Objective-C:

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{ // show loading });

Swift 3: (Note: self cannot be omitted in block)

DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 3) { // show loading }