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lazy-load-images.js
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lazy-load-images.js
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/**
*
* Copyright 2017 Google Inc. All rights reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
'use strict';
import Utils from './utils';
class LazyLoadImages {
static get SUPPORTS_INTERSECTION_OBSERVER () {
return ('IntersectionObserver' in window);
}
static get HANDLED_CLASS () {
return 'js-lazy-image--handled';
}
static get THRESHOLD () {
return 0.01;
}
static init () {
if (this._instance) {
this._instance._disconnect();
}
this._count = 0;
this._instance = new LazyLoadImages();
}
constructor () {
const images = document.querySelectorAll('.js-lazy-image');
const config = {
// If the image gets within 50px in the Y axis, start the download.
rootMargin: '50px 0px',
threshold: LazyLoadImages.THRESHOLD
};
if (!LazyLoadImages.SUPPORTS_INTERSECTION_OBSERVER) {
this._loadImagesImmediately(images);
return;
}
this._count = images.length;
this._onIntersection = this._onIntersection.bind(this);
this._observer = new IntersectionObserver(this._onIntersection, config);
images.forEach(image => {
if (image.classList.contains(LazyLoadImages.HANDLED_CLASS)) {
return;
}
this._observer.observe(image);
});
}
_disconnect () {
if (!this._observer) {
return;
}
this._observer.disconnect();
}
_onIntersection (entries) {
entries.forEach(entry => {
if (entry.intersectionRatio < 0) {
return;
}
this._count--;
this._observer.unobserve(entry.target);
this._preloadImage(entry.target);
});
if (this._count > 0) {
return;
}
this._observer.disconnect();
}
_preloadImage (image) {
const src = image.dataset.src;
if (!src) {
return;
}
return Utils.preloadImage(src).then(_ => this._applyImage(image, src));
}
_loadImagesImmediately (images) {
Array.from(images).forEach(image => this._preloadImage(image));
}
_applyImage (img, src) {
const el = img.querySelector('.js-lazy-image-content');
if (!el) {
return;
}
// Prevent this from being lazy loaded a second time.
img.classList.add(LazyLoadImages.HANDLED_CLASS);
el.style.backgroundImage = `url(${src})`;
el.classList.add('fade-in');
}
}
export default LazyLoadImages;