Content in ngOnInit or ngAfterViewInit should not load until alll images in tags are loaded

Question

I'm using Angular 8 for a blog based web-app. Data is stored in json file for now, even images to be loaded along with the path.

JSON data

[
    {
        "imgSrc": "./assets/images/dalai-hills-1.jpg",
        "destination": "Dalai Hills",
        "introTitle": "through happy valley, to a picturesque place",
        "place": "mussoorie, uttarakhand",
        "description": "Lorem ipsum dolor sit amet, no elitr tation delicata cum, mei in causae deseruisse.",
    }
]

The imgSrc decides which image to load. All the images are already optimized and places inside assets folder.

Template

<article class="blog-card" style="border-top: 0;" *ngFor="let blog of blogsList">
    <div class="blog-img-wrap" style="min-height: 200px;">
        <a href="#"">
            <img loading="lazy" class="img-fluid blog-img" src="{{ blog.imgSrc }}" alt="blog-image-1">
        </a>
    </div>
</article>

Let's say, in the blogs page, on load there are 12 images loading because of , I want to ensure page should load only after all the images are loaded.

I am not getting any concrete answer on stackoverflow. Currently, there is a very small fraction of seconds difference between the text and images getting loaded but it looks weird.

Any solutions on the same?

P.S: I want to avoid jQuery.

score 2 · Accepted Answer · answered May 25 '20 at 10:05

You can;

Create image elements programatically. (use HTMLImageElement)
Track their loading state. (use ReplaySubject and forkJoin)
When all images are loaded show them in page. (use async pipe and Renderer2)

Here is a sample implementation (explanations are in comments);

@Component({
  selector: "my-app",
  templateUrl: "./app.component.html",
  styleUrls: ["./app.component.css"]
})
export class AppComponent implements OnInit {
  data = [
    {
      imgSrc: "https://img.jakpost.net/c/2017/10/27/2017_10_27_34794_1509067747._large.jpg",
      destination: "destination-01",
      introTitle: "introTitle-01",
      place: "place-01",
      description: "description-01"
    },
    {
      imgSrc: "https://lakecomofoodtours.com/wp-content/uploads/gravedona-celiaa-img-0282_orig-800x600.jpg",
      destination: "destination-02",
      introTitle: "introTitle-02",
      place: "place-02",
      description: "description-02"
    },
    {
      imgSrc: "https://italicsmag.com/wp-content/uploads/2020/05/Lake-Como-5-770x550.jpg",
      destination: "destination-03",
      introTitle: "introTitle-03",
      place: "place-03",
      description: "description-03"
    }
  ];

  /* This array holds Observables for images' loading status */
  private tmp: ReplaySubject<Event>[] = [];

  blogData: BlogDataType[] = this.data.map(d => {
    const img = new Image();
    img.height = 200;
    img.width = 300;
    const evt = new ReplaySubject<Event>(1);
    img.onload = e => {
      evt.next(e);
      evt.complete();
    };
    this.tmp.push(evt);
    img.src = d.imgSrc 
    return { ...d, imgElem: img };
  });

  /* 
   * Convert images' loading status observables to a higher-order observable .
   * When all observables complete, forkJoin emits the last emitted value from each.
   * since we emit only one value and complete in img.load callback, forkJoin suits our needs.
   * 
   * when forkJoin emits (i.e. all images are loaded) we emit this.blogData so that we use it
   * in template with async pipe
   */
  blogsList: Observable<BlogDataType[]> = forkJoin(this.tmp).pipe(
    map(() => this.blogData)
  );

  constructor(private renderer: Renderer2) {}

  /* manually append image elements to DOM, used in template */
  appendImg(anchor: HTMLAnchorElement, img: HTMLImageElement) {
    this.renderer.appendChild(anchor, img);
    return "";
  }
}

interface BlogDataType {
  imgElem: HTMLImageElement;
  imgSrc: string;
  destination: string;
  introTitle: string;
  place: string;
  description: string;
}

<div *ngFor="let blog of blogsList | async">
  <div>{{blog.description}}</div>
  <a #aEl href="#">
    {{ appendImg(aEl, blog.imgElem) }}
  </a>
</div>

Here is a working demo: https://stackblitz.com/edit/angular-ivy-ymmfz6

Please note that this implementation is not error-proof. img.onerror should be also handled for use in production, i skipped it for the sake of simplicity.

Content in ngOnInit or ngAfterViewInit should not load until alll images in tags are loaded

JSON data

Template

1 Answers1