Example of modifying synchronous code to asynchronous with GCD.

gistfile1.h

// Before:
- (IBAction) optimizeDataObject: (MyDataObject *) obj
{
    // going to start work, so show the user that something's happening
    [self.progressIndicator startAnimating];
    
    // these two are *usually* quick -- unless the user opens a 1GB file.
    // because this is happening in an event handler, it'll stall the event loop, which
    // causes the good ol' spinning beachball of death
    [self pruneDuplicatesInDataObject: obj];
    [self sortContentsOfDataObject: obj];
    
    // all done, stop showing activity
    [self.progressIndicator stopAnimating];
}

// After:
- (IBAction) optimizeDataObject: (MyDataObject *) obj
{
    // going to start work, so show the user that something's happening
    [self.progressIndicator startAnimating];
    
    // these two are *usually* quick -- unless the user opens a 1GB file.
    // we'll run this bit in the background instead:
    dispatch_async( dispatch_get_global_queue(0, 0), ^{
        [self pruneDuplicatesInDataObject: obj];
        [self sortContentsOfDataObject: obj];
        
        // all done, stop showing activity
        // UI updates should happen on the main thread, so we'll run it there:
        dispatch_async( dispatch_get_main_queue(), ^{
            [self.progressIndicator stopAnimating];
        });
    });
}