图片延时加载

// 图片延时加载(调用lazyLoad.init([html标签])对标记为需要延时加载的图片进行延时加载)
lazyLoad = (function () {
// 存储页面延时加载元素的对象{}
var map_element = {};
// 保存延时加载的元素
var element_obj = [];
//需要延时加载的图片数量
var download_count = 0;
var last_offset = -1;
var doc_body;
var doc_element;
// 延时加载html标签(默认img iframe)
var lazy_load_tag;
// 设置延时加载的html标签(默认img iframe)
function initVar(tags) {
doc_body = document.body;
// 判断当前浏览器采用的渲染方式
doc_element = document.compatMode == ‘BackCompat’ ? doc_body : document.documentElement;
lazy_load_tag = tags || [“img”, “iframe”];
};
function initElementMap() {
var all_element = [];
//从所有相关元素中找出需要延时加载的元素
for (var i = 0,
len = lazy_load_tag.length; i < len; i++) {
var el = document.getElementsByTagName(lazy_load_tag[i]);
for (var j = 0,
len2 = el.length; j < len2; j++) {
if (typeof (el[j]) == “object” && el[j].getAttribute(“lazy_src”)) {
// 向element_obj数组中添加延时加载的元素el[j]
element_obj.push(el[j]);
}
}
}
for (var i = 0,
len = element_obj.length; i < len; i++) {
// o_img 每一张图片
var o_img = element_obj[i];
//得到图片相对document的距上距离
var t_index = getAbsoluteTop(o_img);
if (map_element[t_index]) {
map_element[t_index].push(i);
} else {
//按距上距离保存一个队列
var t_array = [];
t_array[0] = i;
map_element[t_index] = t_array;
download_count++; //需要延时加载的图片数量
}
}
};
// 元素加载的监听事件
function initDownloadListen() {
if (!download_count) return;
var offset = (window.MessageEvent && !document.getBoxObjectFor) ? doc_body.scrollTop : doc_element.scrollTop;
//可视化区域的offtset=document的高+
var visio_offset = offset + doc_element.clientHeight;
if (last_offset == visio_offset) {
setTimeout(initDownloadListen, 200);
return;
}
last_offset = visio_offset;
var visio_height = doc_element.clientHeight;
var img_show_height = visio_height + offset;
for (var key in map_element) {
if (img_show_height > key) {
var t_o = map_element[key];
var img_vl = t_o.length;
for (var l = 0; l < img_vl; l++) {
element_obj[t_o[l]].src = element_obj[t_o[l]].getAttribute(“lazy_src”);
}
delete map_element[key];
download_count–;
}
}
setTimeout(initDownloadListen, 200);
};
// 得到元素(img)相对顶部的绝对高度
function getAbsoluteTop(element) {
if (arguments.length != 1 || element == null) {
return null;
}
var offsetTop = element.offsetTop;
while (element = element.offsetParent) {
offsetTop += element.offsetTop;
}
return offsetTop;
}
function init(tags) {
initVar(tags);
initElementMap();
initDownloadListen();
};
return {
init: init
}
})();