图片懒加载实现的原理

<!--
 * @Descripttion: 
 * @Author: voanit
 * @Date: 2022-09-11 09:07:08
 * @LastEditors: voanit
 * @LastEditTime: 2022-09-11 10:05:18
-->
<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .viewport {
      width: 300px;
      height: 200px;
      border: 1px solid blue;
      overflow: auto;
    }


    .box1 {
      height: 600px;
      width: 100%;
    }


    .observed {
      width: 100px;
      height: 100px;
      border: 1px solid green;
    }

    .imgs {
      width: 100px;
      height: 100px;
    }
  </style>
</head>

<body>
  <div class="viewport" id="viewport">
    <div class="box1">
      <img src="./place.jpg" data-src="http://p8.qhimg.com/bdr/__85/t01e5f605262fb61fb4.jpg" alt="图片" class="imgs" />
      <img src="./place.jpg" data-src="http://p8.qhimg.com/bdr/__85/t01e5f605262fb61fb4.jpg" alt="图片" class="imgs" />
      <img src="./place.jpg" data-src="http://p8.qhimg.com/bdr/__85/t01e5f605262fb61fb4.jpg" alt="图片" class="imgs" />
      <img src="./place.jpg" data-src="http://p8.qhimg.com/bdr/__85/t01e5f605262fb61fb4.jpg" alt="图片" class="imgs" />
      <img src="./place.jpg" data-src="http://p8.qhimg.com/bdr/__85/t01e5f605262fb61fb4.jpg" alt="图片" class="imgs" />
      <img src="./place.jpg" data-src="http://p8.qhimg.com/bdr/__85/t01e5f605262fb61fb4.jpg" alt="图片" class="imgs" />
      <img src="./place.jpg" data-src="http://p8.qhimg.com/bdr/__85/t01e5f605262fb61fb4.jpg" alt="图片" class="imgs" />
      <img src="./place.jpg" data-src="http://p8.qhimg.com/bdr/__85/t01e5f605262fb61fb4.jpg" alt="图片" class="imgs" />
      <img src="./place.jpg" data-src="http://p8.qhimg.com/bdr/__85/t01e5f605262fb61fb4.jpg" alt="图片" class="imgs" />
      <img src="./place.jpg" data-src="http://p8.qhimg.com/bdr/__85/t01e5f605262fb61fb4.jpg" alt="图片" class="imgs" />
      <img src="./place.jpg" data-src="http://p8.qhimg.com/bdr/__85/t01e5f605262fb61fb4.jpg" alt="图片" class="imgs" />
      <img src="./place.jpg" data-src="http://p8.qhimg.com/bdr/__85/t01e5f605262fb61fb4.jpg" alt="图片" class="imgs" />
      <img src="./place.jpg" data-src="http://p8.qhimg.com/bdr/__85/t01e5f605262fb61fb4.jpg" alt="图片" class="imgs" />
      <img src="./place.jpg" data-src="http://p8.qhimg.com/bdr/__85/t01e5f605262fb61fb4.jpg" alt="图片" class="imgs" />
      <img src="./place.jpg" data-src="http://p8.qhimg.com/bdr/__85/t01e5f605262fb61fb4.jpg" alt="图片" class="imgs" />
      <img src="./place.jpg" data-src="http://p8.qhimg.com/bdr/__85/t01e5f605262fb61fb4.jpg" alt="图片" class="imgs" />
    </div>
  </div>

  <script>
    let viewport = document.getElementById("viewport"); // 可视区域
    let imgList = document.querySelectorAll(".imgs"); // 被观察元素
    /* 开启观察者 */
    // var IO = new IntersectionObserver(回调,配置项)
    var IO = new IntersectionObserver(fn, {})
    function fn (entry) {
      console.log(entry, 'fn');
      entry.forEach(item => {
        if (item.isIntersecting) {
          /* 表示进入可是区域 */
          console.log(item.target.getAttribute('data-src'));
          // item.target.src = item.target.getAttribute('data-src')
          item.target.src = item.target.dataset.src
          IO.unobserve(item.target)
        } else {
          /* 离开 */
        }
      })
    }
    // console.dir(IO.observe());
    /* 观察img */
    imgList.forEach(item => {
      // console.log(item);
      IO.observe(item)
    })
  </script>
</body>

</html>
图片懒加载实现的原理

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