Queue presentViewController and dismissViewControllerAnimated with dispatch semaphores.

gistfile1.m

- (void) queueViewControllerTransition:(BOOL)show {
  static dispatch_queue_t presentAndDismissViewControllerQueue;
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
    presentAndDismissViewControllerQueue = dispatch_queue_create("presentAndDismissViewControllerQueue", DISPATCH_QUEUE_SERIAL);
  });

  dispatch_async(presentAndDismissViewControllerQueue, ^{
    dispatch_semaphore_t sema = dispatch_semaphore_create(0);
    dispatch_async(dispatch_get_main_queue(), ^{
      if (show) {
        [self presentViewController:self.yourViewController animated:YES completion:^{
          dispatch_semaphore_signal(sema);
        }];
      } else{
        [self dismissViewControllerAnimated:YES completion:^{
          dispatch_semaphore_signal(sema);
        }];
      }
    });
    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
  });
}