javascript - 用于在视口(viewport)中检测溢出滚动容器中的图像的交集观察器不起作用

Question

交集观察者正在为溢出滚动容器中的所有图像触发事件，而不是在它们进入视口(viewport)时触发事件。

使用交集观察器延迟加载图像: https://deanhume.com/home/blogpost/lazy-loading-images-using-intersection-observer/10163

<ul class="product-horizontal-list">
  <li> <img class="js-lazy-image centered" data-src="https://rukminim1.flixcart.com/image/312/312/j0vb3bk0/t-shirt/h/b/x/m-arek0253grey-melange-arrow-sports-original-imaeskqnukxhvhth.jpeg?q=70"></li>

  <li> <img class="js-lazy-image centered" data-src="https://rukminim1.flixcart.com/image/312/312/j6dxaq80/t-shirt/h/y/u/m-28771-0061blacks-levi-s-original-imaewv46jxf4wyxa.jpeg?q=70"></li>
  <li> <img class="js-lazy-image centered" data-src="https://rukminim1.flixcart.com/image/312/312/j7qi9ow0/t-shirt/x/r/z/m-bts026-billion-original-imaexwxvczbnfz8a.jpeg?q=70"></li>
  <li> <img class="js-lazy-image centered" data-src="https://rukminim1.flixcart.com/image/312/312/j4hc5u80/t-shirt/u/w/j/m-17p3dtpj3033i501-united-colors-of-benetton-original-imaevcrzqas8uwvy.jpeg?q=70"></li>
  <li> <img class="js-lazy-image centered" data-src="https://rukminim1.flixcart.com/image/312/312/j0vb3bk0/t-shirt/n/a/x/m-arek0255me-yellow-arrow-sports-original-imaeskqzm5hrn8hk.jpeg?q=70"></li>
  <li> <img class="js-lazy-image centered" data-src="https://rukminim1.flixcart.com/image/312/312/j6pctjk0/t-shirt/v/e/8/m-akss3356navy-arrow-sport-original-imaex3xgzhjvdzxu.jpeg?q=70"></li>
  <li> <img class="js-lazy-image centered" data-src="https://rukminim1.flixcart.com/image/312/312/j6gs6fk0/t-shirt/e/v/n/m-24608-0004blues-levi-s-original-imaewxcwweyz9fh3.jpeg?q=70"></li>

</ul>

  // Get all of the images that are marked up to lazy load
const images = document.querySelectorAll('.js-lazy-image');
const config = {
  // If the image gets within 50px in the Y axis, start the download.
  rootMargin: '0px',
  threshold: 0.01
};

let imageCount = images.length;
let observer;

// If we don't have support for intersection observer, loads the images immediately
if (!('IntersectionObserver' in window)) {
  loadImagesImmediately(images);
} else {
  // It is supported, load the images
  observer = new IntersectionObserver(onIntersection, config);

  // foreach() is not supported in IE
  for (let i = 0; i < images.length; i++) { 
    let image = images[i];
    if (image.classList.contains('js-lazy-image--handled')) {
      continue;
    }

    observer.observe(image);
  }
}

/**
 * Fetchs the image for the given URL
 * @param {string} url 
 */
function fetchImage(url) {
  return new Promise((resolve, reject) => {
    const image = new Image();
    image.src = url;
    image.onload = resolve;
    image.onerror = reject;
  });
}

/**
 * Preloads the image
 * @param {object} image 
 */
function preloadImage(image) {
  const src = image.dataset.src;
  if (!src) {
    return;
  }

  return fetchImage(src).then(() => { applyImage(image, src); });
}

/**
 * Load all of the images immediately
 * @param {NodeListOf<Element>} images 
 */
function loadImagesImmediately(images) {
  // foreach() is not supported in IE
  for (let i = 0; i < images.length; i++) { 
    let image = images[i];
    preloadImage(image);
  }
}

/**
 * Disconnect the observer
 */
function disconnect() {
  if (!observer) {
    return;
  }

  observer.disconnect();
}

/**
 * On intersection
 * @param {array} entries 
 */
function onIntersection(entries) {
  // Disconnect if we've already loaded all of the images
  if (imageCount === 0) {
    observer.disconnect();
  }

  // Loop through the entries
  for (let i = 0; i < entries.length; i++) { 
    let entry = entries[i];
    // Are we in viewport?
    console.log('in viewport')
    if (entry.intersectionRatio > 0) {
      imageCount--;

      // Stop watching and load the image
      observer.unobserve(entry.target);
      preloadImage(entry.target);
    }
  }
}

/**
 * Apply the image
 * @param {object} img 
 * @param {string} src 
 */
function applyImage(img, src) {
  // Prevent this from being lazy loaded a second time.
  img.classList.add('js-lazy-image--handled');
  img.src = src;
  img.classList.add('fade-in');
}

https://jsfiddle.net/anshuPurohit/h84k9zkv/

Answer 1

好消息是您的交叉口观察器正在工作。我还没有测试您的所有方法，但我确实让它按照您在示例 fiddle 中希望的方式运行。您需要至少设置图像的最小宽度。由于图像是嵌入内容，因此它们的初始宽度将为 0。并且相交观察器在加载图像之前进行计算，这意味着它运行时所有这些元素都将可见。

您目前的阈值也设置得非常低。当前阈值“0.01”意味着只有 1% 的元素需要可见才能执行回调。我建议将其设置为“0.25”并从那里进行试验。

Tl；博士。增加阈值，设置图像的最小宽度，然后看看效果如何。

对图像滚动条的另一项建议是使用固有占位符，这将有助于维护响应式图像，而无需为图像分配显式宽度。 http://daverupert.com/2015/12/intrinsic-placeholders-with-picture/