图片懒加载原理

一、总结

一句话总结：

到达指定加载图片位置时，用data-src的属性值替换img的src的属性值即可，这个时候img就会去请求资源

imgs[i].src = imgs[i].getAttribute('data-src');

二、图片懒加载原理

博客对应课程的视频位置：

 1 <!DOCTYPE html>
 2 <html lang="en">
 3 <head>
 4     <meta charset="UTF-8">
 5     <meta name="viewport" content="width=device-width, initial-scale=1.0">
 6     <meta http-equiv="X-UA-Compatible" content="ie=edge">
 7     <title>图片懒加载</title>
 8     <style>
 9         img {
10             display: block;
11             width: 100%;
12             height: 300px;
13             margin-bottom: 20px;
14         }
15     </style>
16 </head>
17 <body>
18     <img data-src="./images/1.jpg" alt="">
19     <img data-src="./images/2.jpg" alt="">
20     <img data-src="./images/3.jpg" alt="">
21     <img data-src="./images/4.jpg" alt="">
22     <img data-src="./images/5.jpg" alt="">
23     <img data-src="./images/6.jpg" alt="">
24     <img data-src="./images/7.jpg" alt="">
25     <img data-src="./images/8.jpg" alt="">
26     <img data-src="./images/9.jpg" alt="">
27     <img data-src="./images/10.jpg" alt="">
28     <img data-src="./images/1.jpg" alt="">
29     <img data-src="./images/2.jpg" alt="">
30 </body>
31 <script>
32         var imgs = document.querySelectorAll('img');
33 
34         //offsetTop是元素与offsetParent的距离，循环获取直到页面顶部
35         function getTop(e) {
36             var T = e.offsetTop;
37             while(e = e.offsetParent) {
38                 T += e.offsetTop;
39             }
40             return T;
41         }
42 
43         function lazyLoad(imgs) {
44             var H = document.documentElement.clientHeight;//获取可视区域高度
45             var S = document.documentElement.scrollTop || document.body.scrollTop;
46             for (var i = 0; i < imgs.length; i++) {
47                 if (H + S > getTop(imgs[i])) {
48                     imgs[i].src = imgs[i].getAttribute('data-src');
49                 }
50             }
51         }
52 
53         window.onload = window.onscroll = function () { //onscroll()在滚动条滚动的时候触发
54             lazyLoad(imgs);
55         }
56 </script>
57 </html>