一、定义
当打开一个有很多图片的页面时，先只加载页面上看到的图片，等滚动到页面下面时，再加载所需的图片。这就是图片懒加载。
二、作用
减少或延迟请求数，缓解浏览器的压力，增强用户体验。
三、实现方式
- 设置图片src属性为同一张图片，同时自定义一个data-src属性来存储图片的真实地址
- 页面初始化显示的时候或者浏览器发生滚动的时候判断图片是否在视野中
- 当图片在视野中时，通过js自动改变该区域的图片的src属性为真实地址
<!DOCTYPE html>
<html>
<head>
<script src="//code.jquery.com/jquery-2.1.1.min.js"></script>
<meta charset="utf-8">
<title>JS Bin</title>
</head>
<style>
.container{
max-width: 600px;
margin: 0 auto;
}
.container::after{
content: '';
display: block;
clear: both;
}
.container img{
float: left;
width: 50%;
}
h1{
clear: both;
}
/* 因为img都是浮动，如果不清除浮动，h1的值高度就相当于container里面最高的，不是实际的数值 */
</style>
<body>
<div class="container">
<img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="1" data-src="http://cdn.jirengu.com/book.jirengu.com/img/1.jpg">
<img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="2" data-src="http://cdn.jirengu.com/book.jirengu.com/img/2.jpg">
<img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="3" data-src="http://cdn.jirengu.com/book.jirengu.com/img/3.jpg">
<img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="4" data-src="http://cdn.jirengu.com/book.jirengu.com/img/4.jpg">
<img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="5" data-src="http://cdn.jirengu.com/book.jirengu.com/img/5.jpg">
<img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="6" data-src="http://cdn.jirengu.com/book.jirengu.com/img/6.jpg">
<img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="7" data-src="http://cdn.jirengu.com/book.jirengu.com/img/7.jpg">
<img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="8" data-src="http://cdn.jirengu.com/book.jirengu.com/img/8.jpg">
<img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="9" data-src="http://cdn.jirengu.com/book.jirengu.com/img/9.jpg">
<img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="10" data-src="http://cdn.jirengu.com/book.jirengu.com/img/10.jpg">
<img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="11" data-src="http://cdn.jirengu.com/book.jirengu.com/img/11.jpg">
<img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="12" data-src="http://cdn.jirengu.com/book.jirengu.com/img/12.jpg">
<img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="13" data-src="http://cdn.jirengu.com/book.jirengu.com/img/13.jpg">
<img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="14" data-src="http://cdn.jirengu.com/book.jirengu.com/img/14.jpg">
<h1 id="target">hello</h1>
<img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="15" data-src="http://cdn.jirengu.com/book.jirengu.com/img/15.jpg">
<img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="16" data-src="http://cdn.jirengu.com/book.jirengu.com/img/16.jpg">
<img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="17" data-src="http://cdn.jirengu.com/book.jirengu.com/img/17.jpg">
<img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="18" data-src="http://cdn.jirengu.com/book.jirengu.com/img/18.jpg">
<img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="19" data-src="http://cdn.jirengu.com/book.jirengu.com/img/19.jpg">
<img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="20" data-src="http://cdn.jirengu.com/book.jirengu.com/img/20.jpg">
</div>
<script>
start()/* 一开始窗口没有滚动，也需要触发一次 */
$(window).on('scroll', function(){
start()
})
function start(){
/*提高性能，不用每张图片都检查一遍，加载了的图片不检测*/
$('.container img').not('[data-isLoaded]').each(function(){
var $node = $(this)
if(isShow($node)){
//没必要为了为了效果减慢加载速度，懒加载本来就是为了提高响应速度的
//setTimeout(function(){
loadImg($node)
//},500)
}
})
}
function isShow($node){
return $node.offset().top <= $(window).height() + $(window).scrollTop()
}
function loadImg($img){
$img.attr('src', $img.attr('data-src'))
$img.attr('data-isLoaded', 1)
}
</script>
</body>
</html>
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