首页加载图片过多,会造成首页加载慢以致卡顿,这时,我们可用图片懒加载的方式进行性能优化。
原理 将资源路径赋值到img标签的data-xx属性中,而不是src属性 获取img节点距离浏览器顶部的距离,如果小于或等于浏览器窗口的可视高度,那么就将data-xx的值赋值到src里去 用到的API
滚动条距离顶部高度 : document.documentElement.scrollTop || document.body.scrollTop;
可见区域高度 : seeHeight = document.documentElement.clientHeight;
代码:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
img {
display: block;
margin-bottom: 50px;
width: 800px;
height: 400px;
}
</style>
</head>
<body>
<img src="" data-src="https://www.mwcxs.top/static/staticImage/sliders/bg6.png" alt="">
<img src="" data-src="https://www.mwcxs.top/static/staticImage/sliders/bg7.png" alt="">
<img src="" data-src="http://ww1.sinaimg.cn/large/006y8mN6gw1fa7kaed2hpj30sg0l9q54.jpg" alt="">
<img src="" data-src="http://cover.read.duokan.com/mfsv2/download/fdsc3/p01N203pHTU7/Wr5314kcLAtVCi.jpg!t" alt="">
<img src="" data-src="http://77fkxu.com1.z0.glb.clouddn.com/20160308/1457402219_73571.jpg" alt="">
<img src="" data-src="http://pic1.cxtuku.com/00/16/18/b3809a2ba0f3.jpg" alt="">
<img src="" data-src="https://www.mwcxs.top/static/staticImage/sliders/bg9.png" alt="">
<img src="" data-src="http://cover.read.duokan.com/mfsv2/download/fdsc3/p015trgKM7vw/H0iyDPPneOVrA4.jpg!t" alt="">
<img src="" data-src="http://ww1.sinaimg.cn/large/006y8mN6gw1fa7kaed2hpj30sg0l9q54.jpg" alt="">
<img src="" data-src="https://www.mwcxs.top/static/staticImage/sliders/bg8.png" alt="">
<img src="" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
<img src="" data-src="https://www.mwcxs.top/static/staticImage/sliders/bg9.png" alt="">
<img src="" data-src="https://www.mwcxs.top/static/staticImage/sliders/bg10.png" alt="">
<img src="" data-src="https://www.mwcxs.top/static/staticImage/sliders/bg11.png" alt="">
<img src="" data-src="https://www.mwcxs.top/static/staticImage/sliders/bg15.png" alt="">
<img src="" data-src="https://www.mwcxs.top/static/staticImage/sliders/bg16.png" alt="">
<img src="" data-src="https://www.mwcxs.top/static/staticImage/sliders/bg17.png" alt="">
<script>
(function(){
let num = document.getElementsByTagName('img').length;
let img = document.getElementsByTagName("img");
let n = 0; //存储图片加载到的位置,避免每次都从第一张图片开始遍历
lazyload(); //页面载入完毕加载可是区域内的图片
window.onscroll = lazyload;
function lazyload() { //监听页面滚动事件
let seeHeight = document.documentElement.clientHeight; //可见区域高度
let scrollTop = document.documentElement.scrollTop || document.body.scrollTop; //滚动条距离顶部高度
for (let i = n; i < num; i++) {
// 图片未出现时距离顶部的距离大于滚动条距顶部的距离+可视区的高度
if (img[i].offsetTop < seeHeight + scrollTop) {
if (img[i].getAttribute("src") == "") {
img[i].src = img[i].getAttribute("data-src");
}
n = i + 1;
}
}
}
})()
</script>
</body>
</html>