This app was mentioned in 4 comments, with an average of 2.25 upvotes
Xbox 360 SmartGlass for iOS sounds like it may be what you are looking for.
It has controller (or used to) functionality for like menus, etc. But I am not sure you could use it for an actually controller for a game.
not even with smart glass? can you still download it? https://play.google.com/store/apps/details?id=com.microsoft.smartglass
If you have an Android phone or tablet, you can get it from here: https://play.google.com/store/apps/details?id=com.microsoft.smartglass
I don't have an apple link.