PageAsyncTask Update UI

我正在学习asp.net中的异步操作。我在MSDN找到了一篇文章。代码工作，但我希望能够在漫长的过程中更新UI。它只显示过程完成后的结果。我使用相同的代码为AsyncClass在提到的链接。下面是代码:

            AsyncTask slowTask1 = new AsyncTask();
            PageAsyncTask task = new PageAsyncTask(slowTask1.OnBegin, slowTask1.OnEnd, slowTask1.OnTimeout, "Async1", true);
            Page.AsyncTimeout = new TimeSpan(0, 0, 5);
            Page.RegisterAsyncTask(task);
           // Page.ExecuteRegisteredAsyncTasks();
            IAsyncResult result = slowTask1.OnBegin(this.Page,null,null,null);
            WaitHandle waitHandle = result.AsyncWaitHandle;
            waitHandle.WaitOne(2000, false);
            if(result.IsCompleted)
            {
                slowTask1.OnEnd(result);
                lbProgress.Text = slowTask1.GetAsyncTaskProgress();
                Thread.Sleep(1000);
            }
            else
            lbProgress.Text = slowTask1.GetAsyncTaskProgress();