aud_midi_windows

The Windows backend of aud_midi: Windows.Devices.Midi and, when present, Windows MIDI Services through a C++/WinRT shim, Bluetooth LE MIDI pairing and DNS-SD advertising for network sessions.

Part of the aud_midi family, see aud_midi.

Goals

  • Windows.Devices.Midi ports with receive time stamps and DeviceWatcher hotplug
  • Windows MIDI Services (Midi2) detected at runtime: UMP, scheduling, virtual devices, function blocks
  • C++/WinRT shim built by a hook, never blocking an STA thread
  • In-app pairing of BLE-MIDI peripherals
  • Advertising via DnsServiceRegister
  • BLE peripheral through GattServiceProvider

State

Implemented; not yet built or run on Windows in this ticket. All Dart logic is tested on the Dart VM, the portable half of the shim on macOS; the WinRT half is reviewed against the Windows SDK reference and the Windows MIDI Services IDL, but never compiled against the real headers.

  • WindowsMidiBackend (MidiBackend, name winrt) with WindowsMidiApi.auto (default), midi1 or midi2:
    • WinRT MIDI 1.0: every MidiInPort is a byte input with time stamps, every MidiOutPort a byte output that sends at once (the engine schedules in software). Each port is a synthetic one-port device. Ids: winrt:in:<DeviceInformation.Id>, winrt:out:<…>.
    • Windows MIDI Services (when built in, present and not in legacy mode): every endpoint is a UMP input and output on one device, with groups, function blocks and endpoint info; outputs schedule in the service (scheduledSend). Ids: winrt:umpin:<endpoint id>, winrt:umpout:<…>. In hybrid legacy mode both APIs run side by side.
    • Hotplug: DeviceWatcher / MidiEndpointDeviceWatcher → added, removed and changed ports; ports that disappear are closed; a disconnected endpoint becomes MidiPortState.disconnected.
    • Time: WinRT MIDI 1.0 time stamps (time since the port was created) plus a QueryPerformanceCounter base taken at open; Midi2 time stamps are QPC ticks. MidiClockMapper maps QPC to the package clock, every 10 s and on hotplug.
    • bluetooth: scans the BLE-MIDI service with an active advertisement watcher, connects by pairing (confirm-only ceremony) and waits for the WinRT ports of the peripheral; disconnect unpairs.
    • virtualPorts: virtual devices of Windows MIDI Services, one function block each; null without Midi2.
  • WindowsMidiServiceAdvertiser (MidiServiceAdvertiser): DnsServiceRegister / DnsServiceDeRegister of dnsapi.dll through package:win32, completion through NativeCallable.listener.
  • WindowsMidiFakeShim: simulates the shim for tests of apps.

How it is verified:

Check Result
Dart unit tests, macOS VM 100 % line coverage per file from its own test
Portable shim core (tool/test_native_core.sh) ring buffer, event queue, callback gate, worker, BLE filter, clock math, MIDI splitting; ASan/UBSan and TSan clean
Byte format C++ ↔ Dart golden files written by the C++ test, decoded by the Dart tests; the virtual device spec the other way round
WinRT shim compiles with -Wall -Wextra -Wconversion -Werror against stand-in headers of the used projection surface (catches errors inside the shim, not API mismatches)
Hook builds nothing for macOS, Linux, Android and iOS
AOT compile (test/aot/aot_compile_test.dart) dart compile aot-snapshot of the backend and a lazily created advertiser succeeds; guards release builds for other platforms against "Class with illegal cid"

Not verified: the shim against the real Windows SDK and C++/WinRT headers, every native call (FFI glue lib/src/shim/windows_midi_ffi_shim.dart, lib/src/advertiser/windows_dns_sd_ffi_api.dart, both coverage-ignored), real devices, BLE pairing, the Midi2 path and the hook build with MSVC. The Windows tests below cover them.

Build and test on Windows

Needs Windows 10 1607+ (DNS-SD advertising 1903+) and Visual Studio 2022 or its Build Tools with "Desktop development with C++" and a Windows 11 SDK; native_toolchain_c finds cl.exe through vswhere. Then:

dart test

The hook compiles src/aud_midi_windows.cpp (C++20, /EHsc, WindowsApp.lib) into the code asset package:aud_midi_windows/aud_midi_windows_shim. Windows-only tests:

  • test/shim/windows_midi_ffi_shim_test.dart: context, clock, every port opened and closed, UMP loopback through the diagnostic loopback endpoints (Midi2), byte loopback through a loopMIDI-style driver, a note on the Microsoft GS Wavetable Synth (smoke test).
  • test/advertiser/windows_dns_sd_ffi_api_test.dart: registers and withdraws _apple-midi._udp and _midi2._udp services.
  • test/hook/build_test.dart: the hook emits the shim.

