feat: add default sound device variables and switching

AGENTS.md

@@ -13,21 +13,23 @@ src/SystemMedia/
   PluginIntegration.cs      instances (current + one per known app, per-widget cycling), variables, events,
                             issues, the optional settings page
   SettingsConfigFlow.cs     the settings page (exact YouTube info)
-  ShowNextPlayingAppAction  the plugin's own action
+  ShowNextPlayingAppAction  switches "Any app" to the next playing app
+  SetDefaultAudioDevice...  switches a default sound device (also done by writing a device variable)
   Core/                     platform-neutral: the player, snapshot, state mapping, leftover filter and
                             thumbnail history, "any app" picker, instance ids, remembered apps, settings,
                             YouTube correction
   Events/                   event definitions, change detection, the once-a-second watcher
-  Variables/                deck variables for the current session
+  Variables/                deck variables for the current session and the default sound devices
   Platform/
     IMediaPlatform.cs       the interface every OS backend implements
+    IAudioDevices.cs        the default sound devices, read and switched per OS
     CompositeMediaPlatform  the OS backend plus every IMediaSource, routed by app id
-    Windows/                GSMTC sessions, volume mixer (Audio/), app names (Shell/), icons, VLC add-on,
+    Windows/                GSMTC sessions, volume mixer and sound devices (Audio/), app names (Shell/), icons, VLC add-on,
                             browser tabs (Browser/)
     Linux/                  MPRIS over the D-Bus session bus (Tmds.DBus.Protocol), app icons from .desktop
-                            files and the hicolor theme
+                            files and the hicolor theme, sound devices through pactl
     MacOS/                  Now Playing through the user's Homebrew media-control, app names/icons via
-                            mdfind/sips
+                            mdfind/sips, sound devices through Core Audio
   Players/                  players the system does not see: the Player base class, MpvPlayer, SmPlayer,
                             the watcher that polls each, local sockets and the Qt single-app message
 docs/                       adding-a-player.md, the contributor guide for a new Player
@@ -49,6 +51,9 @@ Look there instead of guessing.
   `thumbnail-history.json`
 - the "Any app" widget option names (`cycle`, `cycle-seconds`) and settings field names
   (`exact_youtube_info`), stored in users' widgets and config entry
+- action ids, parameter names and choice values (`set-default-audio-device` with `role` and `device`),
+  and the device ids it stores: the Windows endpoint id, the macOS device UID, the PulseAudio sink or
+  source name
 
 **Instances.** The host only resolves instance ids `GetInstances()` currently lists and uses the first as
 the default. `current` stays first, and an app once seen stays listed (and remembered on disk). Names carry
@@ -69,6 +74,11 @@ single-configuration integration to `default` and keeps only the first.
 - Linux has no system-wide current player; the one that started playing last stands in. MPRIS volume is
   the player's own, so a player without a `Volume` property gets no slider. An app id is the bus name
   after `org.mpris.MediaPlayer2.` without a `.instance...` suffix, so every instance is one app.
+- Sound devices are an `IAudioDevices` behind `IMediaPlatform.AudioDevices`: Core Audio on Windows and
+  macOS, `pactl --format=json` on Linux (it ships with PulseAudio and `pipewire-pulse`, run with
+  `LC_NUMERIC=C` because some locales break its JSON). Only Windows has communication defaults; elsewhere
+  those roles are the plain defaults. Windows switches through the undocumented `IPolicyConfig`, as every
+  device switcher does; keep it confined to `WindowsAudioDevices`.
 - No third-party program is bundled or run unless the user installed it (Store guideline 2); managed NuGet
   libraries such as Tmds.DBus.Protocol are fine. macOS relies on the user installing `media-control`
   from Homebrew; its `stream --micros` output is what `NowPlayingState` parses. Its version is not pinned,

README.md

@@ -73,6 +73,27 @@ For whatever is playing right now (the same as *Any app*):
 | `system_media_volume`                                                  | The app's volume in percent; can be set      |
 | `system_media_shuffle_enabled`, `system_media_repeat_mode`             | `repeat_mode` is `off`, `track` or `context` |
 
