How to use IsKeyboardFocusWithinChanged action mes

问题:

I've noticed sometimes I can not get certain ActionMessages to behave, for example:

<TextBox cal:Message.Attach="[Event IsKeyboardFocusWithinChanged] = [Action Foo()]"/>

Will never call Foo, I'm forced to use code behind:

<TextBox IsKeyboardFocusWithinChanged="Foo"/>