延迟加载

var imagesrcname='a',thissrolltop=300,
	lazyLoad=(function(){
		var map_element    = {},
			element_obj    = [],
			download_count = 0,
			last_offset    = -1,
			doc_body,
			doc_element,
			lazy_load_tag; 
		function initVar(tags) { 
			doc_body      = document.body; 
			doc_element   = document.compatMode == 'BackCompat' ? doc_body: document.documentElement; 
			lazy_load_tag = tags || ["img", "iframe"]; 
		}; 
		function initElementMap() { 
			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(imagesrcname)) { 
						element_obj.push(el[j]); 
					} 
				} 
			} 
			for (var i = 0, len = element_obj.length; i < len; i++) { 
				var o_img = element_obj[i]; 
				var t_index = getAbsoluteTop(o_img);
				if (map_element[t_index]) { 
					map_element[t_index].push(i); 
				} else { 
					var t_array = []; 
					t_array[0] = i; 
					map_element[t_index] = t_array; 
					download_count++;
				} 
			} 
		}; 
		function initDownloadListen() {  
			if (!download_count) return; 
			var getscrolltop = document.body.scrollTop;
			if(getscrolltop == 0)
			{
				getscrolltop = document.documentElement.scrollTop;
			}
			var offset = (window.MessageEvent && !document.getBoxObjectFor) ? getscrolltop: getscrolltop; 
			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 + thissrolltop; 
			var img_show_height = visio_height + offset; 
			//var allurlnum = parseInt(imageurlarray.length);
			//if(allurlnum <= 0)
			//{
			//	allurlnum = 1;
			//}
			var j = 0;
			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++) { 
						//var u_j = j % allurlnum;
						//var thisurl = imageurlarray[u_j];
						//if(element_obj[t_o[l]].getAttribute(imagesrcname).indexOf('http://') != -1) thisurl='';
						//element_obj[t_o[l]].src = thisurl + element_obj[t_o[l]].getAttribute(imagesrcname); 
						element_obj[t_o[l]].src = element_obj[t_o[l]].getAttribute(imagesrcname);
						element_obj[t_o[l]].removeAttribute(imagesrcname);
						j++; 
					} 
					delete map_element[key]; 
					download_count--; 
				}
			} 
			setTimeout(initDownloadListen, 200); 
		}; 
		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 
		} 
})();
lazyLoad.init();