MIDI in + out for Mob apps.

Receive from a controller (keyboard, pads) and send to external synths / DAWs, over USB-MIDI and BLE-MIDI. Unlike MobBluetooth (classic BT, Android only), MIDI is first-class on both platforms, so this is a real cross-platform surface: CoreMIDI on iOS, android.media.midi on Android.

API style

Same as the rest of Mob: callbacks take and return socket unchanged; results arrive in handle_info/2 as messages tagged :midi. Functions return {:error, :unsupported} on the host (no MIDI stack outside a device).

Discovering devices

MobMidi.list_devices(socket)
# => {:midi, :devices, [%{id: 1, name: "Oxygen 49", direction: :input}, ...]}

Hot-plug: {:midi, :device_added, device} / {:midi, :device_removed, id}.

Receiving

MobMidi.open_input(socket, device_id)   # subscribe to that source

Incoming MIDI is delivered to the calling process as raw packets:

{:midi, :raw, %{device: id, bytes: <<0x90, 60, 100>>}}

Parse it with parse/1 (pure, so it's unit-testable and you decide how much to decode):

def handle_info({:midi, :raw, %{bytes: b}}, socket) do
  for ev <- MobMidi.parse(b), do: react(ev)
  {:noreply, socket}
end

parse/1 returns events like %{type: :note_on, channel: 0, note: 60, velocity: 100} (:note_on with velocity 0 is normalised to :note_off), :note_off, :cc, :program_change, :pitch_bend, and %{type: :raw, bytes: ...} for anything it doesn't decode.

Sending

MobMidi.open_output(socket, device_id)
MobMidi.send_note_on(socket, device_id, 0, 60, 100)   # ch 0, middle C, vel 100
MobMidi.send_note_off(socket, device_id, 0, 60, 0)
MobMidi.send_cc(socket, device_id, 0, 7, 90)          # ch 0, CC#7 (volume)
MobMidi.send_raw(socket, device_id, <<0xF0, ...>>)    # SysEx / anything

Channels are 0..15 on the wire (shown as 1..16 in most UIs). Notes / velocities / values are 0..127.