Get document object with current focus from NSCollectionView
<p>I have a (subclassed) NSCollectionView open, containing multiple text views. Each of the text views is mapped to a (subclassed) NSDocument object. (The idea is to use the document architecture's save functions but not its windowing functions, because I need multiple documents in the same window and the traditional document architecture doesn't allow that.) </p>

<p>Now, there's a function I'd like the user to be able to call from the main menu that will affect their currently selected document. That is: the document is currently visible in a text view with current focus, and the menu command should make an alteration to that document. But the sender of the menu command is just the menu. When the window controller handles the command from the menu, how can I tell it what the currently selected document is?</p>