1.懒加载的作用和原理
在我们展示多图片的场景下，类似淘宝或者百度图片，由于图片的数目过多，全部从服务器请求会给用户糟糕的用户体验，为了提升用户体验，我们这里使用懒加载，随着下拉逐步加载。
每个图片的src会有一个get请求，我们把不能看到的图片src设置为相同的图片，这些图片发一次请求即可，设置属性data-src为真正的图片路径。当图片滚动到可视区，我们就用js把data-src 赋值给 src，简单的懒加载就可以实现了。
2.简单的js实现懒加载
//你需要一张timg.png图片
<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><title>Title</title><style>img{display: block;width: 700px;height: 400px;}</style>
</head>
<body><ul class="img-group"><li><img src="timg.jpg" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158197&di=8845befd9fbda1e99e565b9c2298be50&imgtype=0&src=http%3A%2F%2Fuploads.xuexila.com%2Fallimg%2F1503%2F626-15031G42255b3.jpg"></li><li><img src="timg.jpg" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158196&di=4a38d248001c145c5b4dd31474dedf41&imgtype=0&src=http%3A%2F%2Fpic29.nipic.com%2F20130512%2F11178195_152908769116_2.jpg"></li><li><img src="timg.jpg" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158196&di=b39e8b14d214b7aa518d7a8328939efb&imgtype=0&src=http%3A%2F%2Fimage16-c.poco.cn%2Fmypoco%2Fmyphoto%2F20141114%2F12%2F4567377520141114124637053.jpg%3F1024x684_120"></li><li><img src="timg.jpg" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158196&di=f2e7a6ffbfa3f92025c91e6180b5e317&imgtype=0&src=http%3A%2F%2Ffile31.mafengwo.net%2FM00%2F3F%2F26%2FwKgBs1gXBQuAPWGGABOK4vbGpt412.groupinfo.w600.jpeg"></li><li><img src="timg.jpg" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158195&di=6a073a070b9cea1214efc0994ed6dde1&imgtype=0&src=http%3A%2F%2Fimages3.ctrip.com%2Fwri%2Fimages%2F200610%2F100602312604121954734.jpg"></li><li><img src="timg.jpg" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158195&di=a9436d83f55b301d1df9cd24f6e367e7&imgtype=0&src=http%3A%2F%2Fcyjctrip.qiniudn.com%2F69015%2F1379755311203p184vt4juft5p1t8q1andgp5ijn11.jpg"></li><li><img src="timg.jpg" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158195&di=2b6bc6b871d4f7f267169dfcfb727f84&imgtype=0&src=http%3A%2F%2Fcyjctrip.qiniudn.com%2F106357%2F1395933091608p18k21ek6kvj1gv2ta910kn1002m.jpg"></li></ul>
<script type="text/javascript">let imgArr = document.querySelectorAll('img');let len = imgArr.length;window.onscroll = function () {let seeHeight = document.documentElement.clientHeight;console.log("seeHeight ="+seeHeight);let scrollTop = document.body.scrollTop || document.documentElement.scrollTop;console.log("scrollTop ="+scrollTop);for(let i=0; i<len; i++){console.log(imgArr[i].offsetTop);if(imgArr[i].offsetTop < seeHeight + scrollTop){if(imgArr[i].getAttribute('src')=='timg.jpg'){imgArr[i].src = imgArr[i].getAttribute('data-src');}}}}
</script>
</body>
</html>
我们根据js代码分析一下，主要有两部分
1>.如何加载图片?
img.src = img.getAttribute('data-src');
2>.如何判断是否在可视区
就是 图片的 offsetTop < scrollTop + clientHeigth 即可
好了上面就是我们简单的js实现和原理，但是看到各位大佬提到这里有三个需要优化的地方。
1.在初始条件下，应该有图片显示，只要在加载完毕之后滚动之前执行图片的加载即可
2.函数节流，但我们在高频度的滚动时，每隔一段事件开始图片的渲染。实现原理是 加入一个开关变量, 控制每隔固定的一段时间,函数才可能被触发
<!DOCTYPE html> <html lang="en"> <head><meta charset="UTF-8"><title>Title</title><style>img{display: block;width: 700px;height: 400px;}</style> </head> <body><ul class="img-group"><li><img src="timg.jpg" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158197&di=8845befd9fbda1e99e565b9c2298be50&imgtype=0&src=http%3A%2F%2Fuploads.xuexila.com%2Fallimg%2F1503%2F626-15031G42255b3.jpg"></li><li><img src="timg.jpg" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158196&di=4a38d248001c145c5b4dd31474dedf41&imgtype=0&src=http%3A%2F%2Fpic29.nipic.com%2F20130512%2F11178195_152908769116_2.jpg"></li><li><img src="timg.jpg" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158196&di=b39e8b14d214b7aa518d7a8328939efb&imgtype=0&src=http%3A%2F%2Fimage16-c.poco.cn%2Fmypoco%2Fmyphoto%2F20141114%2F12%2F4567377520141114124637053.jpg%3F1024x684_120"></li><li><img src="timg.jpg" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158196&di=f2e7a6ffbfa3f92025c91e6180b5e317&imgtype=0&src=http%3A%2F%2Ffile31.mafengwo.net%2FM00%2F3F%2F26%2FwKgBs1gXBQuAPWGGABOK4vbGpt412.groupinfo.w600.jpeg"></li><li><img src="timg.jpg" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158195&di=6a073a070b9cea1214efc0994ed6dde1&imgtype=0&src=http%3A%2F%2Fimages3.ctrip.com%2Fwri%2Fimages%2F200610%2F100602312604121954734.jpg"></li><li><img src="timg.jpg" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158195&di=a9436d83f55b301d1df9cd24f6e367e7&imgtype=0&src=http%3A%2F%2Fcyjctrip.qiniudn.com%2F69015%2F1379755311203p184vt4juft5p1t8q1andgp5ijn11.jpg"></li><li><img src="timg.jpg" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158195&di=2b6bc6b871d4f7f267169dfcfb727f84&imgtype=0&src=http%3A%2F%2Fcyjctrip.qiniudn.com%2F106357%2F1395933091608p18k21ek6kvj1gv2ta910kn1002m.jpg"></li></ul> <script type="text/javascript">let imgArr = document.querySelectorAll('img');let len = imgArr.length;let n = 0; //记录加载图片的位置，避免从第一张开始加载let canrun = true;let seeHeight = document.documentElement.clientHeight;console.log("seeHeight ="+seeHeight);lazyLoad();window.onscroll = function () {if(!canrun){return ;}canrun = false;setTimeout(function () {console.log('*****');lazyLoad();canrun= true;},1000);}function lazyLoad() {let scrollTop = document.body.scrollTop || document.documentElement.scrollTop;console.log("scrollTop ="+scrollTop);for(let i=0; i<len; i++){console.log(imgArr[i].offsetTop);if(imgArr[i].offsetTop < seeHeight + scrollTop){if(imgArr[i].getAttribute('src')=='timg.jpg'){imgArr[i].src = imgArr[i].getAttribute('data-src');}n = i+1;console.log("n="+n);}}} </script> </body> </html>
这是以上的两个优化之后的代码。
