问如何订阅一次事件发射器？

EN

// Part of service
public someEvent: EventEmitter<number> = new EventEmitter();

....

// Component
@Component({
  selector: 'some-component',
  template: `...`
})
export class SomeComponent {
  constructor(public service: Service) {
    this.service.someEvent.subscribe((x) => {
      // Do something
    });
  }
}

// Can't subscribe after.
ngOnDestroy() {
  this.service.someEvent.unsubscribe();
}