feat(linux): implement an ISystemThemeHelper based on the Settings portal

src/Uno.UI.Runtime.Skia.X11/LinuxSystemThemeHelper.cs

@@ -0,0 +1,109 @@
+using System;
+using System.Threading.Tasks;
+using Tmds.DBus.Protocol;
+using Uno.Foundation.Logging;
+using Uno.Helpers.Theming;
+using Uno.UI.Dispatching;
+using Uno.WinUI.Runtime.Skia.X11.DBus;
+
+namespace Uno.WinUI.Runtime.Skia.X11;
+
+// https://flatpak.github.io/xdg-desktop-portal/docs/doc-org.freedesktop.portal.Settings.html
+
+internal class LinuxSystemThemeHelper : ISystemThemeHelperExtension
+{
+	private const string Service = "org.freedesktop.portal.Desktop";
+	private const string ObjectPath = "/org/freedesktop/portal/desktop";
+
+	public event EventHandler? SystemThemeChanged;
+
+	private SystemTheme _currentTheme = SystemTheme.Light;
+	private SystemTheme CurrentTheme
+	{
+		get => _currentTheme;
+		set
+		{
+			if (NativeDispatcher.Main.HasThreadAccess)
+			{
+				if (_currentTheme != value)
+				{
+					_currentTheme = value;
+					SystemThemeChanged?.Invoke(this, EventArgs.Empty);
+				}
+			}
+			else
+			{
+				NativeDispatcher.Main.Enqueue(() => CurrentTheme = value);
+			}
+		}
+	}
+
+	public SystemTheme GetSystemTheme() => CurrentTheme;
+
+	public static LinuxSystemThemeHelper Instance { get; } = new();
+
+	private LinuxSystemThemeHelper()
+	{
+		_ = Init().ConfigureAwait(false);
+	}
+
+	public async Task Init()
+	{
+		try
+		{
+			var sessionsAddressBus = Address.Session;
+			if (sessionsAddressBus is null)
+			{
+				if (this.Log().IsEnabled(LogLevel.Error))
+				{
+					this.Log().Error($"Can not determine DBus session bus address");
+				}
+				return;
+			}
+
+			var connection = new Connection(sessionsAddressBus);
+			await connection.ConnectAsync();
+
+			var desktopService = new DesktopService(connection, Service);
+			var settings = desktopService.CreateSettings(ObjectPath);
+
+			var version = await settings.GetVersionAsync();
+			if (version != 2)
+			{
+				if (this.Log().IsEnabled(LogLevel.Error))
+				{
+					this.Log().Error($"File pickers are only implemented for version 2 of the Settings portal, but version {version} was found");
+				}
+				return;
+			}
+
+			var result = await settings.ReadOneAsync("org.freedesktop.appearance", "color-scheme");
+			CurrentTheme = result.GetUInt32() == 1 ? SystemTheme.Dark : SystemTheme.Light;
+
+			// ignoring IDisposable return value here since we're watching for the lifetime of the app
+			await settings.WatchSettingChangedAsync((exception, tuple) =>
+			{
+				if (exception is not null)
+				{
+					if (this.Log().IsEnabled(LogLevel.Error))
+					{
+						this.Log().Error($"{nameof(settings.WatchSettingChangedAsync)} threw an exception", exception);
+					}
+					return;
+				}
+
+				if (tuple is { Namespace: "org.freedesktop.appearance", Key: "color-scheme" })
+				{
+					CurrentTheme = tuple.Value.GetUInt32() == 1 ? SystemTheme.Dark : SystemTheme.Light;
+				}
+			});
+		}
+		catch (Exception e)
+		{
+			if (this.Log().IsEnabled(LogLevel.Error))
+			{
+				this.Log().Error($"DBus Settings error, see https://aka.platform.uno/x11-dbus-troubleshoot for troubleshooting information.", e);
+			}
+		}
+	}
+}

src/Uno.UI.Runtime.Skia.X11/X11ApplicationHost.cs

@@ -17,6 +17,7 @@
 using System.Threading.Tasks;
 using System.Runtime.InteropServices.Marshalling;
 using Microsoft.UI.Xaml.Controls;
+using Uno.Helpers.Theming;
 
 namespace Uno.WinUI.Runtime.Skia.X11;
 
@@ -68,6 +69,8 @@ static X11ApplicationHost()
 		ApiExtensibility.Register<DragDropManager>(typeof(Windows.ApplicationModel.DataTransfer.DragDrop.Core.IDragDropExtension), o => new X11DragDropExtension(o));
 
 		ApiExtensibility.Register<XamlRoot>(typeof(Uno.Graphics.INativeOpenGLWrapper), xamlRoot => new X11NativeOpenGLWrapper(xamlRoot));
+
+		ApiExtensibility.Register(typeof(ISystemThemeHelperExtension), xamlRoot => LinuxSystemThemeHelper.Instance);
 	}
 
 	public X11ApplicationHost(Func<Application> appBuilder, int renderFrameRate = 60)