I want to keep a similar syntax used in `@HostListner` and from that event create a stream using `fromEvent()`. Currently scroll isn't triggering the `fromEvent` subscription.

**Question:**
How do I trigger `var source = Observable.fromEvent(target, 'window:scroll');`?

```
import { Directive, HostListener } from '@angular/core';
import {Observable} from "rxjs";
import 'rxjs/add/observable/fromEvent';

@Directive({
  selector: '[mh-scroll]'
})
export class MhScroll {
  lastKnownScrollPosition: number;
  ticking: boolean;

  constructor() {
    this.lastKnownScrollPosition = 0;
    this.ticking = false;
  }

    isElementCloseToTop(target) {

      var source = Observable.fromEvent(target, 'window:scroll'); //this never triggers.

      var subscription = source.subscribe(
          (x) => {
            console.log('Next: Clicked!');
          },
          (err) => {
            console.log('Error: %s', err);
          },
          () => {
            console.log('Completed');
          });

      this.ticking = false;
    }

    @HostListener('window:scroll', ['$event.target'])
    triggeredScroll(target) {
      this.lastKnownScrollPosition = window.scrollY;

      if (!this.ticking) {
        window.requestAnimationFrame(this.isElementCloseToTop.bind(this, target));
      }

      this.ticking = true;
    }
}

```