Recently, I had the need for a function that I could use to guarantee synchronous execution of a given block on a particular serial dispatch queue. There was the possibility that this shared function could be called from something already running on that queue, so I needed to check for this case in order to prevent a deadlock from a synchronous dispatch to the same queue.
I used code like the following to do this:
void runSynchronouslyOnVideoProcessingQueue(void (^block)(void))
dispatch_queue_t videoProcessingQueue = [GPUImageOpenGLESContext sharedOpenGLESQueue];
if (dispatch_get_current_queue() == videoProcessingQueue)
Recommended for debugging and logging purposes only:
The code must not make any assumptions about the queue returned,
unless it is one of the global queues or a queue the code has itself
created. The code must not assume that synchronous execution onto a
queue is safe from deadlock if that queue is not the one returned by
Assign whatever identifier you want using
dispatch_queue_set_specific(). You can then check your identifier using
dispatch_get_specific() is nice because it'll start at the current queue, and then walk up the target queues if the key isn't set on the current one. This usually doesn't matter, but can be useful in some cases.