feat: add RegisterFollowSystemTheme extension.

src/Semi.Avalonia/Extensions/ApplicationExtension.cs

@@ -0,0 +1,59 @@
+using System.Collections.Generic;
+using System.Runtime.InteropServices;
+using Avalonia;
+using Avalonia.Media;
+using Avalonia.Platform;
+using Avalonia.Styling;
+
+namespace Semi.Avalonia;
+
+public static class ApplicationExtension
+{
+    private static Application _app = null!;
+
+    private static readonly Dictionary<Color, ThemeVariant> ColorThemeMap = new()
+    {
+        [Color.Parse("#8EE3F0")] = SemiTheme.Aquatic,
+        [Color.Parse("#903909")] = SemiTheme.Desert,
+        [Color.Parse("#A1BFDE")] = SemiTheme.Dusk,
+        [Color.Parse("#D6B4FD")] = SemiTheme.NightSky
+    };
+
+    public static void RegisterFollowSystemTheme(this Application app)
+    {
+        _app = app;
+        if (!RuntimeInformation.IsOSPlatform(OSPlatform.Windows)) return;
+        if (app.PlatformSettings is null) return;
+        app.PlatformSettings.ColorValuesChanged -= OnColorValuesChanged;
+        app.PlatformSettings.ColorValuesChanged += OnColorValuesChanged;
+        OnColorValuesChanged(null, app.PlatformSettings?.GetColorValues());
+    }
+
+    public static void UnregisterFollowSystemTheme(this Application app)
+    {
+        _app = app;
+        if (!RuntimeInformation.IsOSPlatform(OSPlatform.Windows)) return;
+        if (app.PlatformSettings is null) return;
+        app.PlatformSettings.ColorValuesChanged -= OnColorValuesChanged;
+    }
+
+    private static void OnColorValuesChanged(object? _, PlatformColorValues? args)
+    {
+        ThemeVariant result;
+        if (args?.ContrastPreference is ColorContrastPreference.High)
+        {
+            result = ColorThemeMap.TryGetValue(args.AccentColor1, out var theme) ? theme : ThemeVariant.Default;
+        }
+        else
+        {
+            result = args?.ThemeVariant switch
+            {
+                PlatformThemeVariant.Light => ThemeVariant.Light,
+                PlatformThemeVariant.Dark => ThemeVariant.Dark,
+                _ => ThemeVariant.Default
+            };
+        }
+
+        _app.RequestedThemeVariant = result;
+    }
+}