js实现网页图片延时加载 提升网页打开速度

<script type="text/javascript" src="jquery.js"></script> 
<script type="text/javascript" src="jquery.lazyload.js"></script>

$(document).ready(function($){
$("img").lazyload({
placeholder:"grey.gif", //加载图片前的占位图片
effect:"fadeIn" //加载图片使用的效果(淡入)
});
});

<img lazy_src="图片路径" border="0"/>

 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
 
  }
 
})();

///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");
 
 });