使用 Intersection Observer API 測量元素可見性時間

<div class="wrapper">
  <header>
    <h1>A Fake Blog</h1>
    <h2>Showing Intersection Observer in action!</h2>
  </header>

  <aside>
    <nav>
      <ul>
        <li><a href="#link1">A link</a></li>
        <li><a href="#link2">Another link</a></li>
        <li><a href="#link3">One more link</a></li>
      </ul>
    </nav>
  </aside>

  <main>…</main>
</div>

body {
  font-family: "Open Sans", "Helvetica", "Arial", sans-serif;
  background-color: aliceblue;
}

.wrapper {
  display: grid;
  grid-template-columns: auto minmax(min-content, 1fr);
  grid-template-rows: auto minmax(min-content, 1fr);
  max-width: 700px;
  margin: 0 auto;
  background-color: aliceblue;
}

header {
  grid-column: 1 / -1;
  grid-row: 1;
  background-color: aliceblue;
}

aside {
  grid-column: 1;
  grid-row: 2;
  background-color: cornsilk;
  padding: 5px 10px;
}

aside ul {
  padding-left: 0;
}

aside ul li {
  list-style: none;
}

aside ul li a {
  text-decoration: none;
}

main {
  grid-column: 2;
  grid-row: 2;
  margin: 0;
  margin-left: 16px;
  font-size: 16px;
}

article {
  background-color: white;
  padding: 6px;
}

article:not(:last-child) {
  margin-bottom: 8px;
}

article h2 {
  margin-top: 0;
}

.ad {
  height: 96px;
  padding: 6px;
  border-color: #555555;
  border-style: solid;
  border-width: 1px;
}

.ad:not(:last-child) {
  margin-bottom: 8px;
}

.ad h2 {
  margin-top: 0;
}

.ad div {
  position: relative;
  float: right;
  padding: 0 4px;
  height: 20px;
  width: 120px;
  font-size: 14px;
  bottom: 30px;
  border: 1px solid black;
  background-color: rgb(255 255 255 / 50%);
}

const contentBox = document.querySelector("main");

let nextArticleID = 1;
let visibleAds = new Set();
let previouslyVisibleAds = null;

document.addEventListener("visibilitychange", handleVisibilityChange);

const observerOptions = {
  root: null,
  rootMargin: "0px",
  threshold: [0.0, 0.75],
};
const adObserver = new IntersectionObserver(
  intersectionCallback,
  observerOptions,
);
const refreshIntervalID = setInterval(handleRefreshInterval, 1000);

const loremIpsum =
  "<p>Lorem ipsum dolor sit amet, consectetur adipiscing" +
  " elit. Cras at sem diam. Vestibulum venenatis massa in tincidunt" +
  " egestas. Morbi eu lorem vel est sodales auctor hendrerit placerat" +
  " risus. Etiam rutrum faucibus sem, vitae mattis ipsum ullamcorper" +
  " eu. Donec nec imperdiet nibh, nec vehicula libero. Phasellus vel" +
  " malesuada nulla. Aliquam sed magna aliquam, vestibulum nisi at," +
  " cursus nunc.</p>";

buildContents();

function handleVisibilityChange() {
  if (document.hidden) {
    if (!previouslyVisibleAds) {
      previouslyVisibleAds = visibleAds;
      visibleAds = new Set();
      previouslyVisibleAds.forEach((adBox) => {
        updateAdTimer(adBox);
        adBox.dataset.lastViewStarted = 0;
      });
    }
  } else {
    previouslyVisibleAds.forEach((adBox) => {
      adBox.dataset.lastViewStarted = performance.now();
    });
    visibleAds = previouslyVisibleAds;
    previouslyVisibleAds = null;
  }
}

function intersectionCallback(entries) {
  entries.forEach((entry) => {
    const adBox = entry.target;

    if (entry.isIntersecting) {
      if (entry.intersectionRatio >= 0.75) {
        adBox.dataset.lastViewStarted = entry.time;
        visibleAds.add(adBox);
      }
    } else {
      visibleAds.delete(adBox);
      if (
        entry.intersectionRatio === 0.0 &&
        adBox.dataset.totalViewTime >= 60000
      ) {
        replaceAd(adBox);
      }
    }
  });
}

