things.effects.ts

@Injectable({
  providedIn: 'root'
})
export class ThingsEffects {

  fetchThings$ = createEffect(() => this.actions$.pipe(
    ofType(ThingsActions.fetchThings),
    switchMap(() => this.api.things.fetch()
      .pipe(
        map((data) => ThingsActions.update(data.items))
      ))
  ));

  constructor(
    private actions$: Actions<ThingsActions.All>,
    private api: API
  ) { }
}