ajax滚动条懒加载

<div class="activity_box"> ?<ul class="clearfix" id="contact"> ????<!--内容--> ?</ul> ?<div id="loading" style="text-align: center"> ???<img src="/images/loading.gif" alt="" />正在加载数据,请稍候...
 ?</div></div>

<script> ???var pageindex = 1; ???var DataIsNull = false; ???function GetData(pageindex) { ???????var url = "ajax/do_data.ashx"; ???????var act = "activity"; ???????$.ajax({ ???????????type: "POST", ???????????url: url, ???????????dataType: "html", ???????????async: true, ???????????data: { ???????????????"act": act, ???????????????"page": pageindex ???????????}, ???????????beforeSend: function () { ???????????????ShowDiv(); ???????????}, ???????????complete: function () { ???????????????HiddenDiv(); ???????????}, ???????????success: function (data) { ???????????????if (data == "") { ???????????????????DataIsNull = true; ???????????????} ???????????????$("#contact").append(data); ???????????} ???????}); ???} ???GetData(pageindex); ???//显示加载数据 ???function ShowDiv() { ???????$("#loading").show(); ???} ???//隐藏加载数据 ???function HiddenDiv() { ???????$("#loading").hide(); ???} ???//滚动条滚动事件 ???$(window).scroll(function () { ???????if ($(document).scrollTop() + $(window).height() >= $(document).height()) { ???????????if (!DataIsNull) { ???????????????next(); ???????????} ???????} ???}); ???function next() { ???????pageindex++; ???????GetData(pageindex); ???}</script>

public void activity() ???{ ???????int maxPageSize = 8; ???????int pageID = Convert.ToInt32(HttpContext.Current.Request.Form["page"]); ???????int newsCount = BLLNewsInfo.GetRecordCount("");//新闻总数 ???????int PageCount = newsCount / maxPageSize + (newsCount % maxPageSize == 0 ? 0 : 1);//最大页数 ???????int MaxPerPage1 = pageID == PageCount ? (newsCount - (PageCount - 1) * maxPageSize) : maxPageSize; ???????if (pageID > PageCount) ???????{ ???????????HttpContext.Current.Response.Write(""); ???????} ???????else ???????{ ???????????News_InfoList = BLLNewsInfo.GetListByPage("1=1", " AddDate", MaxPerPage1, maxPageSize * pageID); ???????????StringBuilder html = new StringBuilder(); ???????????foreach (SHLY.Model.News_Info item in News_InfoList) ???????????{ ???????????????html.AppendLine("<li>"); ???????????????html.AppendLine("<a href=\"/news-" + item.Id + ".html\" title=\"" + item.Title + "\" >"); ???????????????html.AppendLine("<img src=\"" + item.TitlePic + "\"/>"); ???????????????html.AppendLine("</a>"); ???????????????html.AppendLine("<p><a href=\"/news-" + item.Id + ".html\" title=\"" + item.Title + "\" >"); ???????????????html.AppendLine("" + item.Title + "</a></p>"); ???????????????html.AppendLine("</li>"); ???????????} ???????????HttpContext.Current.Response.Write(html.ToString()); ???????} ???}