VividTracker is an Amiga Protracker compatible music app for the iPhone and iPad. You can use it to load, edit, save, and create new Protracker music. This tutorial shows how to make a nice drum loop using the echo feature.
VividTracker will be released on App Store in late January 2014 (which is tomorrow, so I hope that this gets out very soon).
Visit http://www.vividtracker.com for more information.