Intersection Observer 观察元素何时进入或退出浏览器的视口

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  <meta charset="utf-8">
  <title>Intersection Observer API</title>
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  <style>
    body {
      height: 1000px;
    }

    .full-img {
      max- 100%;
    }

    .doc {
      position: fixed;
      right: 0;
      top: 0;
      color: #fff;
    }
  </style>
</head>

<body>
  <!--[if lt IE 8]>  <h3>请升级你的浏览器，或更换主浏览器！！!</h3>  <![endif]-->
  <img class="full-img" src="http://per.kelantu.com/photos/1515232896-FnIf9c57Hf1-l7DUdBwlXdN1bcLv-orij0?e=3153600000&token=CT86R8zZYVXDyHWEWFoMX4pz0ksOzxtKCaC80si4:HcaTgLGmJ6IbOt_d9ZGOQIqtv6g="
    alt="">
  <div class="doc"></div>
  <script>
    let img = document.querySelector('.full-img');
    let doc = document.querySelector('.doc');

    var options = {
      root: null,
      rootMargin: '0px',
      threshold: [0, 0.5, 1] // 阈值
    }

    var callback = function (entries, observer) {
      entries.forEach(entry => {
        // 总面积
        var h = entry.boundingClientRect.height;
        // 可视 面积
        var bandH = entry.intersectionRect.height;

        if (bandH == h) {
          console.log('100%');
        }

        if (bandH < h * 0.5 && bandH != 0) {
          console.log('小于 50%');
        }
        if (bandH > h * 0.5 && bandH < h) {
          console.log('大于 50%');
        }

        if (bandH <= 0) {
          console.log('0%');

        }


        if (entry.isIntersecting) {
          Object.assign(doc, {
            ...Object.assign(doc.style, {
              background: 'green'
            }),
            textContent: 'image show '
          });

        } else {
          Object.assign(doc, {
            ...Object.assign(doc.style, {
              background: 'red'
            }),
            textContent: 'image hidden '
          });
        }
      });

    };


    if ('IntersectionObserver' in window) {
      var observer = new IntersectionObserver(callback, options);
      observer.observe(img);
    }
  </script>
</body>

</html>