How to get triggered on scrolling inside cdk-virtual-scroll-viewport?

Question

I'm using virtual scroll with Angular 7. I create a CdkVirtualScrollViewport and I would add a listener to every scroll event. I mean I would like to be notified on scrolling inside that viewport.

I tried to inject scrollDispatcher in my component, but I see that scrolled() is triggered on scrolling on the whole component, then I figure out that it is bind to the component instead of just to CdkVirtualScrollViewport.

Here is my code:

@ViewChild(CdkVirtualScrollViewport) virtualScroll: CdkVirtualScrollViewport;

items: Array<any>;

constructor(private scrollDispatcher: ScrollDispatcher, private cd: ChangeDetectorRef) {
this.items = [];
}

ngOnInit(): void {
  for (let i = 0; i < 100; i++) {
    this.items.push(i);
  }
}

ngAfterViewInit(): void {
this.scrollDispatcher.scrolled()
  .subscribe(event => {
    console.log('scrolled');
  });
}

As you can see CdkVirtualScrollViewport is a child element inside my component:

<div class="header">
  {{header}}
</div>

<div class="container">
  <cdk-virtual-scroll-viewport itemSize="4" class='example-viewport'>
    <li *cdkVirtualFor="let item of items" class='example-item' >{{item}}</li>
  </cdk-virtual-scroll-viewport>
</div>

<div class="footer">
  {{footer}}
</div>

Here is the full code: https://stackblitz.com/edit/angular7-scroll-dispatcher

Maybe I could use register() method of scrollDispatcher... but I should pass a CdkScrollable to it, instead mine is a CdkVirtualScrollViewport element.

Then, how can I listen only to my virtual scroll viewport scrolling events?

Answer 1

The way for listening to scroll events within CdkVirtualScrollViewport is using elementScrolled() method:

this.virtualScroll.elementScrolled()
  .subscribe(event => {
    console.log('scrolled');
  });

So there is no need to inject scrollDispatcher and register any element...