NSDocument subclass "close" method called twice

Question

I have a document-based Cocoa application which subclasses NSDocument as MyDocument. Each MyDocument manages a separate background process (as an NSTask). I want to make sure that the NSTask is terminated when its corresponding MyDocument closes or when the whole application quits.

For the latter, I make the document observe NSApplicationWillTerminateNotification. For the former, I override the close method:

-(void)close {
    // Cleanup code here
    [super close];
}

(Incidentally, I can't put cleanup code in the dealloc method since the project is GC'd.)

The problem is this: If I open a MyDocument, make unsaved changes and then press cmd-Q, the close method is called twice. From the debugger, the call chain is: [MyDocument close] calls [NSDocument close], which calls [NSWindowController _windowDidClose], which calls [MyDocument close] again. (After that call, the application quits).

Is this expected behavior? If so, is there a better way to release document-specific resources? Or should I just make close safe to run multiple times?

Answer 1

I believe I've seen a post to the cocoadev mailing list saying that this is normal behaviour for the frameworks at the moment (but that it might change in future). You should make your -close method robust enough to handle multiple calls since no guarantee is made by AppKit that it will be called only once.

I don't believe you need care about NSApplicationWillTerminateNotification, since if I understand correctly, tasks will automatically be terminated when your app is too. Furthermore, if you support sudden termination, your app can be killed without notice/notification anyway.