  <user-messages
    [user]="user">
  </user-messages>
</div>

<ng-template #loading>
  Loading stuff...
</ng-template>

If this does not work for you, another cleaner solution (IMO) is to use BehaviorSubject. When your dom gets around to subscribing, it will get the last observable data.

0
votes

If you do not want to add everything into your DOM, you can group your observables using forkJoin (RxJS doc) operator. Then you can keep your syntax with *ngIf and the DOM will stay clean.

Solution C

.ts

this.data$ = forkjoin([yourObs1, yourObs2,...]).pipe(
switchMap(myResolvedArray=>{
// Here you must return the object you want
return {
dataObs1: myResolvedArray[0]
...
}
})
)

.html

<div *ngIf='data$|async as data'>
//data.dataObs1 is accessible like other attributes
</div>