Question

I have two GCD blocks that are async. The first is for the background thread, the second runs on the main thread. This works great, but I just saw somewhere talking that I might need to release them using dispatch_release(). E.g.:

// Use gcd
dispatch_queue_t queue = dispatch_queue_create("com.awesome", 0);
dispatch_queue_t main = dispatch_get_main_queue();

//  do the long running work in bg async queue
// within that, call to update UI on main thread.
dispatch_async(queue, ^{ 
   // Do work in the background



    // Release
    dispatch_release(queue);

   dispatch_async(main, ^{ 

       // Main


       // Release
       dispatch_release(main);

   });//end
});//end

Is this true? Do I need to release them here?

Answer 1

Only release queues that you create; don’t release the main queue or the global concurrent queues (or, again, any you did not create yourself). It’s also not a good idea to nest the release within the work block enqueued on that queue, as you’re doing, because that’s doing it in the wrong scope and this:

queue = dispatch_queue_create(...)
dispatch_async(queue, ^{ something; dispatch_release(queue); });
dispatch_async(queue, ^{ something else}); // CRASH!

Won’t work when you later change the code to add that 2nd dispatch_async(). Always pairing your create/release calls in the same scope, assuming that you can, is a better stylistic choice.