GUI don't respond until click

Question

I have a similar problem than here : WPF MVVM Light: Command.RaiseCanExecuteChanged() doesn't work, using commands with WPF and have my GUI not working until I click somewhere in the scren. I don't use MVVM Light.

I call an external DLL to do some action, by calling ExternalDLL.Start(), and call GetStatus() to know if the action started. If I get the correct status in return, I change the actual action, and it have to activate a button on my GUI.

The button don't activate himself until I click somewhere.

I checked for the thread, but it seems it's on the same thread, I tried to put it in the GUI thread to, by using Application.Current.Dispatcher.BeginInvoke, but it didn't work too.

Here is my code :

private async void StartScanCommand_Executed(object sender, ExecutedRoutedEventArgs e)
{  
    ExternalDLL.Start();  
    WaitForStarting();
}
private async void WaitForStarting()
{
    Waiting();
    Stopwatch chrono = new Stopwatch();
    chrono.Start();
    bool started = false;
    while (chrono.ElapsedMilliseconds < 20000)
    {
        if (ExternalDLL.GetStatus() != ExternalDLL.Status.Started)
        {
            await Task.Delay(100);
        }
        else
        {
            started = true;
            chrono.Stop();
            StartedAction();
            break;
        }
    }
    if (!started)
    {
        MessageBox.Show("Error");
    }
}

The Waiting() method call activate a button in the GUI and work. but the StartedAction() have to activate a button too, and doesn't work.

Here is the code for started action :

private void StartedAction()
{
    _actualAction = ActualAction.DoingAction;
}

And here is the button's can execute method :

private void SomeButtonCommand_CanExecute(object sender, 
CanExecuteRoutedEventArgs e) 
{ 
    e.CanExecute = _actualAction == ActualAction.DoingAction; 
}

What am I doing wrong ?

Answer 1

The problem is simply that the bound Command's CanExecute status is not re-evaluted when the ActualAction value changes.

Call CommandManager.InvalidateRequerySuggested() to force re-evaluation.

private void StartedAction()
{
    _actualAction = ActualAction.DoingAction;
    CommandManager.InvalidateRequerySuggested();
}

Answer 2

You are doing the background work on the UI thread. Don't do it there, do it in another thread, and use polling, events or other callback methods to update the UI (on the UI thread).

For example you can do:

Task.Run(() => { OtherDll.DoWork(); };

This will kick off the other work on the external thread.

If you need more control you can wrap the functionality of the other dll in a thread all by itself.

Public Class OtherDLLThread 
{
    Thread _internalThread;

    public OtherDLLThread()
    {
         _internalThread = new Thread(ThreadMainLoop);
    }

    public void ThreadMainLoop()
    {
        OtherDLL.DoWork();
    }

    public static void Start() 
    {
       _internalThread.Start();
    }

}

Use it like this:

OtherDLLThread other = new OtherDLLThread();
other.Start();

Here is another function for bumping code to the UI thread:

    /// <summary>
    ///     Runs the action on UI thread.
    /// </summary>
    /// <param name="action">The action.</param>
    public static void RunOnUIThread(Action action)
    {
        try
        {
            if (Application.Current != null)
                Application.Current.Dispatcher.Invoke(action);
        }
        catch (Exception ee)
        {
            _logger.Fatal("UI Thread Code Crashed. Action detail: " + action.Method, ee);
            //SystemManager.Instance.SendErrorEmailToCsaTeam("Kiosk Application Crashed", "UI Thread Code Crashed. Action detail: " + action.Method);
            throw;
        }
    }

Use it like this:

RunOnUITHread(() => lblStatus.Text = "Working...");