Question

I’m developing a database application for Windows Phone 7.5 (mango). I trying (during tapping on a button) to update a textblock with the text “Searching…” This button performs a rather lengthy search in a big table and thus I want to inform the user. However everything I trying is failed! Here is one of the code snippets that I used. Is there any way to achieve this? Any help helping me understand what’s wrong would be appreciated.

private void btnSearch_Tap(object sender, GestureEventArgs e)
{
    workerThread = new Thread(new ThreadStart(turnVisibilityOn));
    workerThread.Start();
    while (!workerThread.IsAlive) ;
    Thread.Sleep(500);

    //Search database takes about 15 sec on windows phone device!
    Procedures[] results = CSDatabase.RunQuery<Procedures>(@"select Code, Description from tblLibraries where Description like '%" +
    textBox1.Text + "%' or Code like '%" + textBox1.Text + "%'");

    this.MyListBox.ItemsSource = results;

    // Of course this not work
    Search1.Text = ""

}

private void turnVisibilityOn()
{
    // Inform the user updating the Search1 textblock
    // UIThread is a static class -Dispatcher.BeginInvoke(action)-
    UIThread.Invoke(() => Search1.Text = "Searching...");

}


public static class UIThread
{
   private static readonly Dispatcher Dispatcher;

       static UIThread()
       {
       // Store a reference to the current Dispatcher once per application
       Dispatcher = Deployment.Current.Dispatcher;
       }

       /// <summary>
       ///   Invokes the given action on the UI thread - if the current thread is the UI thread this will just invoke the action directly on
       ///   the current thread so it can be safely called without the calling method being aware of which thread it is on.
      /// </summary>
     public static void Invoke(Action action)
     {
         if (Dispatcher.CheckAccess())
         action.Invoke();
         else
         Dispatcher.BeginInvoke(action);
     }
}

Answer 1

I am not sure I understand the problem correctly.
The “Searching…” text does not show up? The

// Of course this not work
Search1.Text = ""

line doesn’t work? (Why do you write “Of course this not work”? Why wouldn’t it work?)

I don’t understand why you change the text to “Searching…” in a background thread. You could do it in the UI thread, and make the time-consuming work in the background thread, something like this (I switched to using a ThreadPool):

private void btnSearch_Tap( object sender, GestureEventArgs e )
{
    Search1.Text = "Searching..."

    ThreadPool.QueueUserWorkItem(p =>
    {
        Procedures[] results = CSDatabase.RunQuery<Procedures>( @"select Code, Description from tblLibraries where Description like '%" +
        textBox1.Text + "%' or Code like '%" + textBox1.Text + "%'" );

        // Dispatch manipulation of UI elements:
        Dispatcher.BeginInvoke( () =>
        {
            this.MyListBox.ItemsSource = results;
            Search1.Text = "";
        } );
    } ) ;
}

You always have to manipulate the UI elements from the UI thread (on which the event handler runs) and you have to do the time-consuming work in a background thread.