<asp:UpdateProgress ID="UpdateProgress1" runat="server" DisplayAfter="0" DynamicLayout="false">
    <ProgressTemplate>
        <span style="color:Red">Loading...</span>
    </ProgressTemplate>
</asp:UpdateProgress>

<asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <ContentTemplate>
        <%= DateTime.Now %><br />
        <asp:Label ID="Label1" runat="server" Text=""></asp:Label><br />
        <asp:Button ID="btnPrecedence" runat="server" Text="优先" OnClick="Button_OnClick" />
        <asp:Button ID="Button2" runat="server" Text="普通" OnClick="Button_OnClick" />
    </ContentTemplate>
</asp:UpdatePanel>

<script type="text/javascript" language="javascript">
    var lastPostBackButtonId = null;
    var btnPrecedenceId = "<%= this.btnPrecedence.ClientID %>";
    
    Sys.WebForms.PageRequestManager.getInstance().add_initializeRequest(
        function(sender, e)
        {
            var prm = Sys.WebForms.PageRequestManager.getInstance();
            if (prm.get_isInAsyncPostBack())
            {
                if (lastPostBackButtonId == btnPrecedenceId)
                {
                    e.set_cancel(true);
                    
                    if (e.get_postBackElement().id == btnPrecedenceId)
                    {
                        showMessage("不可重复发起优先的刷新。");
                    }
                    else
                    {
                        showMessage("请等待优先的刷新结束。");
                    }
                    
                    return;
                }
                else if (e.get_postBackElement().id == btnPrecedenceId)
                {
                    showMessage("发起优先的刷新，普通的刷新将被取消。");
                }
                else
                {
                    showMessage("重新发起普通的刷新，前一次提交将被取消。");
                }
            }
            
            lastP