使异步管道更新Angular 2中的视图

Question

The component seen below would display a list of events in the template upon initialization ( ngOnInit() ). However, when the addEvent() is invoked, the view will not be updated. Why is this the case?

How can I make the view to be updated whenever I publish new data to a data store that's used to populate the observable property ( events: Observable<Event[]> )?

@Component({
    template:
        <ul>
            <li *ngFor="let event of events | async"
{{ event.title }}
</li>
</ul>
<button (click)="addEvent()">Add event</button>

})
export class EventCenterListComponent implements OnInit {
    events: Observable<Event[]>;

    constructor(
        private eventsService: EventsService,
    ) {}


    ngOnInit() {
        this.events = return this.eventsService.getEvents();
    }

    addEvent() {
        this.eventsService.createEvent('New event'); <--this would add one event to the data source
        this.events = this.eventsService.getEvents(); <-- that reassignment would not be seen in the template
    }
}

I've seen that many tutorials, including Angulat's official tutorial convert the Observable into a plain array and then push new values into the array, eg:

heroes: Hero[];

addHero(name: string) {
  this.heroService.create(name)
                   .subscribe(
                     hero  => this.heroes.push(hero), <-- that would work 
                     error =>  this.errorMessage = <any>error);
} 

But is the above really necessary? Cannot I operate directly on observables and have the view updated?

Answer 1

Yeah, better not to subscribe in the component.

Here's one idea. The events service keeps a private array of events, and fires a subject every time it changes. The component can use the async pipe so that its view is updated whenever that subject fires.

@Injectable()
export class EventsService {
  private events: Event[] = [];

  private eventsSubject = new Subject<Event[]>();
  private _events$ = this.eventsSubject.asObservable();

  createEvent(title: string) {
    const newEvent: Event = {title};
    this.events = [...this.events, newEvent];
    this.eventsSubject.next(this.events);
  }

  getEvents$() {
    return this._events$;
  }
}

@Component({
  template: `
    <ul>
      <li *ngFor="let event of events | async">{{ event.title }}</li>
    </ul>
    <button (click)="addEvent()">Add event</button>`
})
export class EventCenterListComponent {
  events: Observable<Event[]>;

  constructor(private eventsService: EventsService) {
    this.events = eventsService.getEvents$();
  }

  addEvent() {
    this.eventsService.createEvent('New event');
  }
}

UPDATE

New idea from a closer reading of your question. Assuming EventService works like this:

createEvent(title: string): Observable<void> // post one new event to server
getEvents(): Observable<Event[]> // get list of events from server

...And you simply want to refetch the list of the events after posting a new one, then implement EventCenterListComponent like this:

export class EventCenterListComponent {
  events: Observable<Event[]>;

  private newEventSubject = new Subject<string>();

  constructor(eventsService: EventsService) {
    const eventWasCreated$ = this.newEventSubject.mergeMap(title =>
      eventsService.createEvent(title));

    const shouldRefresh$ = eventWasCreated$.startWith(undefined);

    this.events = shouldRefresh$.switchMap(() =>
      eventsService.getEvents());

    // optional, if you are going to subscribe to `events` in more
    // than one place:
    this.events = this.events.share();
  }

  addEvent() {
    this.newEventSubject.next('New event');
  }
}