原理：
- 按需加载图片，在图片位置滚动到浏览器窗口中时才去加载图片，这样可以减少网络请求提高网页加载速度
-实现代码
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>lazyload</title>
<style type="text/css">
.content img{
width:200px;
}
</style>
</head>
<body>
<ul class="content">
<li><img src="http://kejian.jirengu.com/data/fe/%E8%AF%BE%E4%BB%B6/32-%E6%87%92%E5%8A%A0%E8%BD%BD/code/blank.jpg" data-src="http://cdn.jirengu.com/book.jirengu.com/img/1.jpg" alt=""></li>
<li><img src="http://kejian.jirengu.com/data/fe/%E8%AF%BE%E4%BB%B6/32-%E6%87%92%E5%8A%A0%E8%BD%BD/code/blank.jpg" data-src="http://cdn.jirengu.com/book.jirengu.com/img/2.jpg" alt=""></li>
<li><img src="http://kejian.jirengu.com/data/fe/%E8%AF%BE%E4%BB%B6/32-%E6%87%92%E5%8A%A0%E8%BD%BD/code/blank.jpg" data-src="http://cdn.jirengu.com/book.jirengu.com/img/3.jpg" alt=""></li>
<li><img src="http://kejian.jirengu.com/data/fe/%E8%AF%BE%E4%BB%B6/32-%E6%87%92%E5%8A%A0%E8%BD%BD/code/blank.jpg" data-src="http://cdn.jirengu.com/book.jirengu.com/img/4.jpg" alt=""></li>
<li><img src="http://kejian.jirengu.com/data/fe/%E8%AF%BE%E4%BB%B6/32-%E6%87%92%E5%8A%A0%E8%BD%BD/code/blank.jpg" data-src="http://cdn.jirengu.com/book.jirengu.com/img/5.jpg" alt=""></li>
<li><img src="http://kejian.jirengu.com/data/fe/%E8%AF%BE%E4%BB%B6/32-%E6%87%92%E5%8A%A0%E8%BD%BD/code/blank.jpg" data-src="http://cdn.jirengu.com/book.jirengu.com/img/6.jpg" alt=""></li>
<li><img src="http://kejian.jirengu.com/data/fe/%E8%AF%BE%E4%BB%B6/32-%E6%87%92%E5%8A%A0%E8%BD%BD/code/blank.jpg" data-src="http://cdn.jirengu.com/book.jirengu.com/img/7.jpg" alt=""></li>
<li><img src="http://kejian.jirengu.com/data/fe/%E8%AF%BE%E4%BB%B6/32-%E6%87%92%E5%8A%A0%E8%BD%BD/code/blank.jpg" data-src="http://cdn.jirengu.com/book.jirengu.com/img/8.jpg" alt=""></li>
<li><img src="http://kejian.jirengu.com/data/fe/%E8%AF%BE%E4%BB%B6/32-%E6%87%92%E5%8A%A0%E8%BD%BD/code/blank.jpg" data-src="http://cdn.jirengu.com/book.jirengu.com/img/9.jpg" alt=""></li>
<li><img src="http://kejian.jirengu.com/data/fe/%E8%AF%BE%E4%BB%B6/32-%E6%87%92%E5%8A%A0%E8%BD%BD/code/blank.jpg" data-src="http://cdn.jirengu.com/book.jirengu.com/img/10.jpg" alt=""></li>
<li><img src="http://kejian.jirengu.com/data/fe/%E8%AF%BE%E4%BB%B6/32-%E6%87%92%E5%8A%A0%E8%BD%BD/code/blank.jpg" data-src="http://cdn.jirengu.com/book.jirengu.com/img/11.jpg" alt=""></li>
<li><img src="http://kejian.jirengu.com/data/fe/%E8%AF%BE%E4%BB%B6/32-%E6%87%92%E5%8A%A0%E8%BD%BD/code/blank.jpg" data-src="http://cdn.jirengu.com/book.jirengu.com/img/12.jpg" alt=""></li>
<li><img src="http://kejian.jirengu.com/data/fe/%E8%AF%BE%E4%BB%B6/32-%E6%87%92%E5%8A%A0%E8%BD%BD/code/blank.jpg" data-src="http://cdn.jirengu.com/book.jirengu.com/img/13.jpg" alt=""></li>
<li><img src="http://kejian.jirengu.com/data/fe/%E8%AF%BE%E4%BB%B6/32-%E6%87%92%E5%8A%A0%E8%BD%BD/code/blank.jpg" data-src="http://cdn.jirengu.com/book.jirengu.com/img/14.jpg" alt=""></li>
<li><img src="http://kejian.jirengu.com/data/fe/%E8%AF%BE%E4%BB%B6/32-%E6%87%92%E5%8A%A0%E8%BD%BD/code/blank.jpg" data-src="http://cdn.jirengu.com/book.jirengu.com/img/15.jpg" alt=""></li>
<li><img src="http://kejian.jirengu.com/data/fe/%E8%AF%BE%E4%BB%B6/32-%E6%87%92%E5%8A%A0%E8%BD%BD/code/blank.jpg" data-src="http://cdn.jirengu.com/book.jirengu.com/img/16.jpg" alt=""></li>
<li><img src="http://kejian.jirengu.com/data/fe/%E8%AF%BE%E4%BB%B6/32-%E6%87%92%E5%8A%A0%E8%BD%BD/code/blank.jpg" data-src="http://cdn.jirengu.com/book.jirengu.com/img/17.jpg" alt=""></li>
<li><img src="http://kejian.jirengu.com/data/fe/%E8%AF%BE%E4%BB%B6/32-%E6%87%92%E5%8A%A0%E8%BD%BD/code/blank.jpg" data-src="http://cdn.jirengu.com/book.jirengu.com/img/18.jpg" alt=""></li>
<li><img src="http://kejian.jirengu.com/data/fe/%E8%AF%BE%E4%BB%B6/32-%E6%87%92%E5%8A%A0%E8%BD%BD/code/blank.jpg" data-src="http://cdn.jirengu.com/book.jirengu.com/img/19.jpg" alt=""></li>
</ul>
<script src="http://code.jquery.com/jquery-3.2.1.slim.min.js"></script>
<script type="text/javascript">
render()
$(window).on('scroll',render)
function isIn(el){
var windowHeight = $(window).height();
var elTop = el.offset().top;
var elHeight = el.outerHeight();
var scrollTop = $(window).scrollTop();
var total = windowHeight+scrollTop;
// console.log(total)
console.log(scrollTop)
if(total>elTop){
return true;
}else{
return false;
}
}
function show(el){
console.log(2)
var dataSrc = el.attr('data-src');
el.attr('src',dataSrc);
el.addClass('show');
}
function render(){
$('.content img').not('.show').each(function(item){
if(isIn($(this))){
show($(this))
}
})
}
</script>
</body>
</html>
- 在滚动加载的情况下若文档高度小于浏览器窗口高度，会无法触发滚动事件，可以判断在浏览器高度大于文档高度的情况下持续触发加载函数
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