aud_midi_linux

The Linux backend of aud_midi in pure Dart: ALSA sequencer (MIDI 1.0 and UMP) through FFI, BLE over BlueZ and Avahi over D-Bus.

Part of the aud_midi family, see aud_midi.

Goals

  • ALSA sequencer ports, virtual ports, hotplug, queues
  • UMP clients on kernel 6.5+
  • BLE MIDI over BlueZ D-Bus
  • Avahi advertising
  • BLE peripheral through the BlueZ GATT server

State

Implemented (plan step 3 and the Linux part of step 7):

  • LinuxMidiBackend (MidiBackend, name alsa): ports of all other sequencer clients (inputs and outputs, transport from client and port type, UMP endpoint and function blocks), dynamic virtual ports, announce-port hotplug, real-time queue for scheduled sends and cancelPending, receive time stamps on the package clock, UMP mode (snd_seq_set_client_midi_version) with MIDI 1.0 fallback, BLE-MIDI peripherals through core's MidiBleBluetoothBackend.
  • MidiBlueZBleTransport (MidiBleTransport): scan, connect, notify and write without response on the BLE-MIDI characteristic over BlueZ.
  • MidiAvahiServiceAdvertiser (MidiServiceAdvertiser): DNS-SD registration through Avahi with collision renaming.

How it is verified:

  • Unit tests on the Dart VM (macOS): 100 % line coverage per file. All logic sits behind small interfaces with fakes; the BlueZ and Avahi code runs over a real D-Bus connection against in-process mock services; a layout test checks the generated structs (snd_seq_event_t 28 bytes, snd_seq_ump_event_t 32 bytes).
  • Not verified on real Linux in this ticket: the FFI glue (lib/src/alsa/ffi_*.dart, coverage-ignored), the reader isolate on a real handle, BlueZ and Avahi daemons. The Linux tests below cover the FFI glue; they are skipped on other systems.

Run the Linux tests

sudo apt-get install libasound2
sudo modprobe snd-seq
dart test test/alsa/ffi_alsa_sequencer_test.dart \
  test/alsa/ffi_alsa_reader_test.dart test/alsa/ffi_alsa_system_test.dart

They loop the backend's own virtual source into its own virtual destination (bytes, order, time stamps, scheduling deviation, cancel, SysEx, UMP when available) and watch hotplug of a helper client; no hardware is needed.

Permissions and packaging

  • libasound2 at runtime, /dev/snd/seq (kernel module snd-seq), on some distributions the user in the group audio.
  • BlueZ 5 with access to the system D-Bus; the Avahi daemon for advertising.
  • Snap: interfaces alsa, bluez, avahi-control, network.
  • Flatpak: --device=all, --system-talk-name=org.bluez, --system-talk-name=org.freedesktop.Avahi.

Installation

dart pub add aud_midi_linux

Documentation

Code Examples

import 'package:aud_midi_core/aud_midi_core.dart';
import 'package:aud_midi_linux/aud_midi_linux.dart';
import 'package:aud_midi_standard/aud_midi_standard.dart';

Future<void> main() async {
  final engine = MidiEngine(backend: LinuxMidiBackend(clientName: 'My App'));
  await engine.open();
  for (final port in engine.ports) {
    print('${port.direction.name}: ${port.name} (${port.id})');
  }
  final out = await engine.createVirtualPort(
    MidiVirtualPortSpec(name: 'My App Out', direction: MidiDirection.output),
  );
  await engine.openOutput(out.id);
  await engine.send(
    out.id,
    const MidiNoteOn(channel: 0, note: 60, velocity: 100),
    at: engine.clock.now() + const Duration(milliseconds: 100),
  );
  await engine.close();
}

How It Works

  • Two sequencer clients: the output client (the app's name) sends directly addressed events, owns the virtual sources and the queue; the input client (<name> (in)) owns the virtual destinations and one input port subscribed to every opened source and to the announce port.
  • A reader isolate blocks in snd_seq_event_input on the input client, copies each event into Dart objects and sends batches to the backend; a wake-up event with a random token stops it.
  • Received events carry the queue's real time, mapped over CLOCK_MONOTONIC to the package clock; due sends go out directly, later ones on the queue with their time; cancelPending removes them by destination (and by tag for own sources).
  • With alsa-lib 1.2.10+ and kernel 6.5+ both clients switch to UMP (MIDI 2.0); the kernel converts to and from MIDI 1.0 clients. Otherwise ports exchange MIDI 1.0 bytes, converted like alsa-lib's snd_midi_event, SysEx in chunks of 256 bytes.
  • Port ids are alsa:<client>:<port>:in|out; a reused client number gets a generation suffix (alsa:20#1:0:in).

Regenerate the ALSA bindings

lib/src/alsa/alsa_bindings.g.dart is generated by ffigen from the alsa-lib headers, parsed for Linux x86_64 against the stub system headers in tool/alsa_bindings/stubs (so the result does not depend on the host; aarch64 has the same layouts):

dart run tool/alsa_bindings/generate.dart              # clones v1.2.16.1
dart run tool/alsa_bindings/generate.dart --alsa-lib ../alsa-lib

It needs libclang (Xcode on macOS, libclang-dev on Linux). Functions newer than alsa-lib 1.2.10 are looked up at run time.

Contributing

See doc/guides/develop-guide.md.

Libraries

aud_midi_linux
The Linux backend of the aud_midi family: the ALSA sequencer through alsa-lib in pure Dart (MIDI 1.0 and UMP), BLE-MIDI over BlueZ and DNS-SD advertising over Avahi.