javascript - Angular: unable to scroll down to bottom in element

Question

I've been postponing fixing this error that I have been having for a while now. I have the below chatwindow:

The window where I display the messages is a separate component (chat-window.component.ts). I want to scroll to the bottom with ngOnChanges.

When we receive the conversation with the messages from the parent component, where it is received from the server via an asynchronous request, we want to scroll to the bottom of the window element. We do this by calling the this.scrollToBottom() method of the class in the ngOnChanges lifecycle hook.

This.scrollToBottom does get called, but it doesn't scroll to the bottom of the element. Can someone see why?

chat-window.component.ts: in ngOnchanges we do some synchronous stuff before we call this.scrollToBottom()

export class ChatboxWindowComponent implements OnChanges, OnInit, AfterViewChecked {

  @Input('conversation') conversation;
  @ViewChild('window') window;

  constructor() { }

  ngOnChanges() {
    // If the date separators have already been added once, we avoid doing it a second time
    const existingDateObj = this.conversation.messages.findIndex((item, i) => item.dateObj);

    if (existingDateObj === -1) {
      this.conversation.messages.forEach( (item, index, array) => {
        if (index !== 0) {
          const date1 = new Date(array[index - 1].date);
          const date2 = new Date(item.date);

          if (date2.getDate() !== date1.getDate() || date2.getMonth() !== date1.getMonth()) {
            this.conversation.messages.splice(index, 0, {date: date2, dateObj: true});
            console.log(this.conversation.messages.length);
          }
        }
      });
    }

    this.scrollToBottom();
  }

  ngOnInit() {
  }

  ngAfterViewChecked() {
  }

  isItMyMsg(msg) {
    return msg.from._id === this.conversation.otherUser.userId;
  }

  scrollToBottom() {
    try {
      console.log('scrollToBottom called');
      this.window.nativeElement.top = this.window.nativeElement.scrollHeight;
    } catch (err) {}
  }
}

chat-window.component.html

<div #window class="window">
  <ng-container *ngFor="let message of conversation.messages">
    <div class="date-container" *ngIf="!message.msg; else windowMsg">
      <p class="date">{{message.date | amDateFormat:'LL'}}</p>
    </div>
    <ng-template #windowMsg>
      <p
        class="window__message"
        [ngClass]="{
    'window__message--left': isItMyMsg(message),
    'window__message--right': !isItMyMsg(message)
    }"
      >
        {{message.msg}}
      </p>
    </ng-template>
  </ng-container>
</div>

Answer 1

The scroll doesn't work because the list of messages is not rendered yet when you call scrollToBottom. In order to scroll once the messages have been displayed, set a template reference variable (e.g. #messageContainer) on the message containers:

<ng-container #messageContainer *ngFor="let message of conversation.messages">
  ...
</ng-container>

In the code, you can then access these elements with ViewChildren and scroll the window when the QueryList.changes event is triggered:

@ViewChildren("messageContainer") messageContainers: QueryList<ElementRef>;

ngAfterViewInit() {
  this.scrollToBottom(); // For messsages already present
  this.messageContainers.changes.subscribe((list: QueryList<ElementRef>) => {
    this.scrollToBottom(); // For messages added later
  });
}