+### Sound devices
+
+Whatever is playing, these name the system's default sound devices:
+
+| Variable                                   | Value                                                  |
+| ------------------------------------------ | ------------------------------------------------------ |
+| `system_media_output_device`               | The default output, e.g. `Speakers (Realtek(R) Audio)` |
+| `system_media_input_device`                | The default input, e.g. `Microphone (USB Audio)`       |
+| `system_media_communication_output_device` | The output Windows uses for calls                      |
+| `system_media_communication_input_device`  | The input Windows uses for calls                       |
+
+Setting one of them to a device's name makes that device the default. The **Set the default audio device**
+action does the same with a device picked from a list.
+
+Only Windows keeps separate devices for calls. On macOS and Linux the communication variables show the
+plain defaults, and setting one changes the plain default.
+
+On Linux the devices are read and switched with `pactl`, which comes with PulseAudio and with PipeWire's
+PulseAudio support (`pipewire-pulse`). Without it the variables stay empty. On Windows, switching uses the
+same undocumented interface as the Sound settings, because Windows has no public one.
+
 ### Events
 
 Each event can be narrowed to one app with its **App** field and carries `app` and `appName`:
@@ -136,11 +157,13 @@ Everything runs on your computer. The plugin sends nothing about you or your med
 - **SMPlayer and mpv** are read and controlled over their local IPC pipe or socket. A cover is a
   screenshot mpv writes to a temporary file, which is deleted right after it is read.
 - **macOS:** Now Playing is read by running your installed `media-control`; app names and icons come
-  from `mdfind`, `plutil` and `sips`. Nothing third-party ships with the plugin.
+  from `mdfind`, `plutil` and `sips`, sound devices from Core Audio. Nothing third-party ships with the
+  plugin.
 - **Linux:** players are read over the D-Bus session bus with the bundled
   [Tmds.DBus.Protocol](https://github.com/tmds/Tmds.DBus) library (MIT). A player that reports its cover
   as a web address (Spotify does) has the cover downloaded from there, as the desktop's own media controls
-  do. App icons come from the apps' `.desktop` files and the icon theme.
+  do. App icons come from the apps' `.desktop` files and the icon theme. Sound devices are read and
+  switched by running `pactl`.
 
 ## Development
 

src/SystemMedia/Localization/Strings.resx

@@ -97,6 +97,18 @@
   <data name="Variables.RepeatMode" xml:space="preserve">
     <value>Repeat mode</value>
   </data>
+  <data name="Variables.OutputDevice" xml:space="preserve">
+    <value>Default output device</value>
+  </data>
+  <data name="Variables.InputDevice" xml:space="preserve">
+    <value>Default input device</value>
+  </data>
+  <data name="Variables.CommunicationOutputDevice" xml:space="preserve">
+    <value>Default communication output device</value>
+  </data>
+  <data name="Variables.CommunicationInputDevice" xml:space="preserve">
+    <value>Default communication input device</value>
+  </data>
   <data name="Events.TrackChanged.Name" xml:space="preserve">
     <value>Track changed</value>
   </data>
@@ -199,6 +211,45 @@
   <data name="Actions.ShowNextPlayingApp.NothingElsePlaying" xml:space="preserve">
     <value>No other app is playing right now.</value>
   </data>
+  <data name="Actions.SetDefaultAudioDevice.Name" xml:space="preserve">
+    <value>Set the default audio device</value>
+  </data>
+  <data name="Actions.SetDefaultAudioDevice.Description" xml:space="preserve">
+    <value>Makes a speaker, headset or microphone the system's default.</value>
+  </data>
+  <data name="Actions.SetDefaultAudioDevice.Role.Label" xml:space="preserve">
+    <value>Default for</value>
+  </data>
+  <data name="Actions.SetDefaultAudioDevice.Role.Description" xml:space="preserve">
+    <value>Only Windows has separate defaults for calls; elsewhere they are the plain defaults.</value>
+  </data>
+  <data name="Actions.SetDefaultAudioDevice.Role.Output" xml:space="preserve">
+    <value>Output</value>
+  </data>
+  <data name="Actions.SetDefaultAudioDevice.Role.Input" xml:space="preserve">
+    <value>Input</value>
+  </data>
+  <data name="Actions.SetDefaultAudioDevice.Role.CommunicationOutput" xml:space="preserve">
+    <value>Communication output</value>
+  </data>
+  <data name="Actions.SetDefaultAudioDevice.Role.CommunicationInput" xml:space="preserve">
+    <value>Communication input</value>
+  </data>
+  <data name="Actions.SetDefaultAudioDevice.Device.Label" xml:space="preserve">
+    <value>Device</value>
+  </data>
+  <data name="Actions.SetDefaultAudioDevice.NotSupported" xml:space="preserve">
+    <value>Audio devices cannot be changed on this system.</value>
+  </data>
+  <data name="Actions.SetDefaultAudioDevice.NoDevice" xml:space="preserve">
+    <value>Choose a device first.</value>
+  </data>
+  <data name="Actions.SetDefaultAudioDevice.DeviceNotFound" xml:space="preserve">
+    <value>That device is not connected.</value>
+  </data>
+  <data name="Actions.SetDefaultAudioDevice.Failed" xml:space="preserve">
+    <value>The default device could not be changed.</value>
+  </data>
   <data name="ConfigFlow.Settings.Title" xml:space="preserve">
     <value>System Media settings</value>
   </data>

