price-consumer.component.ts

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css'],
  changeDetection: ChangeDetectionStrategy.OnPush
})
export class AppComponent {
  public price$: Observable<string | undefined>;
  public currency$ = new BehaviorSubject<string | undefined>(undefined);

  constructor(private api: PriceApiService) { }

  ngOnInit() {
    this.price$ = this.currency$.pipe(
      mergeMap((currency: string | undefined) => {
        return currency ? this.api.getPrice(currency) : of(undefined);
      }),
      shareReplay(1)
    );
  }

  onCryptoSelected(currency: string) {
    if (this.currency$.value) {
      this.api.unsubscribe();
    }

    this.currency$.next(currency);
  }
}