Why UITextView/UITextField can't implement the UITextInput on iOS

Apple supply an example to implement the UITextInput based on a UIView开发者_运维知识库. I tried to do the same thing based on the UITextView/UITextField but failed. That's why?

Thanks

Apple uses a private class as delegate for UITextInput on these classes. That means you cannot hook your own code in there because you cannot subclass a private class.