Question

I have an NSOperation subclass which uses Core Data.

It has a custom init method like this:

- (id)initWithManagedObjectContext:(NSManagedObjectContext*)moc;

As you may know, Core Data managed object contexts are not thread-safe and every thread needs it’s own managed object context. When creating this NSOperation, I create a new NSManagedObjectContext and pass it as the moc argument to the initializer.

Here’s the important part: When initializing, I register for a notification:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(managedObjectContextDidSave:)  name:NSManagedObjectContextDidSaveNotification object:moc];

In -dealloc I unregister from the NotificationCenter:

- (void)dealloc {
    [[NSNotificationCenter defaultCenter] removeObserver:self name:nil object:nil];
}

Before I migrated to ARC, this code worked perfectly fine.

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Now under ARC, I get a stack trace like this:

1) The NSOperation object receives -dealloc

2) -removeObserver:name:object: is called on the NSNotificationCenter.

3) __arclite_objc_release

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4) The NSOperation object receives -dealloc

5) -removeObserver:name:object: is called on the NSNotificationCenter.

6) __arclite_objc_release

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7) The NSOperation object receives -dealloc

8) -removeObserver:name:object: is called on the NSNotificationCenter.

9) __arclite_objc_release

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This continues forever. -dealloc, unregister from notification center, __arclite_objc_release, -dealloc, …

It seems like if __arclite_objc_release triggeres a call to -dealloc even though the NSOperation object has been destroyed!

How is this possible?

Update: For some insanely weird reason the infinite loop to -dealloc disappeared after I changed my notification center unregistration code to this:

[[NSNotificationCenter defaultCenter] removeObserver:self name:nil object:self.managedObjectContext];

So instead of simply unregistering for any notification sent by any sender, I explicitely unregister for any notification sent by a particular sender.

The obvious problem is this: Since ARC inserts code into -dealloc, I don’t know if it is inserted before my -dealloc code, or after my -dealloc code.

Answer 1

Reading your update, it seems you should call [super dealloc] in a different place than where you are (if you are). Try calling it first thing in the dealloc method, then unregister your NSNotification