either (older) apple 30pin cck + usb class compliant midi device
or a bespoke 30pin one like line6 midi mobilizer2 cheap or irig etc
then there’s newer apple lightning stuff
new apple ccl with lightning and and class compliant device
or a new lightning midi device, i’m sure those exist, probably iconnect stuff, at top end
strat here >? http://www.thomann.de/gb/line6_midi_mobilizer_ii_new.htm