With PatternSequencer you can create a simple MIDI pattern/step sequencer on Mac OS X using Cocoa and Objective-C.
PatternSequencer features up to 16 parts, each of which can have 8 different patterns. You can set MIDI channel, volume, bank and program number for each part. Patterns can be up to 128 steps long. Timing can be 4, 8 or 16 steps per beat.
