*对于来自BehaviorSubject的observable的异步管道，ngIf似乎在加载数据之前不正确地激活

Question

If the array in response is empty I want to display a message, otherwise the elements of the array.

In case the response has a non-empty array the *ngIf condition evalutes to true and the message is displayed until the data is loaded.

Html template snippet:

<div *ngIf="personalAndStarredBookmarks$ && (personalAndStarredBookmarks$ | async)?.size === 0; else personalBookmarksList" class="missing-category-bookmarks-message  alert alert-info">
  <p>No bookmarks yet</p>
</div>
<ng-template #personalBookmarksList>
  <app-async-bookmark-list [bookmarks]="personalAndStarredBookmarks$" [shownSize]="10" [userData]="userData"></app-async-bookmark-list>
</ng-template>

Component snippet:

import { List } from 'immutable';

export class PersonalBookmarksListComponent implements OnInit {

  personalAndStarredBookmarks$: Observable<List<Bookmark>>;

  ngOnInit(): void {

    this.personalAndStarredBookmarks$ = this.personalBookmarksStore.getPersonalBookmarks();
  }
  ...
}

The store uses a BehaviourSubject where it holds the response from a service that actually does the HTTP call.

Store snippet

@Injectable()
export class PersonalBookmarksStore {

  private _personalBookmarks: BehaviorSubject<List<Bookmark>> = new BehaviorSubject(List([]));

  constructor(private personalBookmarkService: PersonalBookmarkService,
              private keycloakService: KeycloakService
  ) {
    keycloakService.loadUserProfile().then(keycloakProfile => {
      this.userId = keycloakProfile.id;
      this.loadInitialData();
    });
  }

  private loadInitialData() {
    this.personalBookmarkService.getAllPersonalBookmarks(this.userId)
      .subscribe(
        data => {
          let bookmarks: Bookmark[] = <Bookmark[]>data;
          this._personalBookmarks.next(List(bookmarks));
        },
        err => console.error('Error retrieving bookmarks', err)
      );
  }

  getPersonalBookmarks(): Observable<List<Bookmark>> {
    return this._personalBookmarks.asObservable();
  }
  ...
}

If I am calling directly the Service (not the Store) in the component it behaves as expected...

Answer 1

You should restructure your template to handle the async calls efficiently by assigning a local variable as

<ng-content*ngIf="personalAndStarredBookmarks$ | async as personalAndStarredBookmarks">
    <div *ngIf="personalAndStarredBookmarks.size === 0; else personalBookmarksList" 
        class="missing-category-bookmarks-message  alert alert-info">
        <p>No bookmarks yet</p>
    </div>
    <ng-template #personalBookmarksList>
        <app-async-bookmark-list [bookmarks]="personalAndStarredBookmarks" [shownSize]="10"
            [userData]="userData">
        </app-async-bookmark-list>
    </ng-template>
<ng-content>

With this, you can efficiently handle all the conditions and the main content loads only after the async call is resolved.

See a DEMO HERE

Answer 2

The culprit is the initialisation of the BehaviorSubject with an empty list, which gets emitted and the condition evaluates to true:

private _personalBookmarks: BehaviorSubject<List<Bookmark>> = new BehaviorSubject(List([]));

Fix - init the BehaviorSubject with a null or undefined

private _personalBookmarks: BehaviorSubject<List<Bookmark>> = new BehaviorSubject(null);