ASP.NET

function SimpleCallback() {
   var lCurrentStatus = PageMethods.GetCurrentStatus(ProcessResults);
}

function ProcessResults(pCurrentStatus) {
   if (pCurrentStatus != "Completed") {
       window.setTimeout("SimpleCallback()", 100);
   }
}

function SaveButtonClick(pArgument) {
   var lAjaxManager = $find("<%= MyAjaxManager.ClientId %>");
   lAjaxManager.ajaxRequest(pArgument);
   SimpleCallback();
}

<asp:Button ID="SaveButton" runat="server" Text="Save" OnClientClick="return SaveButtonClick('SaveStuff');"

[WebMethod]
public static string GetCurrentStatus()
{
   return FCurrentStatus;
}

protected void MyAjaxManager_AjaxRequest(object sender, AjaxRequestEventArgs e)
{
    if (e.Argument == "SaveStuff")
    {
        FCurrentStatus = "ProcessRunning";
        SaveInformation();
    }
}

//While this is being executed the webmetod, GetCurrentStatus, is ignored until this process is completed
delegate void MyLongRunningProcess();

private void SaveInformation()
{
  var lLongRunningMethod = new MyLongRunningProcess(NextClass.Save());
  
  IAsyncResult lResult = lLongRunningMethod.BeginInvoke(null, null);

  while (!lResult.IsCompleted)
  {
     //Update progressbar with new percentage
  }
 
  lLongRunningMethod.EndInvoke(lResult);
}