A little example on how to easily make (UIKit's) Buttons and Sliders MIDI controllable.
The UI elements register to NSNotificationCenter to receive either control messages with values or note on/off depending on whether it is a button or a slider. You can register a target in your main view controller to further process the incoming information.
This project partly relies on PGMIDI for the lower level MIDI work.
Parts of this code have been used for Circula.