Angular Material VirtualScrollViewPort - load data async

Question

I find different samples for the VirtualScrollViewPort but with all I have the problem how to use them in real life. The samples load the whole data at once from the server - but since they are too large, I wanna load them individually. My main requirements are:

Search mask - user enters some data
Display progress bar...
Query a Search on the server
If no result is found ==> display a message
If something is found ==> display the first n (=10) items
After the user scrolls down and only e.g. 5 items are left ==> load additionally 10 items
- continue with 5.
- if only e.g. 3 are left ==> end the scrolling

I tried already the approach from Specifying data section - but there I fail how to recognize that no data is loaded AND I fail to initiate the view - especially when the user resets the content.

I tried as well with

<cdk-virtual-scroll-viewport itemSize="itemHeight" (scrolledIndexChange)="nextBatch($event,(resultList[resultList.length-1].total) )"
class="scroll-container">
  <div *cdkVirtualFor="let search of resultList"  class="card-item" >

which works for the first requirements but finally I fail with scrolledIndexChange is fired only on the very first item on the list. I have no clue how to track if user already is displaying already item 6 (which would load additional data). On the API page I do not see any @Output() beside the scrolledIndexChange.

Any hint how to track the events properly?

UPDATE First problem I figured out was the incorrect syntax of setting the height, i.e. [itemSize]="itemHeight" is the appropriate syntax otherwise it remains always to zero ==> all elements are rendered!

btx · Accepted Answer · 2020-05-08 13:05:07Z

After some work my final solution looks like:

<ng-container *ngIf="lstSearchResults|async as resultList; else searching">
    <cdk-virtual-scroll-viewport [itemSize]="itemHeight" (scrolledIndexChange)="nextBatch()"
         class="scroll-container">
       <div *cdkVirtualFor="let search of resultList"  class="card-item">

where it is noteworthy that my list is an async list, named lstSearchResults and in the the ts code I have:

// for proper height and caching... (in pixels)
itemHeight = 174;

search(searchConfig: SearchOptions): void {
    //    ....
    this.lstSearchResults = new BehaviorSubject<SearchResult[]>(null);
    // call the REST service
    this.searchService.doSearch(searchConfig).subscribe((foundEntry) => {
        if (!this.resultList) {
            // first list - nothing found up till now
            this.resultList = foundEntry;
        } else {
            if (!this.resultList[this.resultList.length - 1]) {
                //remove the marker (which was added below/previously)
                this.resultList.pop();
            }
            foundEntry.map((item) => this.resultList.push(item));
        }
        if (this.resultList[0] && this.resultList[0].total > this.resultList.length + 1) {
            //some more elements could be fetched from the server ==> add a dummy entry for rendering
            this.resultList.push(undefined);
        }
        // notify the search list to be updated
        this.lstSearchResults.next(this.resultList);
    });
}

and for the scrolling I have the following code:

nextBatch(): void {
    if (this.theEnd) {
        return;
    }

    if (this.resultList[0]) {
        // something was found
        if (
            this.viewport.getRenderedRange().end ===
            this.viewport.getDataLength()
        ) {
            // since we scrolled to the very end of the rendered display
            // ==> check if further search is required (and do so...)
            const searchTotal = this.resultList[0].total;
            this.mySearchConfig.posOffset += this.mySearchConfig.noOfElements;
            // some basic check if the total counter exceeds the current offset
            // i.e. no further search required
            if (this.mySearchConfig.posOffset <= searchTotal) {
                this.search(this.mySearchConfig, true);
            } else {
                this.theEnd = true;
            }
        }
    } else {
        // nothing found ==> mark the end
        this.theEnd = true;
    }
}

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