本文介绍了一个使用ASP.NET AJAX实现异步回发的示例，通过客户端JavaScript控制异步请求的过程，包括取消回发、显示加载提示及处理回发后的更新。
//pageLoad函数在成功更新后隐藏提示面板 function pageLoad() 
{
showmessage("false","");
} //设定当引发initializeRequest事件时便执行事件处理函数CheckStatus Sys.WebForms.PageRequestManager.getInstance().add_initializeRequest(CheckStatus); function CheckStatus(sender,args) { //取得PageRequestManager类的实例 var prm=Sys.WebForms.PageRequestManager.getInstance(); //判断是否在进行异步回发且点击了“取消”按钮，就调用PageRequestManager的abortPostBack方法取消异步回发 if(prm.get_isInAsyncPostBack() & args.get_postBackElement().id=='LinkButton1')
{ prm.abortPostBack(); //隐藏提示面板 showmessage("false","");
} //判断是否在进行异步回发并且又单击了发送异步回发按钮此时取消新的异步回送并显示正在处理先前的请求 else if(prm.get_isInAsyncPostBack() & args.get_postBackElement().id=='Button1') { args.set_cancel(true); showmessage("true","仍在处理先前的请求"); } //提示正在取得数据 else if(!prm.get_isInAsyncPostBack() & args.get_postBackElement().id=='Button1') { showmessage("true","正在取得数据"); } } //显示与隐藏提示信息 function showmessage(visiable,msg) { if(visiable=="true") { var loading=$get("loading"); loading.style.visibility='visible'; $get("message").innerHTML=msg; } if(visiable=="false") { var loading=$get("loading"); loading.style.visibility='hidden'; $get("message").innerHTML=msg; } } if (typeof(Sys) !== 'undefined') Sys.Application.notifyScriptLoaded();前台页面主要代码：
<body> <form id="form1" runat="server"> <div align="center" > <asp:ScriptManager ID="ScriptManager1" runat="server" > <Scripts> <asp:ScriptReference Path="ClientControl.js" /> </Scripts> </asp:ScriptManager> <asp:Button ID="Button1" runat="server" Text="获取时间" onclick="Button1_Click" /> <br /> <div id="loading" align="left" style=" visibility: hidden;"> <img alt="" src="progressbar_long.gif" /><span id="message"></span> <asp:LinkButton ID="LinkButton1" runat="server">取消</asp:LinkButton> </div> <asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional"> <ContentTemplate> <span>当前时间：</span><asp:Label ID="Label1" runat="server" Text=""></asp:Label> </ContentTemplate> <Triggers> <asp:AsyncPostBackTrigger ControlID="Button1" EventName="Click"> </asp:AsyncPostBackTrigger> <asp:AsyncPostBackTrigger ControlID="LinkButton1" EventName="Click" /> </Triggers> </asp:UpdatePanel> </div> </form></body>Button1_Click事件代码：
protected void Button1_Click(object sender, EventArgs e) { //刻意暂停6S System.Threading.Thread.Sleep(6000); Label1.Text = DateTime.Now.ToString(); }
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