Scroll to bottom with cdk-virtual-scroll (Angular 8)

Question

Goal

Display a list of messages and scroll to the bottom when a new message is received, even when I am at the top. I would like to scroll fully bottom even with elements of different heights.

Problem

With virtual scroll, I have to set the [itemSize] property, but for me it is not a static value:

• When a message is too long for one line it breaks in multiple, so its height changes.
• I have different types of messages with different heights (system messages for example).

Also, I am using ng-content to insert a button from the parent to load previous messages. What I see is that, when _scrollToBottom is invoked, instead of taking me to the bottom, it takes me to a bit higher. I suspect this is because of the different heights of elements inside virtual-scroll.

I have read this autosize scroll strategy issue from Angular: https://github.com/angular/components/issues/10113; but I am not sure this will solve my problem.

Any idea of what I could do will be welcome.

Test

Codesandbox: https://codesandbox.io/s/angular-virtual-scroll-biwn6

• When messages are loaded, scroll up.
• Send message. (When the new message is loaded, instead of scrolling to bottom, the virtual-scroll stops a little higher)
• Repeat

Video with the error: https://gofile.io/d/8NG9HD

---

Solution

The solution given by Gourav Garg works. Simply by executing twice the scroll function.

I am doing this now:

  private _scrollToBottom() {
    setTimeout(() => {
      this.virtualScrollViewport.scrollTo({
        bottom: 0,
        behavior: 'auto',
      });
    }, 0);
    setTimeout(() => {
      this.virtualScrollViewport.scrollTo({
        bottom: 0,
        behavior: 'auto',
      });
    }, 50);
  }

I think it is not very elegant but works fine.

Answer 1

You can use if you are using cdk > 7

this.virtualScrollViewport.scrollToIndex(messages.length-1);

As this will move to the top of last item. You need to call scrollIntoView for that item.

this.virtualScrollViewport.scrollToIndex(this.numbers.length - 1);
setTimeout(() => {
  const items = document.getElementsByClassName("list-item");
  items[items.length - 1].scrollIntoView();
}, 10);

<cdk-virtual-scroll-viewport #virtualScroll style="height: 500px" itemSize="90">
  <ng-container *cdkVirtualFor="let n of numbers">
    <li class="list-item"> {{n}} </li>
  </ng-container>
</cdk-virtual-scroll-viewport>

I have updated your sandbox

Answer 2

There's an alternative to cdk scroll.

Here's a genric version of code to scroll to a given HTML element. It can be used as a service function in angular or as a function in javascript.

scroll(el: HTMLElement, behaviour: any = "smooth", block: any = "start", inline: any = "nearest") {
    el.scrollIntoView({ behavior: behaviour, block: block, inline: inline })
  }

Sample HTML:

<div class="scroll-to-top" [ngClass]="{'show-scrollTop': windowScrolled}">
    Back to top<button mat-button mat-icon-button (click)="scrollToTop()">
        <mat-icon>keyboard_arrow_up</mat-icon>
    </button>
</div>

Sample Typescript component code:

@HostListener("window:scroll")
  onWindowScroll() {
    if (window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop > 100) {
      this.windowScrolled = true;
    }
    else if (this.windowScrolled && window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop < 10) {
      this.windowScrolled = false;
    }
  }
  scrollToTop() {
    (function smoothscroll() {
      var currentScroll = document.documentElement.scrollTop || document.body.scrollTop;
      if (currentScroll > 0) {
        window.requestAnimationFrame(smoothscroll);
        window.scrollTo(0, currentScroll - (currentScroll / 8));
      }
    })();
  }