Update ASP.NET panel asynchronously

I need some help with this problem:

Situation: I've got a usercontrol (in SharePoint) that reads query string and processes it with an asynchronous event. While it's busy, a spinner is shown. After the event is finished, the updatepanel inside the usercontrol should update and show the message (+ hide the spinner)

Code: I've got a function that's called asynchronously on the UserControl_Unload event.

private delegate void AsyncFunction(string activation);

void UserControl_Unload(object sender, EventArgs e)
{
    if (!Page.IsPostBack)
    {
        AsyncFunction dlgt = new AsyncFunction(this.CheckUrl);
        AsyncCallback callback = new AsyncCallback(FunctionCallBack);
        IAsyncResult ar = dlgt.BeginInvoke(activationcode, callback, null);
    }
}
private void CheckUrl(string lalala)
{
   // Some code
}

User control markup:

<asp:UpdatePanel runat="server" id="pnlContent" updatemode="Conditional"      ChildrenAsTriggers="true">
    <ContentTemplate>
         <asp:UpdatePanel runat="server" id="pnlStatus" UpdateMode="Conditional"  ChildrenAsTriggers="false">
            <ContentTemplate>
                <asp:Label runat="server" ID="lblMessage" />
                <asp:LinkButton runat="server" ID="btnHome" Text="Terug naar welkom-pagina" PostBackUrl="<% $SPUrl:~sitecollection %>"  />
            </ContentTemplate>
        </asp:UpdatePanel>
        <asp:UpdatePanel runat="server" id="pnlGegevens" UpdateMode="Conditional" ChildrenAsTri开发者_如何学Goggers="false">
            <ContentTemplate>
                <div><asp:Image runat="server" ID="imgLoading" AlternateText="Loading..." CssClass="gb_pl_loadingImage" ImageUrl="<% $SPUrl:~sitecollection/Style Library/GB-VW Styles/Images/ajax-loader.gif %>"/></div>
                <div class="gb_pl_loading">Even geduld aub. De gebruiker wordt geactiveerd...</div>
            </ContentTemplate>
        </asp:UpdatePanel>
    </ContentTemplate>
</asp:UpdatePanel>

This all works great, but when I need to update the panel, it doesn't work.

private void FunctionCallBack(IAsyncResult test)
{
    pnlContent.Update()
}

Anyone who knows how to solve this? (if it's possible only use asp, c# or javascript)

Is it possible to fire off the asynchronous operation from the client? That is, display your page but include javascript that makes a webservice call? That way you at least have something to wait for, and your client will get notified becuase it initiated the request.

Otherwise I don't see how the page, which is already gone to the client, could be updated by the server once the async op finishes.