GCD Thread Redirecting

gistfile1.m

/* say you have a process driven by the main thread, such as
 * a UI control, and you want the process to execute on a
 * background thread, but once it's done, you want to update
 * some UI element. that must happen on the main thread, so
 * here's an example of how you might do that
 */

- (IBAction)buttonClicked
{
  dispatch_async(backgroundProcessingQueue, ^{
    dispatch_async(dispatch_get_main_queue(), ^{
      // show activity indicator
      [delegate willProcessInBackground]; // if you want to follow Apple's naming convention
    });

    // perform the long-duration task here

    dispatch_async(dispatch_get_main_queue(), ^{
      // update UI element
      [delegate didProcessInBackground]; // if you want to follow Apple's naming convention
    });
  });
}