图片懒加载
在实际的项目开发中,我们经常遇到这样的情况:一个页面要展示的图片很多,但是在首屏出现的图片只有几张。如果我们一次性把所有的图片都加载出来,会影响页面的渲染速度,还浪费带宽。我们需要做的就是把首屏的图片先展示,不可视区域的图片不展示,等图片到达可视区域的时候再加载,这就是我们图片懒加载。
思路:
将页面里所有img属性src属性用data-xx代替,当页面滚动直至此图片出现在可视区域时,用js取到该图片的data-xx的值赋给src,同时移除data-xxx属性。
懒加载判断条件如下图所示
js懒加载图片.png
html结构
![](https://img.haomeiwen.com/i3453618/e31c0108123ec0b7.gif?imageMogr2/auto-orient/strip)
1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>1<br/>
![](https://img.haomeiwen.com/i3453618/e31c0108123ec0b7.gif?imageMogr2/auto-orient/strip)
![](https://img.haomeiwen.com/i3453618/e31c0108123ec0b7.gif?imageMogr2/auto-orient/strip)
![](https://img.haomeiwen.com/i3453618/e31c0108123ec0b7.gif?imageMogr2/auto-orient/strip)
![](https://img.haomeiwen.com/i3453618/e31c0108123ec0b7.gif?imageMogr2/auto-orient/strip)
![](https://img.haomeiwen.com/i3453618/e31c0108123ec0b7.gif?imageMogr2/auto-orient/strip)
![](https://img.haomeiwen.com/i3453618/e31c0108123ec0b7.gif?imageMogr2/auto-orient/strip)
![](https://img.haomeiwen.com/i3453618/e31c0108123ec0b7.gif?imageMogr2/auto-orient/strip)
这里用多行1来实现部分图片在不可视区域,以上src的属性用占位图来替代,实际的图片地址绑定在data-src属性上。这样首屏的时候实际就加载了一张图片。
第一种(javascript)
(function(){
var imgList,
time = 250,
delay,imgH,
offset = 100;
function _delay(obj){
clearTimeout(delay);
delay = setTimeout(function(){
_imgload(obj);
},time);
}
function _imgload(obj){
imgH = obj[0].offsetHeight;
// console.log(imgH);
for(var i = 0,len = obj.length ; i < len ; i ++){
if(obj[i].getAttribute('data-src')){
if(_isShow(obj[i])){
obj[i].src = obj[i].getAttribute('data-src');
obj[i].removeAttribute('data-src');
}
}
}
}
function _isShow(el){
var coords = el.getBoundingClientRect();
//console.log(coords);
var left = coords.left ,
top = coords.top,
winH = document.documentElement.clientHeight || window.innerHeight;
// return left >=0 && ( top >= 0 || top > - imgH ) && ( top <= winH + parseInt(offset) ); //只加载可视区域的图片
return left >=0 && top <= winH + parseInt(offset) //可视区域以上的图片也加载
}
function imgLoad(selector){
var selector = selector || '.imgLazyLoad',
node = document.querySelectorAll(selector),
imgList = Array.apply(null,node);
_delay(imgList);
window.addEventListener('scroll',function(){
_delay(imgList);
},false);
}
imgLoad('.imgLazyLoad');
})();
第二种(jquery)
!(function($){
/*
options是个对象 非必填
*/
$.fn.imgLazyLoad = function(options){
var _windowH = $(window).height(),
_delay, //函数节流
_scrollTop = $(window).scrollTop();
//默认参数
var defaults = {
time:500, //图片延迟加载的时间
offset:250, //图片预加载的距离,
className:"data-src"
} ;
var that = this;
var options = $.extend({},defaults,options);
function picLazyLoad(){
clearTimeout(_delay);
_delay = setTimeout(function(){
that.each(function(index,value){
//console.log($(value).offset().top);
if($(value).attr(options.className)){
_scrollTop = $(window).scrollTop();
if($(value).offset().top <= _windowH + _scrollTop + options.offset){
$(value).attr("src",$(value).attr(options.className));
$(value).removeAttr(options.className);
}
}
});
},options.time);
}
$(window).on('scroll',function(){
picLazyLoad();
});
picLazyLoad();
};
})(jQuery);
使用方法:
(function(){
$('.imgLazyLoad').imgLazyLoad({
time:50, //间隔时间
offset:100,//图片距离可视区域的偏移距离
class:"data-src"
});
})();
源码参考 https://github.com/chunxiao1991/imgLazyLoad
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