chats.service.ts

@Injectable()
export class ChatsService {
  constructor(
    private http: HttpClient,
    private store: Store<AppState>
  ) {}

  getChats() {
    return this.store
      .select(state => state.chats)
      .pipe(
        mergeMap(chats => {
          // Returns available data
          if (chats && chats.length) {
            return of(chats);
          }

          // Fetches a list of chats
          return this.fetchChats().pipe(
            mergeMap(chats =>
              combineLatest(chats.map(chat => this.resolveChat(chat))),
            ),
          );
        }),
      );
  }
}