An example AUv3 MIDI plugin for macOS.
You will need a host application that understands this format, e.g. Logic X.
Run the app to install the audio unit. Then launch your host and then load the AU into it.
I do not have the current version of Logic X. (lack of disposable income).
My version is before MIDI was added to AUv3, so I cannot test yet.
When I run my version of Logic X, it does not validate
The plugin does validate though via the command line program auval.
If you do have the current version of Logic X (or GB, or Ableton), please test it and let me know what happens.
Even better, if you can find what's wrong, let me know!
If you find one, please add it to issues
Please read the LICENSE for details.