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import _ from 'underscore';
export const placeholderImage = 'data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==';
const SCROLL_THRESHOLD = 300;
export default class LazyLoader {
constructor(options = {}) {
this.lazyImages = [];
this.observerNode = options.observerNode || '#content-body';
const throttledScrollCheck = _.throttle(() => this.scrollCheck(), 300);
const debouncedElementsInView = _.debounce(() => this.checkElementsInView(), 300);
window.addEventListener('scroll', throttledScrollCheck);
window.addEventListener('resize', debouncedElementsInView);
const scrollContainer = options.scrollContainer || window;
scrollContainer.addEventListener('load', () => this.loadCheck());
}
searchLazyImages() {
this.lazyImages = [].slice.call(document.querySelectorAll('.lazy'));
if (this.lazyImages.length) {
this.checkElementsInView();
}
}
startContentObserver() {
const contentNode = document.querySelector(this.observerNode) || document.querySelector('body');
if (contentNode) {
const observer = new MutationObserver(() => this.searchLazyImages());
observer.observe(contentNode, {
childList: true,
subtree: true,
});
}
}
loadCheck() {
this.searchLazyImages();
this.startContentObserver();
}
scrollCheck() {
requestAnimationFrame(() => this.checkElementsInView());
}
checkElementsInView() {
const scrollTop = window.pageYOffset;
const visHeight = scrollTop + window.innerHeight + SCROLL_THRESHOLD;
// Loading Images which are in the current viewport or close to them
this.lazyImages = this.lazyImages.filter((selectedImage) => {
if (selectedImage.getAttribute('data-src')) {
const imgBoundRect = selectedImage.getBoundingClientRect();
const imgTop = scrollTop + imgBoundRect.top;
const imgBound = imgTop + imgBoundRect.height;
if (scrollTop < imgBound && visHeight > imgTop) {
LazyLoader.loadImage(selectedImage);
return false;
}
return true;
}
return false;
});
}
static loadImage(img) {
if (img.getAttribute('data-src')) {
img.setAttribute('src', img.getAttribute('data-src'));
img.removeAttribute('data-src');
img.classList.remove('lazy');
img.classList.add('js-lazy-loaded');
}
}
}
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