Question

Sometimes Cocoa allows a user to pass either NSOperationQueue or dispath_queue_t to the async method:

+[NSURLConnection sendAsynchronousRequest:queue:completionHandler:]
-[AVPlayer addBoundaryTimeObserverForTimes:queue:usingBlock:]
-[NSNotificationCenter addObserverForName:object:queue:usingBlock:]

Sometime doesn’t:

-[NSDocument continueActivityUsingBlock:]
-[NSSavePanel beginWithCompletionHandler:]
-[GKAchievementDescription loadImageWithCompletionHandler:]

To me, passing a queue to the method looks ambiguous, because you can always catch desired queue when you create a block. E.g.:

NSOperationQueue *q = …;
[aDocument continueActivityUsingBlock:^{
    [q addOperationWithBlock:^{
        // Do actual work here.
    }];
}];

Maybe I’m missing something and passing queues makes sense?

Answer 1

Depends.

Do you need a callback on completion?

If so, you might need to specify a queue. The type — NSOperationQueue vs. GCD queue — is up to you.

However, if you define your callback as either always being invoked on the main queue (which would be appropriate for a callback that is always going to update the UI extensively) or is always going to be called asynchronously (i.e. the callback is dispatched onto an asynchronous queue, typically), there there is no reason to pass the targeted queue.

If your callback is one where the expectation is that it will potentially interact with data whose manipulation must be serialized, defining an API that provides a serialization primitive — a queue — can be a powerful shortcut.