grand-central-dispatch

I have an asynchronous method longRunningMethodOnObject:completion: this method receives an object of type 'Object' - does work with its data and then calls the completion handler. I need to call many different "longRunningMethods" and wait for all to complete. I would like all of the "longRunningMethodOnObject" to run asynchronously (parallel) to each other in the "for" loop. (I am not certain if the "longRunningMethodOnObject" runs in serial to each other but this is more of a general question) I'm not sure I have created a proper semaphore and would appreciate an explanation on the proper

I need a clarifications on how dispatch_queue s is related to reentrancy and deadlocks. Reading this blog post Thread Safety Basics on iOS/OS X , I encountered this sentence: All dispatch queues are non-reentrant, meaning you will deadlock if you attempt to dispatch_sync on the current queue. So, what is the relationship between reentrancy and deadlock? Why, if a dispatch_queue is non-reentrant, does a deadlock arise when you are using dispatch_sync call? In my understanding, you can have a deadlock using dispatch_sync only if the thread you are running on is the same thread where the block is

I have this code in the - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath delegate call: dispatch_async( dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ AVPlayerItem *playerItem = [AVPlayerItem playerItemWithURL:[webUrls objectAtIndex:indexPath.row]]; CMTime timeduration = playerItem.duration; float seconds = CMTimeGetSeconds(timeduration); NSString *duration = [NSString stringWithFormat:@"%f", seconds]; dispatch_async( dispatch_get_main_queue(), ^{ UITableViewCell *updatecell = [tblView cellForRowAtIndexPath:indexPath];

I'm hitting a race-condition in my app, where all or all but 1 threads get stuck on syscall_thread_switch whenever I pause debugging. It reproduces much more often on the simulator, but also on the iPad Air. There is ALWAYS at least 2 threads stuck in CocoaLumberjack's queueLogMessage: -- see screenshots. I've never seen this before on 8.1 and 8.2, but i'm hitting it often on 8.3. I'm not claiming this is an 8.3 bug :) This is a level of complexity i've never had to debug before, so i'm not sure what to do. I hope I'm providing enough information, please let me know if you need more (please be

I am trying to understand the proper way of using Grand Central Dispatch (GCD) to implement concurrent read exclusive write model of controlling access to a resource. Suppose there is a NSMutableDictionary that is read a lot and once in awhile updated. What is the proper way of ensuring that reads always work with consistent state of the dictionary? Sure I can use a queue and serialize all read and write access to the dictionary, but that would unnecessarily serialize reads which should be allowed to access the dictionary concurrently. At first the use of groups here sounds promising. I could