Enable Windows MIDI Services

The Midi2 path is compiled only with AUD_MIDI_WITH_MIDI2, because the Windows.Devices.Midi2 projection comes from NuGet, not from the Windows SDK:

  1. Get the NuGet packages Windows.Devices.Midi2 (winmd) and Microsoft.Windows.CppWinRT 3.x (cppwinrt.exe).

  2. Generate the projection, including the Windows SDK it refers to:

    cppwinrt -input <package>\ref\native\Windows.Devices.Midi2.winmd `
  -reference sdk -output third_party\midi2

  3. Turn it on in the pubspec.yaml of the app:

    hooks:
  user_defines:
    aud_midi_windows:
      midi2: true
      midi2_include: third_party/midi2

At runtime the shim asks for MidiApi (try_get_activation_factory), EnsureServiceAvailable and the API mode; without them it reports byte ports only. Where the API is not in-box yet, Windows.Devices.Midi2.dll of the package (runtimes\win-<arch>\native) must lie next to the executable. Windows MIDI Services before the November 2026 release can hang when a virtual device is torn down (microsoft/MIDI#1047).

Permissions and packaging

  • Unpackaged apps need nothing.
  • MSIX: <DeviceCapability Name="bluetooth" /> for BLE-MIDI, and <Capability Name="internetClient" /> plus <Capability Name="privateNetworkClientServer" /> for network sessions; the firewall asks for UDP 5004/5005 on first use.

Regenerate

dart run ffigen --config ffigen.yaml  # bindings of src/aud_midi_windows.h
tool/test_native_core.sh              # native core test, checks the goldens
tool/test_native_core.sh --update     # rewrites test/goldens

See the plan in aud_midi_pm.

Installation

dart pub add aud_midi_windows

Code Examples

Open every input and, where Windows MIDI Services offers virtual devices, play a MIDI 2.0 note on a virtual port; runs on Windows:

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

Future<void> main() async {
  final backend = WindowsMidiBackend();
  await backend.start(_PrintingHost());

  await Future.wait([
    for (final port in backend.ports)
      if (port.isInput) backend.openPort(port.id),
  ]);

  final source = await backend.virtualPorts?.create(
    MidiVirtualPortSpec(
      name: 'aud_midi_windows example',
      direction: MidiDirection.output,
      protocol: MidiProtocol.midi2,
    ),
  );
  if (source != null) {
    const noteOn = MidiNoteOn2(
      channel: 0,
      note: MidiNoteNumbers.c4,
      velocity: 0xc800,
    );
    await backend.send(
      source.id,
      MidiUmpPacket(
        words: UmpEncoder.encode(noteOn).expand((ump) => ump.words),
        time: const MidiSystemClock().now(),
      ),
    );
  }
  await backend.stop();
}

final class _PrintingHost implements MidiBackendHost {
  @override
  final MidiClock clock = const MidiSystemClock();

  @override
  void portsChanged(List<MidiPortEvent> events) => events.forEach(print);

  @override
  void received(MidiPortId port, MidiPacket packet) => print('$port $packet');

  @override
  void diagnostic(MidiDiagnostic diagnostic) => print(diagnostic);
}

The complete example, which also prints the ports, is example/aud_midi_windows_example.dart.

How It Works

  • src/aud_midi_windows.h: a plain C API with opaque handles; results are HRESULTs, events and data leave the shim as little-endian byte buffers.
  • src/aud_midi_windows.cpp: a worker thread in the multithreaded apartment owns every WinRT object, so callers never block an STA thread. WinRT raises events on thread pool threads; they copy messages into a ring buffer per port and events into a queue, then call a NativeCallable.listener once until the backend drains. A callback gate makes amw_destroy wait until no callback runs before the Dart side closes the callable.
  • src/aud_midi_windows_core.h: the portable parts, tested natively.
  • The Dart side decodes the buffers and owns all logic.

Contributing

See doc/guides/develop-guide.md.

Libraries

aud_midi_windows
The Windows backend of the aud_midi family: WinRT MIDI 1.0 (Windows.Devices.Midi) and, when present, Windows MIDI Services (Windows.Devices.Midi2) through a C++/WinRT shim, Bluetooth LE MIDI pairing and DNS-SD advertising for network sessions.