WPF Routed Command only fires sometimes

Question

I have some RoutedCommands for commands like control-A, copy paste and they all work fine. Then I added 4 more routedcommands to move object up down left and right in the canvas using arrowkeys, they sometimes works and sometime doesn't. At first I thought it was a Focus issue on the Canvas but I just found out that at the same time, all the other routedcommands like control-A works but arrowkeys doesn't. I really have no idea what's going on here, they are identical routedcommands with different variable names, how come one works 100% of time and one only work 50% of time?

Working RoutedCommand:

_bindings.Add(new CommandBinding(DesignerCanvas.SelectAll, SelectAll_Executed));
SelectAll.InputGestures.Add(new KeyGesture(Key.A, ModifierKeys.Control));

private void SelectAll_Executed(object sender, ExecutedRoutedEventArgs e)
{
    SelectionService.SelectAll();
}

Malfunctioning RoutedCommand:

_bindings.Add(new CommandBinding(DesignerCanvas.MoveDown, MoveDown_Executed));
MoveDown.InputGestures.Add(new KeyGesture(Key.Down));

private void MoveDown_Executed(object sender, ExecutedRoutedEventArgs e)
{
    e.Handled = true;
    var selectedItems = from item in SelectionService.CurrentSelection.OfType<DesignerItem>()
                            select item;

    if (selectedItems.Count() > 0)
    {
        for (int i = 0; i < selectedItems.Count(); i++)
            selectedItems.ElementAt(i).Top += Option.OptionSingleton.Sensitivity;
    }
}

The malfunctioning RoutedCommand is just not firing sometimes, especially after I open some other window and come back to the canvas, then it will stop firing while other routedcommands are unaffected. Any ideas what's causing this weird behavior?

Answer 1

It turns out that it was a focus issue, I just set the focus to the canvas whenever mouse enters, now it's sort of fixed. Thanks everybody for answering.

Answer 2

This may be due to the fact that the key you are using is the "Down" key. I suspect that if you used a different key, it would work.

Some controls consume the arrow keys and pageup/pagedown keys. For example, TextBox does this. If your Canvas is in a scrollviewer, the scrollviewer might be eating it.

There are two workarounds for this:

1. Add a binding to the control that is eating the key gesture.
2. Handle KeyPreview for the Canvas (or any parent of the control that is eating the keystroke) and execute the command from there.

The answer to this question shows how you can do #2 without writing specific code in the KeyPreview handler for each command.

Answer 3

You can sometiems use very inclusive class event handlers to trace the route of an event:

EventManager.RegisterClassHandler(typeof(FrameworkElement), CommandManager.CanExecuteEvent, 
     new CanExecuteRoutedEventHandler((s, e) => Debug.WriteLine("CanExecute: " + s)), true);
EventManager.RegisterClassHandler(typeof(FrameworkElement), CommandManager.ExecutedEvent, 
     new CanExecuteRoutedEventHandler((s, e) => Debug.WriteLine("Executed:" + s)), true);
EventManager.RegisterClassHandler(typeof(FrameworkElement), CommandManager.ExecutedEvent, 
     new CanExecuteRoutedEventHandler((s, e) => Debug.WriteLine("KeyDown:" + s)), true);

In your case the KeyDown may be handled before it reaches the command binding or the CanExecute event may not reach it for some other reason.

Hopefully this will help you debug the problem