第一个盒子对每个可见度百分比都有一个阈值；也就是说，IntersectionObserver.thresholds 数组是 [0.00, 0.01, 0.02, ..., 0.99, 1.00] 。第二个盒子有一个阈值，即 50%。第三个盒子每 10% 的可见度（0%、10%、20% 等）设置阈值。最后一个盒子有每个 25% 的阈值。 - 在线编辑器

          <style>
main {
  position: relative;
  height: 500px;
  overflow: scroll;
}

.contents {
  position: absolute;
  width: 700px;
  height: 1725px;
}

.wrapper {
  position: relative;
  top: 600px;
}

.sampleBox {
  position: relative;
  left: 175px;
  width: 150px;
  background-color: rgb(245, 170, 140);
  border: 2px solid rgb(201, 126, 17);
  padding: 4px;
  margin-bottom: 6px;
}

#box1 {
  height: 200px;
}

#box2 {
  height: 75px;
}

#box3 {
  height: 150px;
}

#box4 {
  height: 100px;
}

.label {
  font: 14px "Open Sans", "Arial", sans-serif;
  position: absolute;
  margin: 0;
  background-color: rgba(255, 255, 255, 0.7);
  border: 1px solid rgba(0, 0, 0, 0.7);
  width: 3em;
  height: 18px;
  padding: 2px;
  text-align: center;
}

.topLeft {
  left: 2px;
  top: 2px;
}

.topRight {
  right: 2px;
  top: 2px;
}

.bottomLeft {
  bottom: 2px;
  left: 2px;
}

.bottomRight {
  bottom: 2px;
  right: 2px;
}
</style>

<template id="boxTemplate">
  <div class="sampleBox">
    <div class="label topLeft"></div>
    <div class="label topRight"></div>
    <div class="label bottomLeft"></div>
    <div class="label bottomRight"></div>
  </div>
</template>

<main>
  <div class="contents">
    <div class="wrapper">
    </div>
  </div>
</main>

<script>
let observers = [];

startup = () => {
  let wrapper = document.querySelector(".wrapper");

  // 观察者的选项
  let observerOptions = {
    root: null,
    rootMargin: "0px",
    threshold: []
  };

  // 每个盒子的一组阈值集。
  // 第一个盒子的阈值以编程方式设置，因为会有很多（每个百分点都有）。
  let thresholdSets = [
    [],
    [0.5],
    [0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0],
    [0, 0.25, 0.5, 0.75, 1.0]
  ];

  for (let i=0; i<=1.0; i+= 0.01) {
    thresholdSets[0].push(i);
  }

  // 添加每个盒子，为每个盒子创建一个新的观察者
  for (let i=0; i<4; i++) {
    let template = document.querySelector("#boxTemplate").content.cloneNode(true);
    let boxID = "box" + (i+1);
    template.querySelector(".sampleBox").id = boxID;
    wrapper.appendChild(document.importNode(template, true));

    // 为这个盒子设置观察者
    observerOptions.threshold = thresholdSets[i];
    observers[i] = new IntersectionObserver(intersectionCallback, observerOptions);
    observers[i].observe(document.querySelector("#" + boxID));
  }

  // 滚动到起始位置
  document.scrollingElement.scrollTop = wrapper.firstElementChild.getBoundingClientRect().top + window.scrollY;
  document.scrollingElement.scrollLeft = 750;
}

intersectionCallback = (entries) => {
  entries.forEach((entry) => {
    let box = entry.target;
    let visiblePct = (Math.floor(entry.intersectionRatio * 100)) + "%";

    box.querySelector(".topLeft").innerHTML = visiblePct;
    box.querySelector(".topRight").innerHTML = visiblePct;
    box.querySelector(".bottomLeft").innerHTML = visiblePct;
    box.querySelector(".bottomRight").innerHTML = visiblePct;
  });
}

startup();
</script>

        