src/SystemMedia/Platform/CompositeMediaPlatform.cs

@@ -95,6 +95,8 @@ public Task<bool> SetVolumePercentAsync(AppIdentity app, int volumePercent, Canc
 			: _system.SetVolumePercentAsync(app, volumePercent, cancellationToken);
 	}
 
+	public IAudioDevices? AudioDevices => _system.AudioDevices;
+
 	public IReadOnlyList<IntegrationIssue> GetIssues() =>
 		[.. _system.GetIssues(), .. _sources.SelectMany(source => source.GetIssues())];
 

src/SystemMedia/Platform/IAudioDevices.cs

@@ -0,0 +1,41 @@
+namespace SystemMedia.Platform;
+
+// Only Windows keeps separate defaults for calls; elsewhere the communication roles are the plain defaults.
+internal enum AudioDeviceRole
+{
+	Output,
+	Input,
+	CommunicationOutput,
+	CommunicationInput,
+}
+
+internal sealed record AudioDevice(string Id, string Name);
+
+// Reads return no device when the system has none; only a platform-level failure throws.
+internal interface IAudioDevices
+{
+	Task<IReadOnlyList<AudioDevice>> GetDevicesAsync(bool input, CancellationToken cancellationToken);
+
+	Task<AudioDevice?> GetDefaultAsync(AudioDeviceRole role, CancellationToken cancellationToken);
+
+	// false when no device has that id.
+	Task<bool> SetDefaultAsync(AudioDeviceRole role, string deviceId, CancellationToken cancellationToken);
+}
+
+internal static class AudioDeviceRoles
+{
+	internal static bool IsInput(this AudioDeviceRole role) =>
+		role is AudioDeviceRole.Input or AudioDeviceRole.CommunicationInput;
+
+	internal static bool IsCommunication(this AudioDeviceRole role) =>
+		role is AudioDeviceRole.CommunicationOutput or AudioDeviceRole.CommunicationInput;
+
+	// A stored id first, then a name, so a variable can be set to what it shows.
+	internal static async Task<AudioDevice?> FindAsync(this IAudioDevices devices, AudioDeviceRole role, string idOrName,
+		CancellationToken cancellationToken)
+	{
+		var candidates = await devices.GetDevicesAsync(role.IsInput(), cancellationToken);
+		return candidates.FirstOrDefault(device => string.Equals(device.Id, idOrName, StringComparison.Ordinal))
+			?? candidates.FirstOrDefault(device => string.Equals(device.Name, idOrName.Trim(), StringComparison.OrdinalIgnoreCase));
+	}
+}

src/SystemMedia/Platform/IMediaPlatform.cs

@@ -56,6 +56,9 @@ internal interface IMediaPlatform : IDisposable
 
 	Task<bool> SetVolumePercentAsync(AppIdentity app, int volumePercent, CancellationToken cancellationToken);
 
+	// The system's default sound devices; null where the platform cannot reach them.
+	IAudioDevices? AudioDevices => null;
+
 	IReadOnlyList<IntegrationIssue> GetIssues();
 
 	Task<IssueResolution?> ResolveIssueAsync(string issueId, CancellationToken cancellationToken);

src/SystemMedia/Platform/Linux/LinuxMediaPlatform.cs

@@ -138,6 +138,8 @@ public async Task<bool> SetVolumePercentAsync(AppIdentity app, int volumePercent
 		Find(app.AppId) is { } player && player.CanControl && player.Has("Volume") &&
 		await TryAsync(bus => bus.SetAsync(player.BusName, "Volume", Math.Clamp(volumePercent, 0, 100) / 100d), cancellationToken);
 
+	public IAudioDevices AudioDevices { get; } = new PulseAudioDevices();
+
 	// Not "not started": the host asks for issues while the first start is still running.
 	public IReadOnlyList<IntegrationIssue> GetIssues() =>
 		!_startFailed ? [] :