图片的延迟加载

gistfile1.js

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;
    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(all_element[key]);
        }
      }
    }

    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);
  };

  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
  }
})();

//usage:

lazyLoad.init()

//如果页面上存在有内容切换的栏目的话，可能在切换时切换的内容里的图片可能会不显示，处理的方法是在内容时单独图片加载处理，如：
//切换内容的代码…

chlid.find("img[init_src]").each(function(){ 
    $(this).attr("src",$(this).attr("init_src")); 
    $(this).removeAttr("init_src"); 
 });