Question

// Method called when a button is clicked

- (void)handleClickEvent {
      dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
         [self backgroundProcessing];
      });
      // Some code to update the UI of the view
      ....
      [self updateUI];
      ....
}

1) handleClickEvent is called on the main thread when a button on the view is pressed.

2) I used dispatch_sync() because the following code that updates the UI of the view cannot be done until a variable in the backgroundProcessing method is calculated.

3) I used dispatch_get_global_queue in order to get the backgroundProcessing off the main thread. (following the rule: generally put background processing off main thread and generally put code that affect the UI on the main thread).

My question is: Does the backgroundProcessing method “hang” the main thread until it is complete since I am using dispatch_sync?

EDIT:
Based on the answer below i have implemented this solution:

- (void)handleClickEvent {
      dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
         [self backgroundProcessing];
         dispatch_async(dispatch_get_main_queue(), ^{
            [self updateUI];
         });
      });
}

solution from this link: Completion Callbaks

Answer 1

Yes, dispatch_sync will block until the task is complete. Use dispatch_async and when the job is complete have it post a block back to the main queue to update the view.