Is it dangerous to set off an autoreleased NSOperationQueue?

Question

I have a task that takes a rather long time and should run in the background. According to the documentation, this can be done using an NSOperationQueue. However, I do not want to keep a class-global copy of the NSOperationQueue since I really only use it for that one task. Hence, I just set it to autorelease and hope that it won't get released before the task is done. It works.
like this:

NSInvocationOperation *theTask = [NSInvocationOperation alloc];
theTask = [theTask initWithTarget:self
                         selector:@selector(doTask:)
                           object:nil];
NSOperationQueue *operationQueue = [[NSOperationQueue new] autorelease];
[operationQueue addOperation:theTask];
[theTask release];

I am kind of worried, though. Is this guaranteed to work? Or might operationQueue get deallocated at some point and take theTask with it?

Answer 1

There's nothing in the documentation to say what happens when the NSOperationQueue is released. It would be safest to assume there's no guarantee that theTask will get executed.

Answer 2

I would have guessed that an NSOperationQueue releases its tasks when it's released, but I've noticed that the tasks do complete and dealloc even if I release the queue immediately after adding the task. That said, I don't think I'd rely on that behavior - there's more to gain by storing the NSOperationQueue in an instance variable (and releasing it in dealloc). An instance variable will give you a way to call other methods on the queue (cancelAllOperations, setSuspended, etc).

Answer 3

Can't you use the [NSOperation mainQueue] object so that you don't need to worry about autoreleasing it? If you only need to add one task that seems to make the most sense to me.

http://developer.apple.com/mac/library/documentation/Cocoa/Reference/NSOperationQueue_class/Reference/Reference.html#//apple_ref/doc/uid/TP40004592-RH2-SW21

Answer 4

There's no guarantee that it's safe to release an NSOperationQueue while it's still working. I suspect it probably is safe and this guarantee will probably be added someday, but it isn't there now. However, the equivalent Grand Central Dispatch API does guarantee that you can safely release its queues when you're done using them and it will keep them around as long as it needs them. So if you're on a platform with GCD, you can use that to be sure it won't blow up in the meantime.

Alternatively, you could create a wrapper class that checks if a queue is finished and releases both the queue and itself when the queue is finished.