implement CanExecute MVVM light using ICommand - wpf

Question

I have a program, where a button should only be active if the corresponding userControl has focus.
I am using MVVM light, and got a command implementing the ICommand interface.
I have tried using the Keyboard.FocusedElement, but this returns nothing.
This is the code for the command (Note that it just returns true right now to get it working, this sis of course what i'm trying to fix):
class AddItemToNodeCommand<T> : ICommand
{
public bool CanExecute(object parameter)
{
Debug.WriteLine("fokuselement er: " + Keyboard.FocusedElement);
return true;
// throw new NotImplementedException();
}
public event EventHandler CanExecuteChanged;
public void Execute(object parameter)
{
Debug.WriteLine("Parameter er: " + parameter);
Debug.WriteLine("fokuselement er: " + Keyboard.FocusedElement);
//throw new NotImplementedException();
}
}
From the viewmodel:
public ICommand AddItemToNodeCommand { get; private set; }
AddItemToNodeCommand = new AddItemToNodeCommand<object>();
And finally some of the XAML:
<RibbonButton SmallImageSource="../Images/whatever.png" Label="Attribute" Command="{Binding AddItemToNodeCommand}" CommandParameter="Attribute"/>
I haven't posted the xaml for the userControl, but the idea is that when the userControl has focus, the CanExecute should be true.. I thought it would work with the Keyboard.FokusedElement, but I was wrong. What can I do?
Thank you in advance.

Answer 1

Seems that Keyboard.FocusedElement is a bit fickly.
Have a look here for a solution involving attached behavior and overriding the Keyboard.GotKeyboardFocusEvent. I tried it, and it seems to work.
Otherwise, you can bind to IsKeyboardFocused or IsKeyboardFocusWithin. Just put this in your xaml for a quick example:
<StackPanel>
<TextBox Name="test_txtbx" >Hullo</TextBox>
<TextBox Name="test_txtbx_2">Hullo 2</TextBox>
<Label Content="{Binding ElementName=test_txtbx, Path=IsKeyboardFocused}"></Label>
<Label Content="{Binding ElementName=test_txtbx_2,Path=IsKeyboardFocusWithin }"></Label>
<Button Click="TestClick">test me </Button>
</StackPanel>
The label shows if the matching textbox has focus.
(Here's an old article which says focus is funny as well .. dunno if still relevant though).