weilun

懒加载是一种延迟加载的技术。初始时，只加载可视区域内的目标元素，滚动页面时，动态加载可见区域的目标元素。

常见的实现方式有 IntersectionObserver 和 scroll两种，推荐使用 IntersectionObserver，因为他有更好的性能表现。

下面以图片懒加载为例，看看如何实现。

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      src="data:image/png;base64,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"
      data-src="http://e.hiphotos.baidu.com/image/pic/item/4e4a20a4462309f7e41f5cfe760e0cf3d6cad6ee.jpg"
    />
  </div>
  <div class="list-item">
    <img
      src="data:image/png;base64,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"
      data-src="http://b.hiphotos.baidu.com/image/pic/item/9d82d158ccbf6c81b94575cfb93eb13533fa40a2.jpg"
    />
  </div>
  <div class="list-item">
    <img
      src="data:image/png;base64,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"
      data-src="http://e.hiphotos.baidu.com/image/pic/item/4bed2e738bd4b31c1badd5a685d6277f9e2ff81e.jpg"
    />
  </div>
</div>

小细节优化
给src属性添加一张默认占位图（如果有使用组件库的话可以用骨架组件），体验上更加友好。

IntersectionObserver

var io = new IntersectionObserver(callback, [option]);

callback 是可见性变化时的回调函数，option 是配置对象。

相比 `scroll`，性能更好：
    - 节流处理：用户快速滚动页面，也只会触发少量的回调
    - 异步处理：回调函数是由浏览器在适当的时机（requestIdleCallback）调用的，不会阻塞页面渲染和用户交互
    - 跨线程处理：观察过程由浏览器内部线程完成，不会阻塞主线程
    - 精准度高：能够精准地检测目标元素和文档视口之间的交叉状态
判断目标元素是否在是视野内的逻辑简单，根据 item.isInterSection 布尔值判断即可

const lazyImages = document.querySelectorAll('img[data-src]');
const options = {
  root: null,
  rootMargin: '0px',
  threshold: 0.1,
};

const loadImage = function (image) {
  image.src = image.dataset.src;
  image.removeAttribute('data-src');
};

const observer = new IntersectionObserver(function (entries, observer) {
  entries.forEach(function (entry) {
    // isIntersecting是一个Boolean值，判断目标元素当前是否可见
    if (entry.isIntersecting) {
      loadImage(entry.target);
      // 图片加载后即停止监听该元素
      observer.unobserve(entry.target);
    }
  });
}, options);

lazyImages.forEach(function (image) {
  observer.observe(image);
});

scroll（不推荐）

scroll 事件频繁触发，可能会影响页面的性能和用户体验
判断目标元素是否在视野内的计算麻烦

document.addEventListener('DOMContentLoaded', function () {
  function lazyload() {
    const lazyImages = document.querySelectorAll('.lazy[data-src]');

    // 遍历检查所有懒加载图片
    lazyImages.forEach(function (img) {
      const rect = img.getBoundingClientRect();
      if (rect.top >= 0 && rect.bottom <= window.innerHeight) {
        img.src = img.getAttribute('data-src');
        img.removeAttribute('data-src');
      }
    });

    // 无未加载图片时，移除相关事件监听
    if (lazyImages.length == 0) {
      document.removeEventListener('scroll', lazyload);
      window.removeEventListener('resize', lazyload);
      window.removeEventListener('orientationChange', lazyload);
    }
  }
  // 添加事件监听
  document.addEventListener('scroll', lazyload);
  window.addEventListener('resize', lazyload);
  window.addEventListener('orientationChange', lazyload);
});

小结

懒加载是一种动态加载技术，只有当目标元素进入视野时才会去加载目标元素。通常的实现方法有两种，scroll和IntersectionObserver，监听 scroll 事件，会频繁触发回调函数，可能会造成页面卡顿，性能上和用户体验上不好。IntersectionObserver 利用 requestIdelCallback 在程序空闲时调用回调函数，异步处理，不会阻塞页面渲染和用户交互，还能够精准检测目标元素和文档视口之间的可见性。