Shutting down multithreaded NSDocument

I have an NSDocument-based Cocoa app and I have a couple of secondary threads that I need to terminate gracefully (wait for t开发者_StackOverflowhem to run through the current loop) when the users closes the document window or when the application quits. I'm using canCloseDocumentWithDelegate to send a flag to the threads when the document is closing and then when they're done, one of them calls [NSDocument close]. This seems to work peachy keen when the user closes the document window, but when you quit the app, it goes all kinds of wrong (crashes before it calls anything). What is the correct procedure for something like this?

The best possible way is for the threads to own the objects necessary for the thread to finish doing whatever it is doing to the point of being able to abort processing and terminate as quickly as possible.

Under non-GC, this means a -retain that the thread -releases when done. For GC, it is just a hard reference to the object(s) desired.

If there is some kind of lengthy processing that must go on and must complete before the document is closed, then drop a sheet with a progress bar and leave the document modal until done (both Aperture and iPhoto do exactly this).