iOS: How to get background thread to redraw view on main thread

Question

My application will query a database 50 times and will then react accordingly by adding the right UIImageView to another UIImageView, which I then want the application to display immediately at each loop.

Alas, after many nights I have failed to get it to work. The application will not immediately display the UIImageView. Having said that, when I zoom in or zoom out during mid loop the UIImageViews will appear! I must be missing something...

So far every code works except the last part [UIScrollView performSelector....

Please help and thank you in advance.

    UIScrollView *firstView;
    UIImageView *secondView;
    UIImageView *thirdView;
    NSOperationQueue *queue;
    NSInvocationOperation *operation;

    - (void)viewDidAppear:(BOOL)animated
    {
    queue = [NSOperationQueue new];
    operation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(getData) object:nil];
    [queue addOperation:operation];
    }

    - (void) getData
    {
     for (i=0 ; i < 50 ; i++)

         {

            //All the codes to facilitate XML parsing here
            [nsXMLParser setDelegate:parser];
            [BOOL success = [nsXMLParser parse];

            if (success) 
            {   

                if ([parser.currentValue isEqualToString:@"G"])

                thirdView.image = greenTick.jpg;
                [secondView addSubview:thirdView];

            } 
            else 
            {
                NSLog(@"Error parsing document!");
            }
    } 
    [thirdView performSelectorOnMainThread:@selector(setNeedsDisplay) withObject:nil waitUntilDone: YES];
}

Answer 1

I discovered the solution and truly hope this will help someone...

        if (success) 
        {   

            if ([parser.currentValue isEqualToString:@"G"])

            // Make the changes here - start
            dispatch_queue_t main_queue = dispatch_get_main_queue();
            dispatch_async(main_queue, ^{
            thirdView.image = greenTick.jpg;
            [secondView addSubview:thirdView];
            });
            // Make the changes here - end
        } 
        else 
        {
            NSLog(@"Error parsing document!");
        }
} 
// Remove performSelectorOnMainThread