lazy-load-images#lazy-load-method.ts

@Directive({
  selector: 'img[appLazyLoad]'
})
export class LazyLoadDirective implements AfterViewInit {
  // ...
  constructor(private el: ElementRef) {}

  private lazyLoadImage() {
    const obs = new IntersectionObserver(entries => {
      entries.forEach(({ isIntersecting }) => {
        if (isIntersecting) {
          this.loadImage();
          obs.unobserve(this.el.nativeElement);
        }
      });
    });
    obs.observe(this.el.nativeElement);
  }

  private loadImage() {
    this.srcAttr = this.src;
  }
}