The meaning of lazy loading (why use lazy loading)
The biggest impact on the page loading speed is the picture. An ordinary picture can reach the size of several megabytes, and the code may only be dozens of KB. When there are many pictures on the page, the loading speed of the page is slow, and the page is not loaded within a few seconds, and many users may be lost.
Therefore, for pages with too many pictures, in order to speed up the page loading speed, many times we need to load the pictures that do not appear in the visible area on the page first, and then load them after scrolling to the visible area. This will greatly improve the page loading performance and improve the user experience.
principle
Point the img tag src in the page to a small image or the src is empty, and then define the data-src (this attribute can be named by yourself, I just use the data-src) attribute to point to the real image. src points to a default image, otherwise a request will be sent to the server when src is empty. Can point to the loading address.
Note: The picture should specify the width and height
<img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" />
When the page is loaded, the value of the data-src attribute of the img tag in the visible area is first negative to src, and then the scroll event is monitored to load the image that the user is about to see. This enables lazy loading.
code
Before writing code, you need to understand the various heights. Look at this article first scrollTop,offsetTop,scrollLeft,offsetLeft
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
img {
display: block;
margin-bottom: 50px;
width: 400px;
height: 400px;
}
</style>
</head>
<body>
<img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
<img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
<img src="default.jpg" data-src="http://ww1.sinaimg.cn/large/006y8mN6gw1fa7kaed2hpj30sg0l9q54.jpg" alt="">
<img src="default.jpg" data-src="http://ww1.sinaimg.cn/large/006y8mN6gw1fa7kaed2hpj30sg0l9q54.jpg" alt="">
<img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
<img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
<img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
<img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
<img src="default.jpg" data-src="http://ww1.sinaimg.cn/large/006y8mN6gw1fa7kaed2hpj30sg0l9q54.jpg" alt="">
<img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
<img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
</body>
JavaScript
<script>
var num = document.getElementsByTagName('img').length;
var img = document.getElementsByTagName("img");
var 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
var seeHeight = document.documentElement.clientHeight; //可见区域高度
var scrollTop = document.documentElement.scrollTop || document.body.scrollTop; //滚动条距离顶部高度
for (var i = n; i < num; i++) {
if (img[i].offsetTop < seeHeight + scrollTop) {
if (img[i].getAttribute("src") == "default.jpg") {
img[i].src = img[i].getAttribute("data-src");
}
n = i + 1;
}
}
}
</script>
jQuery
<script>
var n = 0,
imgNum = $("img").length,
img = $('img');
lazyload();
$(window).scroll(lazyload);
function lazyload(event) {
for (var i = n; i < imgNum; i++) {
if (img.eq(i).offset().top < parseInt($(window).height()) + parseInt($(window).scrollTop())) {
if (img.eq(i).attr("src") == "default.jpg") {
var src = img.eq(i).attr("data-src");
img.eq(i).attr("src", src);
n = i + 1;
}
}
}
}
</script>
Using throttling functions for performance optimization
If the function is directly bound to the scroll event, when the page is scrolled, the function will be triggered frequently, which greatly affects the performance of the browser.
I want to limit the trigger frequency to optimize performance.
Throttling functions: Only allow a function to execute once in N seconds. Here is a simple throttle function:
// 简单的节流函数
//fun 要执行的函数
//delay 延迟
//time 在time时间内必须执行一次
function throttle(fun, delay, time) {
var timeout,
startTime = new Date();
return function() {
var context = this,
args = arguments,
curTime = new Date();
clearTimeout(timeout);
// 如果达到了规定的触发时间间隔，触发 handler
if (curTime - startTime >= time) {
fun.apply(context, args);
startTime = curTime;
// 没达到触发间隔，重新设定定时器
} else {
timeout = setTimeout(fun, delay);
}
};
};
// 实际想绑定在 scroll 事件上的 handler
function lazyload(event) {}
// 采用了节流函数
window.addEventListener('scroll',throttle(lazyload,500,1000));