How to use IsKeyboardFocusWithinChanged action message with caliburn?

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"/>

来源：https://stackoverflow.com/questions/20753248/how-to-use-iskeyboardfocuswithinchanged-action-message-with-caliburn