lazyLoad = (function () {
            var map_element = {};
            var element_obj = [];
            var download_count = 0;
            var last_offset = -1;
            var doc_body;
            var doc_element;
            var lazy_load_tag;
             
            function initVar(tags) {
                doc_body = document.body;
                //判断是否为IE的"怪异模式"
                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")) {
                            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); //得到图片相对document的距上距离  
                    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  
                    }
                }
 
            };
 
            function initDownloadListen() {
                if (!download_count) return;
                var offset = (window.MessageEvent && !document.getBoxObjectFor) ? doc_body.scrollTop : doc_element.scrollTop;
                //可视化区域的高=offtset+document的高  
                var visio_offset = offset + doc_element.clientHeight;
                if (last_offset == visio_offset) {
                    setTimeout(initDownloadListen, 200);//每隔0.2秒检测一次，这玩意儿感觉有些耗资源的
                    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);
            };
 
            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
            }
        })();