mob_midi

MIDI in + out for Mob apps, over USB-MIDI and BLE-MIDI. Unlike classic Bluetooth, MIDI is first-class on both platforms, so this is a real cross-platform surface: CoreMIDI on iOS, android.media.midi on Android.

# Discover
MobMidi.list_devices(socket)
# => {:midi, :devices, [%{id: 7, name: "Oxygen 49", direction: :input}, ...]}

# Receive
MobMidi.open_input(socket, device_id)
# => {:midi, :raw, %{device: 7, bytes: <<0x90, 60, 100>>}}
#    parse it: MobMidi.parse(bytes) -> [%{type: :note_on, channel: 0, note: 60, velocity: 100}]

# Send: open the output, then send. The caller gets
#   {:midi, :opened, %{device: 7, direction: :output}} once the port is ready, or
#   {:midi, :error, %{device: 7, op: :open_output, reason: :no_such_device, dropped: 0}}
MobMidi.open_output(socket, device_id)

def handle_info({:midi, :opened, %{device: id}}, socket) do
  MobMidi.send_note_on(socket, id, 0, 60, 100)
  MobMidi.send_cc(socket, id, 0, 7, 90)
  {:noreply, socket}
end

Waiting for :opened is optional: Android opens the port asynchronously and queues sends made before it opens (up to 256 per device), writing them in order the moment it does. If the open fails, the queue is discarded and the error's dropped counts the lost messages (close/2 during the open reports reason: :closed the same way). send_* return socket when the message was written or queued and {:error, reason} when it wasn't (:not_open, :queue_full, :no_such_device, :too_large for iOS sends over 256 bytes, :send_failed), so bind the result instead of piping it on as the socket.

The NIF layer is deliberately thin (device enumeration + raw byte I/O); message encode/parse lives in Elixir (MobMidi) where it's pure and unit-tested.

Demo screens (tier 3)

Two screens the host auto-lists via Mob.Plugins.screens():

• MobMidi.KeyboardScreen (/midi_keyboard) — a two-octave keyboard; tap a key to send a note to the selected output. Meant to be landscape; currently portrait-stubbed (white keys, two rows) until Mob.Device.lock_orientation/1 lands in mob core (mob#49).
• MobMidi.InputScreen (/midi_input) — pick a source, watch incoming notes light up plus a decoded event log. Visual-first; an audible sine per note is a fast follow once Mob.Audio grows a tone primitive.

Status: experimental

The Elixir surface (API, parser, screens) is unit-tested. The native layers (iOS priv/native/ios/mob_midi_nif.m over CoreMIDI; Android priv/native/android/MobMidiBridge.kt + priv/native/jni/mob_midi_nif.zig over MidiManager) are experimental.

Verified for 0.1.2, on an Android emulator against virtual MIDI devices (android.media.midi.MidiDeviceService loopbacks in a test host app): list_devices/1, open_output/2 with its :opened / :error events, sends queued before the port opened arriving first and in order, the 256-message queue bound, send_* errors for unopened or closed outputs, open_input/2 receive, and one device opened in both directions. On the iOS simulator against CoreMIDI virtual endpoints (a MIDIDestinationCreate synth echoing to a MIDISourceCreate source): list_devices/1, open_output/2 events, send_* (including the :not_open error) and open_input/2 receive.

Not verified: USB-MIDI and BLE-MIDI hardware on either platform (including the MobMidi.Ble peripheral). Android assumes a single port per device (port 0).

Activating both mob_midi and mob_bluetooth: both declare NSBluetoothAlwaysUsageDescription, and mob_dev (0.7.14) refuses the native build when two activated plugins declare the same key. Set it in the host's ios/Info.plist: the host's value wins and the plugins no longer conflict.