下拉放大图片实现

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先说核心的代码：

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var scale = 1 + Math.abs(scrollY / imgHeight);
imgRef.style.transform = `scale(${scale})`

scrollY是监听滚动的垂直距离，向上滑动scrollY为正，向下拉scrollY为负，推荐配合better-scroll插件进行开发

利用scall，进行scrollY和heigth的占比加上1的值，即可平滑放大

以下为demo代码（配合等比图片实现）：

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<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>下拉放大图片</title>
  <style>
    * {
      padding:  0;
      margin: 0;
    }

    .container {
      height: 300vh;
      overflow: hidden;
    }

    .img-wrapper {
      width: 100%;
      position: relative;
      padding-top: 56.25%;
      font-size: 0;
      background-image: url('./157b6e43fbf2b2110279cbfcdd8065380dd78ef4.jpg');
      background-size: cover;
      transform-origin: top;
    }
  </style>
</head>

<body>
  <div class="container">
    <div id="img" class="img-wrapper"></div>
  </div>
  <script>
    var imgRef = document.getElementById('img');
    document.addEventListener('scroll', (e) => {
      var imgHeight = imgRef.clientHeight;
      var scrollY = document.documentElement.scrollTop;
      var scale = 1 + Math.abs(scrollY / imgHeight);
      scale = Math.min(scale, 1.5)
      imgRef.style.transform = `scale(${scale})`
    })
  </script>
</body>

</html>