图片懒加载原理及如何实现

图片懒加载其实就是用一个自定义属性存储图片的src，当且仅当图片出现或进入可视范围才去加载它。下面是两种方法实现图片懒加载：

/* getBoundClientRect 的实现方式，监听 scroll 事件 */
let imgList = [...document.querySelectorAll('img')];
let num = imgList.length;

let lazyLoad = (function() {
  let count = 0;
  return function() {
    let deleteIndexList = [];
    imgList.forEach((img, index) => {
      let rect = img.getBoundingClientRect();
      if (rect.top < window.innerHeight) {
        img.src = img.dataset.src;
        deleteIndexList.push(index);
        count++;
        if (count === num) {
          document.removeEventListener(
            'scroll',
            lazyLoad
          );
        }
      }
    });
    imgList = imgList.filter(
      (_, index) => !deleteIndexList.includes(index)
    );
  };
})();

/* intersectionObserver 实现方式，实例化一个 IntersectionObserver，并使其观擦所有 img 标签 */
let imgList = [...document.querySelectorAll('img')];

let lazyLoad = function() {
  let observer = new IntersectionObserver(entries => {
    entries.forEach(entry => {
      if (entry.intersectionRatio > 0) {
        entry.target.src = entry.target.dataset.src;
        observer.unobserve(entry.target);
      }
    });
  });
  imgList.forEach(img => {
    observer.observe(img);
  });
};

intersectionObserver 详情请看： https://developer.mozilla.org/zh-CN/docs/Web/API/IntersectionObserver

daily-interview / fe-interview

图片懒加载原理及如何实现 #51