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 
} 
})();