Feat: Implement ThemeListener for winui3

components/Helpers/src/CommunityToolkit.WinUI.Helpers.csproj

@@ -12,7 +12,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <ProjectReference Include="$(ToolkitExtensionsSourceProject)"/>
+    <ProjectReference Include="$(ToolkitExtensionsSourceProject)" />
   </ItemGroup>
 
   <ItemGroup>
@@ -21,6 +21,13 @@
       <PackagePath>\</PackagePath>
     </None>
   </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.Windows.CsWin32" Version="0.3.106">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+  </ItemGroup>
 
   <!-- Sets this up as a toolkit component's source project -->
   <Import Project="$(ToolingDirectory)\ToolkitComponent.SourceProject.props" />

components/Helpers/src/MultiTarget.props

@@ -6,4 +6,4 @@
         -->
         <MultiTarget>uwp;wasdk;wpf;wasm;linuxgtk;macos;ios;android;</MultiTarget>
     </PropertyGroup>
-</Project>
+</Project>

components/Helpers/src/NativeMethods.json

@@ -0,0 +1,9 @@
+{
+  "$schema": "https://aka.ms/CsWin32.schema.json",
+  "allowMarshaling": false,
+  "comInterop": {
+    "preserveSigMethods": [
+      "*"
+    ]
+  }
+}

components/Helpers/src/NativeMethods.txt

@@ -0,0 +1,4 @@
+RegisterClassEx
+DefWindowProc
+CreateWindowEx
+WM_SETTINGCHANGE

components/Helpers/src/ThemeListener.cs

@@ -4,6 +4,8 @@
 
 using Windows.Foundation.Metadata;
 using Windows.UI.ViewManagement;
+using CommunityToolkit.WinUI.HelpersRns;
+
 
 #if !WINAPPSDK
 using Windows.System;
@@ -83,6 +85,15 @@ public ThemeListener(DispatcherQueue? dispatcherQueue = null)
 
             Window.Current.CoreWindow.Activated += CoreWindow_Activated;
         }
+#if WINAPPSDK
+        ThemeListenerHelperWindow.Instance.ThemeChanged += this.Instance_ThemeChanged;
+#endif
+    }
+
+    private void Instance_ThemeChanged(ApplicationTheme theme)
+    {
+        CurrentTheme = theme;
+        ThemeChanged?.Invoke(this);
     }
 
     private async void Accessible_HighContrastChanged(AccessibilitySettings sender, object args)

components/Helpers/src/ThemeListenerHelperWindow.cs

@@ -0,0 +1,79 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using Microsoft.Win32;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+using Windows.Win32;
+using Windows.Win32.Foundation;
+using Windows.Win32.UI.WindowsAndMessaging;
+
+namespace CommunityToolkit.WinUI.HelpersRns;
+
+#if WINAPPSDK
+internal class ThemeListenerHelperWindow
+{
+    public delegate void ThemeChangedHandler(ApplicationTheme theme);
+
+    public static ThemeListenerHelperWindow Instance = new();
+
+    public event ThemeChangedHandler? ThemeChanged;
+
+    private static string s_className = "ThemeListenerHelperWindow";
+
+    private HWND m_hwnd;
+
+    private ThemeListenerHelperWindow()
+    {
+        s_className = "ThemeListenerHelperWindow";
+        RegisterClass();
+        m_hwnd = CreateWindow();
+    }
+
+    private static unsafe void RegisterClass()
+    {
+        WNDCLASSEXW wcx = default;
+        wcx.cbSize = (uint)sizeof(WNDCLASSEXW);
+        fixed (char* pClassName = s_className)
+        {
+            wcx.lpszClassName = pClassName;
+            wcx.lpfnWndProc = &WndProc;
+            if (PInvoke.RegisterClassEx(in wcx) is 0)
+            {
+                Marshal.ThrowExceptionForHR(Marshal.GetLastPInvokeError());
+            }
+        }
+    }
+
+    private static unsafe HWND CreateWindow()
+    {
+        HWND hwnd = PInvoke.CreateWindowEx(0, s_className, string.Empty, 0, 0, 0, 0, 0, default, null, null, null);
+        if (hwnd == HWND.Null)
+        {
+            Marshal.ThrowExceptionForHR(Marshal.GetLastPInvokeError());
+        }
+        return hwnd;
+    }
+
+    [UnmanagedCallersOnly(CallConvs = [typeof(CallConvStdcall)])]
+    private static LRESULT WndProc(HWND hWnd, uint msg, WPARAM wParam, LPARAM lParam)
+    {
+        if (msg is PInvoke.WM_SETTINGCHANGE)
+        {
+            // REG_DWORD
+            object? value = Registry.GetValue(
+                @"HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Themes\Personalize",
+                "AppsUseLightTheme",
+                true);
+
+            if (value is int dword)
+            {
+                Instance.ThemeChanged?.Invoke(dword is 1 ? ApplicationTheme.Light : ApplicationTheme.Dark);
+            }
+        }
+
+        return PInvoke.DefWindowProc(hWnd, msg, wParam, lParam);
+    }
+}
+#endif