function handleRefreshInterval() {
  const redrawList = [];

  visibleAds.forEach((adBox) => {
    const previousTime = adBox.dataset.totalViewTime;
    updateAdTimer(adBox);

    if (previousTime !== adBox.dataset.totalViewTime) {
      redrawList.push(adBox);
    }
  });

  if (redrawList.length) {
    window.requestAnimationFrame((time) => {
      redrawList.forEach((adBox) => {
        drawAdTimer(adBox);
      });
    });
  }
}

function updateAdTimer(adBox) {
  const lastStarted = adBox.dataset.lastViewStarted;
  const currentTime = performance.now();

  if (lastStarted) {
    const diff = currentTime - lastStarted;

    adBox.dataset.totalViewTime =
      parseFloat(adBox.dataset.totalViewTime) + diff;
  }

  adBox.dataset.lastViewStarted = currentTime;
}

function drawAdTimer(adBox) {
  const timerBox = adBox.querySelector(".timer");
  const totalSeconds = adBox.dataset.totalViewTime / 1000;
  const sec = Math.floor(totalSeconds % 60);
  const min = Math.floor(totalSeconds / 60);

  timerBox.innerText = `${min}:${sec.toString().padStart(2, "0")}`;
}

function buildContents() {
  for (let i = 0; i < 5; i++) {
    contentBox.appendChild(createArticle(loremIpsum));

    if (!(i % 2)) {
      loadRandomAd();
    }
  }
}

function createArticle(contents) {
  const articleElem = document.createElement("article");
  articleElem.id = nextArticleID;

  const titleElem = document.createElement("h2");
  titleElem.innerText = `Article ${nextArticleID} title`;
  articleElem.appendChild(titleElem);

  articleElem.innerHTML += contents;
  nextArticleID += 1;

  return articleElem;
}

function loadRandomAd(replaceBox) {
  const ads = [
    {
      bgcolor: "#cceecc",
      title: "Eat Green Beans",
      body: "Make your mother proud—they're good for you!",
    },
    {
      bgcolor: "aquamarine",
      title: "MillionsOfFreeBooks.whatever",
      body: "Read classic literature online free!",
    },
    {
      bgcolor: "lightgrey",
      title: "3.14 Shades of Gray: A novel",
      body: "Love really does make the world go round…",
    },
    {
      bgcolor: "#ffeeee",
      title: "Flexbox Florist",
      body: "When life's layout gets complicated, send flowers.",
    },
  ];
  let adBox, title, body, timerElem;

  const ad = ads[Math.floor(Math.random() * ads.length)];

  if (replaceBox) {
    adObserver.unobserve(replaceBox);
    adBox = replaceBox;
    title = replaceBox.querySelector(".title");
    body = replaceBox.querySelector(".body");
    timerElem = replaceBox.querySelector(".timer");
  } else {
    adBox = document.createElement("div");
    adBox.className = "ad";
    title = document.createElement("h2");
    body = document.createElement("p");
    timerElem = document.createElement("div");
    adBox.appendChild(title);
    adBox.appendChild(body);
    adBox.appendChild(timerElem);
  }

  adBox.style.backgroundColor = ad.bgcolor;

  title.className = "title";
  body.className = "body";
  title.innerText = ad.title;
  body.innerHTML = ad.body;

  adBox.dataset.totalViewTime = 0;
  adBox.dataset.lastViewStarted = 0;

  timerElem.className = "timer";
  timerElem.innerText = "0:00";

  if (!replaceBox) {
    contentBox.appendChild(adBox);
  }

  adObserver.observe(adBox);
}

function replaceAd(adBox) {
  updateAdTimer(adBox);

  const visibleTime = adBox.dataset.totalViewTime;
  console.log(
    `Replacing ad: ${
      adBox.querySelector("h2").innerText
    } - visible for ${visibleTime}`,
  );

  loadRandomAd(adBox);
}