Ng-lazyload-image: Feature request: Get access to the image in observable creation

Motivation and implementation

Right now it's possible to pass an observable which is a great way to extend the library. However, it is a bit limited since the uses can't get access to the image in a simple way.

One of the most common issues is when the user doesn't use the window to scroll or using a 3-part library that adds extra scroll layers (eg. ionic, ng material design, etc), and it can sometimes be hard to find out which element that actually scrolls. One way to solve this could be to use an Intersection Observer (which also can give a performance boost). Eg.

@Component({
  selector: 'intersection-observer',
  template: `<img #img [defaultImage]="defaultImage" [lazyLoad]="image" [scrollObservable]="subject">`,
})
export class IntersectionObserverComponent {
  @ViewChild('img') img;
  subject = new Subject();
  defaultImage = 'https://www.placecage.com/1000/1000';
  image = 'https://images.unsplash.com/photo-1467932760935-519284fc87fa?dpr=2&auto=compress,format&fit=crop&w=1199&h=800&q=80';

  ngAfterViewInit() {
    const config = {
      rootMargin: '50px 50px',
      threshold: 0.01
    };

    function onIntersection(entries) {
      entries.forEach(entry => {
        if (entry.intersectionRatio > 0) {
          observer.unobserve(entry.target);
          this.subject.next();
        }
      });
    }
    const observer = new IntersectionObserver(onIntersection, config);
    observer.observe(this.img.nativeElement);
  }
}

This is however quite cumbersome.

What we can do is to accept a function as scrollObservable, something like:

if (!this.scrollObservable) {
  const windowTarget = isWindowDefined() ? window : undefined;
  scrollObservable = getScrollListener(this.scrollTarget || windowTarget);
} else if (typeof this.scrollObservable === 'function') {
  scrollObservable = this.scrollObservable(image, offset) // and maybe more
} else {
  scrollObservable = this.scrollObservable.startWith('');
}

We can not rewrite the component above to:

@Component({
  selector: 'intersection-observer',
  template: `<img [defaultImage]="defaultImage" [lazyLoad]="image"
 [scrollObservable]="startLazyload" offest="50">`,
})
export class IntersectionObserverComponent {
  defaultImage = 'https://www.placecage.com/1000/1000';
  image = 'https://images.unsplash.com/photo-1467932760935-519284fc87fa?dpr=2&auto=compress,format&fit=crop&w=1199&h=800&q=80';

  startLazyload(image, offset) {
    Observable.create(observer => {
      const intersectionObserver = new IntersectionObserver(entries => {
        entries.forEach(entry => {
          if (entry.intersectionRatio > 0) {
            observer.next();
          }
        });
      }, {
        rootMargin: offset ? `${offset}px` : undefined,
        threshold: 0.01
      });
      intersectionObserver.observe(image);
      return () => observer.unobserve(image);
    });
  }
}

Better but still a bit noisy, especially when the user wants to use IntersectionObserver on all images.

A way to solve this could be too pass a default Observable in NgModule:

import { intersectionObserverLazyLoad } from 'some-place';

@NgModule({
    declarations: [ AppComponent ],
    imports: [ BrowserModule, lazyLoadImage({
      observable: intersectionObserverLazyLoad
    })],
    bootstrap: [ AppComponent ]
})
export class MyAppModule {}

We can no rewrite the component above to:

@Component({
  selector: 'intersection-observer',
  template: `<img [defaultImage]="defaultImage" [lazyLoad]="image" offest="50">`,
})
export class IntersectionObserverComponent {
  defaultImage = 'https://www.placecage.com/1000/1000';
  image = 'https://images.unsplash.com/photo-1467932760935-519284fc87fa?dpr=2&auto=compress,format&fit=crop&w=1199&h=800&q=80';
}

The ability to pass default functions in NgModule could also open up doors to create custom functions for isVisible and loadImage but that is a later story.

Would solve: #287, #195, #286, #285, #275, #259 and more. Most of the issues are solved in other ways but it would be a nice if we could give the user more flexibility when creating an observable.

What do you think @sapierens, @rimlin?