A lot of Apple stuff may be a black box but it is designed to be subclassed.
Aside from a small number of top-level classes[1] UIKit isn't designed to be subclassed. When you subclass things like UIAlertView, UIButton, UISwitch, UITextField, UIWebView… etc, you're in for a world undocumented gotchas and spending hours to do small simple things.
1. UIView, UIControl, UITableViewCell, UIScrollView, UIGestureRecognizer, UIViewController, UIApplication and maybe one or two other classes.