[Hedda]

Apple introduced an official Game Controller Framework + API from iOS 7 and OS X 10.9
http://www.mactrast.com/2013/06/apple-to...mavericks/
http://www.raywenderlich.com/66532/ios-7...r-tutorial

Would it be possible for you to add joystick / gamepad support of it in RetroPlayer?

You can find that developers documentation in the Apple Game Controller Programming Guide here
https://developer.apple.com/library/ios/...ction.html
https://developer.apple.com/library/ios/...efColl.pdf

This should enable native support for joysticks and gamepads under iOS and Mac OS X