Angular cdk scrolling: Updating Data Source not represented in view

Question

All my previous attempts of building an infinite scroller failed so far. The component I am currently working on uses Angular's virtual scrolling and should update the data source when a certain index of the virtual scroll viewport is reached. While the BehaviorSubject does indeed get updated, i cannot see the new version in the view.

My component looks like this:

import {Day, Month, Selected, Weekday} from './datepicker';
import {CdkVirtualScrollViewport} from '@angular/cdk/scrolling';
import {BehaviorSubject} from 'rxjs';

@Component({
  selector: 'app-calendar-datepicker',
  templateUrl: './calendar-datepicker.component.html',
  styleUrls: ['./calendar-datepicker.component.scss']
})
export class CalendarDatepickerComponent implements OnInit {
  months: BehaviorSubject<any[]> = new BehaviorSubject<any[]>([]);

  previousScrollIndex: number;

  @ViewChild('dateScroller') dateScroller: CdkVirtualScrollViewport;

  constructor() {
  }

  ngOnInit() {
    /*initialization of the array*/
    const initialMonths = [];
    const month = new Month(this.currentDate);
    const date = moment(this.currentDate);
    const nextMonth = new Month(date.add(1, 'M'));
    const nextDate = moment(date);
    const monthAfterNextMonth = new Month(nextDate.add(1, 'M'));
    initialMonths.push(month);
    initialMonths.push(nextMonth);
    initialMonths.push(monthAfterNextMonth);
    this.months.next(initialMonths);
  }

  public onScroll(index) {
    if (typeof(this.previousScrollIndex) === 'undefined') {
      this.previousScrollIndex = index;
    } else if (this.previousScrollIndex < index) {
      if (index % 2 === 0) {
        this.dateScroller.scrollToIndex(index);
      } else {
        this.dateScroller.scrollToIndex(index + 1);

        /*This is the place where the BehaviorSubject is updated*/
        const date = moment(this.months.value[index].date);
        const monthToFetch = new Month(date.add(2, 'M'));
        const fetchedMonths = this.months.value;
        fetchedMonths.push(monthToFetch);
        this.months.next(fetchedMonths);
      }
      this.previousScrollIndex = index;
    } else {
      if (index % 2 === 0) {
        this.dateScroller.scrollToIndex(index);
      } else {
        this.dateScroller.scrollToIndex(index - 1);
      }
     this.previousScrollIndex = index;
    }
  }
}

And the view is as follows:

<cdk-virtual-scroll-viewport itemSize="92" #dateScroller class="scroll-container"
                             (scrolledIndexChange)="onScroll($event)">
  <div *cdkVirtualFor="let month of months | async">
    <div class="month">{{month.date.format('MMMM')}}</div>
    <div class="days">
      <div class="day previousMonth" *ngFor="let prevMonthDay of month.prevMonthDays"></div>
      <div class="day" *ngFor="let day of month.monthDays" (click)="onSelect(day)"
           [ngClass]="{ 'selected': day.selected, 'past': day.past, 'today': day.today,
       'first': day.firstSelected && isRangeSelected, 'last':day.lastSelected,
       'onlyOneSelected': day.firstSelected && !isRangeSelected}">
        <span *ngIf="day.firstSelected || day.lastSelected" class="borderDay"></span>
        <span class="dayText">{{day.number}}</span>
      </div>
    </div>
  </div>
</cdk-virtual-scroll-viewport>

So far I have tried using the ChangeDetectorRef and triggering it manually or putting my code to update months in a setTimeout()-function, but neither did work.

Answer 1

I think your problem is that you are appending the new months to the same array, so the object reference doesn't change. As a result, Angular does not see any need to re-render the data.

try:

const fetchedMonths = [...this.months.value, monthToFetch]