AudioKit: MIDICallbackInstrument not triggering a callback when triggered from Sequencer - audiokit

Question

I'm trying to get a callback when a midi note is triggered so I can display a graphical representation of the current beat in the bar. Nothing I do seems to callback. This is the simplest version of the code that I have tried - audio plays back fine but the callback is never triggered. I have enabled Audio in Background Modes.
Here is the code:
class ViewMain: UIViewController {
let engine = AudioEngine()
let kicks = Sampler()
let claps = Sampler()
let sequencer = Sequencer()
let mixer = Mixer()
let midiCallbackInstrument = MIDICallbackInstrument()
override func viewDidLoad() {
super.viewDidLoad()
midiCallbackInstrument.callback = myCallBack
mixer.addInput(midiCallbackInstrument)
mixer.addInput(kicks)
mixer.addInput(claps)
engine.output = mixer
do {
try engine.start()
} catch {
Log("AudioKit did not start! \(error)")
}
kicks.unloadAllSamples()
claps.unloadAllSamples()
guard let url = Bundle.main.resourceURL?.appendingPathComponent("SamplesSFZ/sfz/909A.sfz") else { return }
kicks.loadSFZ(url: url)
guard let url = Bundle.main.resourceURL?.appendingPathComponent("SamplesSFZ/sfz/909B.sfz") else { return }
claps.loadSFZ(url: url)
sequencer.addTrack(for: midiCallbackInstrument)
sequencer.tracks[0].add(noteNumber: 36, velocity: 120, channel: 0, position: 0.0, duration: 1.0)
sequencer.tracks[0].add(noteNumber: 37, velocity: 120, channel: 1, position: 0.5, duration: 1.0)
sequencer.tracks[0].add(noteNumber: 38, velocity: 120, channel: 2, position: 1.0, duration: 1.0)
sequencer.tracks[0].add(noteNumber: 39, velocity: 120, channel: 3, position: 0.5, duration: 1.0)
sequencer.tracks[0].add(noteNumber: 40, velocity: 120, channel: 4, position: 2.0, duration: 1.0)
sequencer.tracks[0].add(noteNumber: 41, velocity: 120, channel: 5, position: 0.5, duration: 1.0)
sequencer.tracks[0].add(noteNumber: 42, velocity: 120, channel: 6, position: 3.0, duration: 1.0)
sequencer.tracks[0].add(noteNumber: 43, velocity: 120, channel: 7, position: 0.5, duration: 1.0)
sequencer.addTrack(for: kicks)
sequencer.tracks[1].add(noteNumber: 24, velocity: 120, channel: 0, position: 0.0, duration: 2.0)
sequencer.addTrack(for: claps)
sequencer.tracks[2].add(noteNumber: 25, velocity: 120, channel: 0, position: 1.0, duration: 0.1)
sequencer.tracks[2].add(noteNumber: 25, velocity: 120, channel: 0, position: 3.0, duration: 0.1)
sequencer.loopEnabled = true
sequencer.tempo = 120.0
sequencer.length = 4.0
sequencer.play()
}
func myCallBack(a: UInt8, b:MIDINoteNumber, c:MIDIVelocity) -> () {
// THIS IS NEVER CALLED
print("*** callback invoked")
}
}
Thanks in advance - I really appreciate any help here.

Answer 1

Use sequencer.setGlobalMIDIOutput(midiCallbackInstrument.midiIn) or sequencer.tracks[0].setMIDIOutput(midiCallbackInstrument.midiIn) to connect sequencer and midiCallbackInstrument.

Answer 2

I have had good results with the newer AudioKit CallbackInstrument, which is intended as a replacement for MIDICallbackInstrument.

To get CallbackInstrument working with your code, you only need to make two changes:
// replace let midiCallbackInstrument = MIDICallbackInstrument() with
var midiCallbackInstrument = CallbackInstrument()
// replace midiCallbackInstrument.callback = myCallBack with
midiCallbackInstrument = CallbackInstrument(midiCallback: { [self] (event, beat, velocity) in
myCallBack(a: event, b: beat, c: velocity) } )
CallbackInstrument is documented here:
https://github.com/AudioKit/AudioKit/wiki/CallbackInstrument
and the Shaker Metronome recipe in the AudioKit Cookbook is an example of how it is used.
https://github.com/AudioKit/Cookbook