How to wait for dispatch to be completed before selecting from a store. Ngrx related issue

Question

How can I wait for a dispatch to be completed before I select from a store. Do not have any luck in Googling? In this case, how do I wait for the dispatch to be done first before selecting from store?

My codes, appreciate the help supported.

**team-list.component.ts**

 teamsState: Observable<{teams: Team[]}>;

  constructor(private store: Store<fromApp.AppState>) { }

  ngOnInit() {
    this.store.dispatch(new TeamActions.GetTeams({
      search: this.search,
      limit: this.limit,
      skip: this.skip,
      sortBy: this.sortBy
    }));
    this.teamsState = this.store.select('teams');
  }

**team-list.component.html**

<mat-expansion-panel
    *ngFor="let team of (teamsState | async).teams; let i = index">
    <mat-expansion-panel-header>
      <div class="container-fluid">
        <div class="row">
          <div class="col-md-1">{‌{ i+1 }}</div>
          <div class="col-md-1">
              <div class="post-image">
                <img [src]="imageUrl+team.imagePath" [alt]="team.name" style>
              </div>
          </div>
          <div class="col-md-10"> {‌{ team.name }} </div>
        </div>
      </div>
    </mat-expansion-panel-header>

effects
@Effect() // If you do not want to dispatch any actions, if need to modify store state then remove
    teamList = this.actions$.pipe(
        ofType(TeamActions.GET_TEAMS),
            map((action: TeamActions.GetTeams) => {
              return action.payload;
            }),
            switchMap((params: {search: string, limit: number, skip: number, sortBy: string}) => {
                return this.httpClient.get<Team[]>(
                  `${BACKEND_URL}?search=${params.search}&&limit=${params.limit}&&skip=${params.skip}&&sortBy=${params.sortBy}`);
            }),
            map((teams: Team[]) => {
                return {
                    type: TeamActions.SET_TEAMS,
                    payload: teams
                };
            }),
            catchError((err, caught) => {
              // console.log(err.error.errors);
              this.snackBarService.showSnackBar('Unable to Get Teams', true);
              return caught;
            })
        );

Currently during first load, the dispatching action is not completed yet and when i select item from store. It is currently empty.

Solution 1

You can't, a dispatch is a fire and forget that you can't wait on.

Luckily this isn't needed because this.store.select('teams') is an observable. This means that if it changes, the observable will be emitted a new value and this will cause your component to re-render.

If the list stays empty, you can check if your state is in fact updated this can be done with @ngrx/store-devtools. If the state is updated but it doesn't show in the component, make sure you don't modify the state directly, but that you're creating a new reference to the array.

Solution 2

What you can do is to clear the selector just before dispatch and filter the response :

fromApp.GetStuff.release();
// the state is now empty
this.store.dispatch(fromApp.FetchMyStuff);
// any previous value has been forgotten. The next defined value is just what you want
this.stuff = this.store.select(fromApp.GetStuff).pipe(
    filter(stuff => !!stuff)
);

See https://ngrx.io/guide/store/selectors#resetting-memoized-selectors

Comments

[Sian20R]

Hi, Currently when I first load the page, the list is empty as it does not set any state in the store yet but I need to refresh then the items will be in the store. effects

[timdeschryver]

Yea the items will be automatically updated on the page when you set your state. If you first want to load the items and then make the page visible, take a look at guards -ultimatecourses.com/blog/preloading-ngrx-store-route-guards

[Xartrick]

Store.dispatch returns void, you can't subscribe to it.

[Experimenter]

Good solution but in my case .release() function doesn't work. Not sure why... maybe something wrong on NgRx side, using 11.1.1 version.