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.
If you find one, please add it to issues
Please read the LICENSE for details.