title: Sharing NSDocument subclass between multiple NSWindowControllers
body: <p>I have a application that isn't currently a document-based application (because I thought it would be more trouble than its worth). However I've been thinking it may possibly be worth it now, but there is one issue I haven't worked out yet. Sharing <code>NSDocument</code> subclasses between multiple windows.</p>
  
  <p>So do subclasses of <code>NSDocument</code> have to be tied to a single <code>NSWindowController</code>, or can I share these between multiple windows? The reason I ask is my applications files are likely to up 100MiB (or larger) and its also likely that a user will open the same file in more than one window. Also these files take a relatively long time to process, so allowing multiple windows access to the same <code>NSDocument</code> would be excellent. Also, the files are updated very quickly with lots of data, so synchronizing multiple instances of the same document would use a lot of CPU time.</p>
  
  <p>Has anybody done this, or can it even be done with a Document-based application? Any advice on this topic is greatly appreciated.</p>