Performing a Completion Block When a Task Is Done (!!!)

Executing a completion callback after a task :

void average_async(int *data, size_t len,
   dispatch_queue_t queue, void (^block)(int))
{
   // Retain the queue provided by the user to make
   // sure it does not disappear before the completion
   // block can be called.
   dispatch_retain(queue);
   // Do the work on the default concurrent queue and then
   // call the user-provided block with the results.
   dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),
^{
      int avg = average(data, len);
      dispatch_async(queue, ^{ block(avg);});
      // Release the user-provided queue when done
      dispatch_release(queue);
   });
}