图片懒加载 - 码hey

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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>原生js实现图片懒加载</title>
  <style>
    *{
      margin: 0;
      padding: 0;
      box-sizing: border-box;
    }
    ul{
      overflow: hidden;
      list-style: none;
    }
    li{
      float: left;
      width: 50%;
      height: 200px;
      padding: 10px;
    }
    li img{
      display: inline-block;
      width: 100%;
      height: 100%;
    }
  </style>
</head>
<body>
<div class="container">
  <ul>
    <li><img src="http://cdn.jirengu.com/book.jirengu.com/img/1.jpg" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/1.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/2.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/3.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/4.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/5.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/6.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/7.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/8.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/9.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/10.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/11.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/12.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/13.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/14.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/15.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/16.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/16.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/16.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/16.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/16.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/16.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/16.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/16.jpg"></li>
    <li><img src="" alt="" data-src="http://cdn.jirengu.com/book.jirengu.com/img/16.jpg"></li>
  </ul>
</div>
<script>
  // window绑定滚动事件
  window.addEventListener('scroll',function(){
    // 遍历所有的img标签
    Array.prototype.slice.apply(document.getElementsByTagName('li')).forEach((li)=>{
      let img=li.getElementsByTagName('img')[0];
      // 判断当前img是否出现在了视野中
      // 判断当前img是否被加载过了
      if(checkShow(img) && !isLoaded(img)){
        loadImg(img);
      }
    })
  });

  // 判断img是否出现浏览器视野中
  function checkShow(img) {
    let scrollTop=document.documentElement.scrollTop; // 页面向上滚动的高度
    let windowHeight=window.innerHeight; //浏览器自身高度
    let offsetTop=img.offsetTop; //目标标签相对于document的高度
    return (offsetTop > scrollTop && offsetTop <(windowHeight + scrollTop));
  }

  // 判断是否已经加载过
  function isLoaded(img) {
    return img.getAttribute('src')===img.getAttribute('data-src');
  }

  // 加载图片
  function loadImg(img) {
   img.setAttribute('src',img.getAttribute('data-src'));
  }
</script>
</body>
</html>