交叉观察器 API

let options = {
  root: document.querySelector("#scrollArea"),
  rootMargin: "0px",
  threshold: 1.0,
};

let observer = new IntersectionObserver(callback, options);

let target = document.querySelector("#listItem");
observer.observe(target);

// the callback we set up for the observer will be executed now for the first time
// it waits until we assign a target to our observer (even if the target is currently not visible)

let callback = (entries, observer) => {
  entries.forEach((entry) => {
    // Each entry describes an intersection change for one observed
    // target element:
    //   entry.boundingClientRect
    //   entry.intersectionRatio
    //   entry.intersectionRect
    //   entry.isIntersecting
    //   entry.rootBounds
    //   entry.target
    //   entry.time
  });
};

<div class="demo">
  <div id="container">
    <div id="elem"></div>
    <div id="gutter"></div>
  </div>
  <div id="marginIndicator"></div>
</div>
<div class="controls">
  <label>
    Set the right margin of the root:
    <input id="margin" type="number" value="0" step="5" />px
  </label>
  <label>
    You can also use this slider to scroll the container:
    <input id="scrollAmount" type="range" min="0" max="300" value="0" />
  </label>
  <p>Current intersection ratio: <span id="output"></span></p>
</div>

.demo {
  display: flex;
}

.controls {
  display: flex;
  flex-direction: column;
}

#container {
  position: relative;
  width: 200px;
  height: 100px;
  overflow-x: scroll;
  border: 1px solid black;
}

#marginIndicator {
  position: relative;
  height: 100px;
  background-color: blue;
  opacity: 0.5;
}

#elem {
  background-color: red;
  width: 100px;
  height: 100px;
  position: absolute;
  left: 200px;
}

#gutter {
  width: 500px;
  height: 100px;
}

let observer;
function createObserver() {
  if (observer) {
    observer.disconnect();
  }
  observer = new IntersectionObserver(
    (entries) => {
      entries.forEach((entry) => {
        output.textContent = entry.intersectionRatio.toFixed(2);
      });
    },
    {
      threshold: Array.from({ length: 1000 }, (_, i) => i / 1000),
      root: container,
      rootMargin: `0px ${margin.value}px 0px 0px`,
    },
  );
  if (margin.valueAsNumber < 0) {
    marginIndicator.style.width = `${-margin.valueAsNumber}px`;
    marginIndicator.style.left = `${margin.valueAsNumber}px`;
    marginIndicator.style.backgroundColor = "blue";
  } else {
    marginIndicator.style.width = `${margin.valueAsNumber}px`;
    marginIndicator.style.left = "0px";
    marginIndicator.style.backgroundColor = "green";
  }
  observer.observe(elem);
}
createObserver();
margin.addEventListener("input", () => {
  createObserver();
});
scrollAmount.addEventListener("input", () => {
  container.scrollLeft = scrollAmount.value;
});

<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>

.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: rgb(255 255 255 / 70%);
  border: 1px solid rgb(0 0 0 / 70%);
  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;
}

let observers = [];

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

  // Options for the observers

  let observerOptions = {
    root: null,
    rootMargin: "0px",
    threshold: [],
  };

  // An array of threshold sets for each of the boxes. The
  // first box's thresholds are set programmatically
  // since there will be so many of them (for each percentage
  // point).

  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);
  }

  // Add each box, creating a new observer for each

  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));

    // Set up the observer for this box

    observerOptions.threshold = thresholdSets[i];
    observers[i] = new IntersectionObserver(
      intersectionCallback,
      observerOptions,
    );
    observers[i].observe(document.querySelector(`#${boxID}`));
  }

  // Scroll to the starting position

  document.scrollingElement.scrollTop =
    wrapper.firstElementChild.getBoundingClientRect().top + window.scrollY;
  document.scrollingElement.scrollLeft = 750;
};

intersectionCallback = (entries) => {
  entries.forEach((entry) => {
    const box = entry.target;
    const visiblePct = `${Math.floor(entry.intersectionRatio * 100)}%`;

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

startup();

const intersectionCallback = (entries) => {
  entries.forEach((entry) => {
    if (entry.isIntersecting) {
      let elem = entry.target;

      if (entry.intersectionRatio >= 0.75) {
        intersectionCounter++;
      }
    }
  });
};

<div id="box">
  <div class="vertical">Welcome to <strong>The Box!</strong></div>
</div>

#box {
  background-color: rgb(40 40 190 / 100%);
  border: 4px solid rgb(20 20 120);
  transition:
    background-color 1s,
    border 1s;
  width: 350px;
  height: 350px;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 20px;
}

.vertical {
  color: white;
  font: 32px "Arial";
}

.extra {
  width: 350px;
  height: 350px;
  margin-top: 10px;
  border: 4px solid rgb(20 20 120);
  text-align: center;
  padding: 20px;
}

const numSteps = 20.0;

let boxElement;
let prevRatio = 0.0;
let increasingColor = "rgb(40 40 190 / ratio)";
let decreasingColor = "rgb(190 40 40 / ratio)";

// Set things up
window.addEventListener(
  "load",
  (event) => {
    boxElement = document.querySelector("#box");

    createObserver();
  },
  false,
);

function createObserver() {
  let observer;

  let options = {
    root: null,
    rootMargin: "0px",
    threshold: buildThresholdList(),
  };

  observer = new IntersectionObserver(handleIntersect, options);
  observer.observe(boxElement);
}

function buildThresholdList() {
  let thresholds = [];
  let numSteps = 20;

  for (let i = 1.0; i <= numSteps; i++) {
    let ratio = i / numSteps;
    thresholds.push(ratio);
  }

  thresholds.push(0);
  return thresholds;
}

function handleIntersect(entries, observer) {
  entries.forEach((entry) => {
    if (entry.intersectionRatio > prevRatio) {
      entry.target.style.backgroundColor = increasingColor.replace(
        "ratio",
        entry.intersectionRatio,
      );
    } else {
      entry.target.style.backgroundColor = decreasingColor.replace(
        "ratio",
        entry.intersectionRatio,
      );
    }

    prevRatio = entry.intersectionRatio;
  });
}