If you are writing your own MIDI synth from scratch, you could add a notification callback for each MIDI note. Due to real-time behavior concerns, none of the Apple AU instruments currently provide public notification of note on/off changes. If you are attempting to use one of these AUs, I would suggest filing a radar asking for this ability.
Ok fair enough, I would like to implement something that displays the MIDI note being played and acts like a progress bar at the same time.
Thank you for the reply.