Component Events from NGXS

Developers always use the @Output decorator in conjunction with the EventEmitter. The below code has been seen by any Angular developer:

@Output() search = new EventEmitter<string>();

The secret is that the @Output can decorate any "observable" property. The Angular compiler emits needful information for the Angular itself that says "hey, please subscribe to the search class property and dispatch CustomEvent any time the observable emits".

Let's imagine that we're a part of the A team. We develop custom element that uses NGXS and we want to provide this component to the team B. The team B doesn't know anything about NGXS, they cannot use our API. Our element is just a black box that exposes data via @Output.

We develop the app-email-list custom element that emits messagesLoaded DOM event and gives the data to the team B for analytics. Given the following code:

@Component({
  selector: 'app-email-list',
  template: `
    <app-message *ngFor="let message of messages$ | async" [message]="message"></app-message>
    <app-button (click)="refresh()">Refresh messages</app-button>
  `,
  changeDetection: ChangeDetectionStrategy.OnPush
})
export class EmailListComponent {
  @Select(MessagesState.getMessages) messages$: Observable<Message[]>;

  @Output() messagesLoaded = new EventEmitter<Message[]>();

  constructor(private store: Store) {}

  refresh(): void {
    this.store.dispatch(new LoadMessages()).subscribe(() => {
      const messages = this.store.selectSnapshot(MessagesState.getMessages);
      this.messagesLoaded.emit(messages);
    });
  }
}

The above code is very simple and is used for demonstrating purposes only! As you can see we dispatch the LoadMessages action every time the user clicks "Refresh messages" button. After the LoadMessages action handler has completed his asynchronous job we emit the messagesLoaded event. Let's be more declarative:

@Component({
  selector: 'app-email-list',
  template: `
    <app-message *ngFor="let message of messages$ | async" [message]="message"></app-message>
    <app-button>Refresh messages</app-button>
  `,
  changeDetection: ChangeDetectionStrategy.OnPush
})
export class EmailListComponent {
  @Select(MessagesState.getMessages) messages$: Observable<Message[]>;

  @ViewChild(ButtonComponent, { static: true }) button: ButtonComponent;

  @Output() messagesLoaded = this.button.click.pipe(
    switchMap(() => this.store.dispatch(new LoadMessages())),
    map(() => this.store.selectSnapshot(MessagesState.getMessages))
  );

  constructor(private store: Store) {}
}

Assume that ButtonComponent.click is an EventEmitter. Wow, we've done it in a more declarative and reactive way. So every time the user clicks the app-button our switchMap will produce the next store.dispatch subscribe and unsubscribe from the previous one. Next we use the map operator that will map our stream value to the Message[] array from our state.

Now let's take away that idea with A and B teams. As our store is a single source of truth thus we can listen to any action from any part of our application. DOM events can be handy to use with the Actions stream. Assume we've got a component that emits booksLoaded event every time when different genre of books are loaded:

// books.state.ts
const enum Genre {
  Novel,
  Detective,
  Horror
}

export class LoadBooks {
  static readonly type = '[Books] Load books';
  constructor(public genre: Genre) {}
}

export class BooksState {
  static getBooks(genre: Genre) {
    return createSelector(
      [BooksState],
      (books: Book[]) => books.filter(book => book.genre === genre)
    );
  }
}

// books.component.ts
export class BooksComponent {
  @Output() booksLoaded = this.actions$.pipe(
    ofActionSuccessful(LoadBooks),
    map((action: LoadBooks) => this.store.selectSnapshot(BooksState.getBooks(action.genre)))
  );

  constructor(private store: Store, private actions$: Actions) {}
}

This might significantly reduce your code business logic and do it in a more declarative and reactive way.