Implement adapting to system base theme mode on Windows

Material.Styles/Themes/SystemThemeProbe.cs

@@ -0,0 +1,55 @@
+using System;
+using System.Runtime.InteropServices;
+using Avalonia.Logging;
+using Material.Styles.Themes.Base;
+
+namespace Material.Styles.Themes;
+
+public static class SystemThemeProbe {
+    /// <summary>
+    /// Tries to resolve base theme mode for the current system
+    /// </summary>
+    /// <returns>Base theme (<c>Dark</c>/<c>Light</c>) or <c>null</c></returns>
+    public static BaseThemeMode? GetSystemBaseThemeMode() {
+        try {
+            if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+                return GetWindowsBaseThemeMode();
+        }
+        catch (Exception e) {
+            Logger.TryGet(LogEventLevel.Error, "Material.Themes")
+                ?.Log("SystemThemeProbe", "Failed to get system base theme: {Exception}", e);
+        }
+
+        return null;
+    }
+
+
+    [DllImport("advapi32.dll", EntryPoint = "RegOpenKeyEx")]
+    private static extern int RegOpenKeyEx_DllImport(UIntPtr hKey, string lpSubKey, uint ulOptions, int samDesired, out UIntPtr phkResult);
+    [DllImport("advapi32.dll", EntryPoint = "RegQueryValueEx")]
+    private static extern int RegQueryValueEx_DllImport(UIntPtr hKey, string lpValueName, int lpReserved, out uint lpType, byte[] lpData, ref int lpcbData);
+    private static readonly UIntPtr HKEY_CURRENT_USER = (UIntPtr)0x80000001;
+
+    /// <summary>
+    /// Retrieving windows base theme from registry
+    /// </summary>
+    /// <remarks>
+    /// Relies on the <c>HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Themes\Personalize</c> and <c>AppsUseLightTheme</c> key
+    /// </remarks>
+    /// <exception cref="Exception">
+    /// Can probably throw exception if something gone wrong :shrug:
+    /// </exception>
+    /// <returns>Base theme (<c>Dark</c>/<c>Light</c>)</returns>
+    public static BaseThemeMode GetWindowsBaseThemeMode() {
+        var infoDataLength = 4;
+        var infoBytes = new byte[infoDataLength];
+
+        // I implemented it via P/Invoke cuz i dont want to see additional package reference to Microsoft.Win32.Registry in Material.Avalonia
+        var o0 = RegOpenKeyEx_DllImport(HKEY_CURRENT_USER, "Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize", 0, 0x1, out var hKeyVal);
+        if (o0 != 0) throw new Exception("Something went wrong when opening \"Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize\" registry entry");
+        var o1 = RegQueryValueEx_DllImport(hKeyVal, "AppsUseLightTheme", 0, out _, infoBytes, ref infoDataLength);
+        if (o1 != 0) throw new Exception("Something went wrong when reading \"AppsUseLightTheme\" registry entry value");
+
+        return BitConverter.ToBoolean(infoBytes, 0) ? BaseThemeMode.Dark : BaseThemeMode.Light;
+    }
+}

Material.Styles/Themes/Theme.cs

@@ -45,11 +45,6 @@ namespace Material.Styles.Themes {
         public Color TextAreaInactiveBorder { get; set; }
         public Color DataGridRowHoverBackground { get; set; }
 
-        public static BaseThemeMode? GetSystemTheme() {
-            // TODO Implement getting system themes
-            return null;
-        }
-
         public static Theme Create(IBaseTheme baseTheme, Color primary, Color accent) {
             if (baseTheme is null) throw new ArgumentNullException(nameof(baseTheme));
             var theme = new Theme();

Material.Styles/Themes/ThemeExtensions.cs

@@ -17,13 +17,10 @@ namespace Material.Styles.Themes {
 
         public static IBaseTheme GetBaseTheme(this BaseThemeMode baseThemeMode) {
             return baseThemeMode switch {
-                BaseThemeMode.Dark  => Theme.Dark,
-                BaseThemeMode.Light => Theme.Light,
-                BaseThemeMode.Inherit => Theme.GetSystemTheme() switch {
-                    BaseThemeMode.Dark => Theme.Dark,
-                    _                  => Theme.Light
-                },
-                _ => throw new InvalidOperationException()
+                BaseThemeMode.Dark    => Theme.Dark,
+                BaseThemeMode.Light   => Theme.Light,
+                BaseThemeMode.Inherit => (SystemThemeProbe.GetSystemBaseThemeMode() ?? BaseThemeMode.Light).GetBaseTheme(),
+                _                     => throw new InvalidOperationException()
             };
         }