img

图片懒加载

2020-05-14 0条评论 2k次阅读 JavaScript

网页加载时,图片是最容易占流的资源;我们可以使用懒加载来减少网络请求,提升网站性能


推荐使用data-src的时候图片不要加src属性

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>
<body>
    <style>
        img[data-src] {
            display: block;
        }
    </style>

    <img class="loading" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i39.jpg" alt="">
    <img class="loading" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i40.jpg" alt="">
    <img class="loading" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i41.jpg" alt="">
    <img class="loading" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i42.jpg" alt="">
    <img class="loading" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i43.jpg" alt="">
    <img class="loading" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i44.jpg" alt="">
    <img class="loading" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i45.jpg" alt="">
    <img class="loading" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i46.jpg" alt="">
    <img class="loading" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i47.jpg" alt="">
    <img class="loading" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i48.jpg" alt="">
    <img class="loading" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i49.jpg" alt="">
    <img class="loading" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i50.jpg" alt="">
    <img alt="" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i1.jpg">
    <img alt="" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i2.jpg">
    <img alt="" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i3.jpg">
    <img alt="" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i4.jpg">
    <img alt="" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i5.jpg">
    <img alt="" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i6.jpg">
    <img alt="" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i7.jpg">
    <img alt="" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i8.jpg">
    <img alt="" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i9.jpg">
    <img alt="" data-src="https://8150e502a00d512ce440-4f545a264b21ec0a641efaa20af32482.ssl.cf4.rackcdn.com/d300x200/i10.jpg">
</body>
<script type="text/javascript">
    window.onscroll = function () {
        Limg()
    }
    window.onload = function () {
        var img = document.querySelectorAll("img[data-src]")
        for (var i = 0; i < img.length; i++) {
            img[i].style.opacity = "0"
        }
        Limg();
    }

    function Limg() {
        var viewHeight = document.documentElement.clientHeight


        var t = document.documentElement.scrollTop || document.body.scrollTop;

        var limg = document.querySelectorAll("img[data-src]")

        Array.prototype.forEach.call(limg, function (item, index) {
            var rect;
            if (item.getAttribute("data-src") === ""){
                return
            }
            //getBoundingClientRect
            rect = item.getBoundingClientRect();
            if (rect.bottom >= 0 && rect.top < viewHeight) {
                (function () {
                    var img = new Image()
                    img.src = item.getAttribute("data-src")
                    item.src = img.src
                    var j = 0
                    setInterval(function () {
                        j += 0.2
                        if (j <= 1) {
                            item.style.opacity = j
                            return
                        }
                    }, 100)
                    item.removeAttribute('data-src')
                })()
            }
        })
    }

</script>
</html>

如果面试官问你如何实现图片懒加载

window.addEventListener("scroll", lazyload);
function lazyload() {
  let viewHeight = document.body.clientHeight; //获取可视区高度
  let imgs = document.querySelectorAll("img[data-src]");
  imgs.forEach((item, index) => {
    if (item.dataset.src === "") return;

    // 用于获得页面中某个元素的左,上,右和下分别相对浏览器视窗的位置
    let rect = item.getBoundingClientRect();
    if (rect.bottom >= 0 && rect.top < viewHeight) {
      item.src = item.dataset.src;
      item.removeAttribute("data-src");
    }
  });
}
🏷️ #懒加载

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