Subclass WinUI Window to hook native win32 events

src/Core/src/LifecycleEvents/Windows/WindowsLifecycle.cs

@@ -7,6 +7,7 @@ public static class WindowsLifecycle
 		public delegate void OnLaunched(UI.Xaml.Application application, UI.Xaml.LaunchActivatedEventArgs args);
 		public delegate void OnLaunching(UI.Xaml.Application application, UI.Xaml.LaunchActivatedEventArgs args);
 		public delegate void OnVisibilityChanged(UI.Xaml.Window window, UI.Xaml.WindowVisibilityChangedEventArgs args);
+		public delegate void OnNativeMessage(UI.Xaml.Window window, WindowsNativeMessageEventArgs args);
 
 		// Internal events
 		internal delegate void OnMauiContextCreated(IMauiContext mauiContext);

src/Core/src/Platform/Windows/IWindowNative.cs

@@ -0,0 +1,13 @@
+using System;
+using System.Runtime.InteropServices;
+
+namespace Microsoft.Maui
+{
+	[ComImport]
+	[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
+	[Guid("EECDBF0E-BAE9-4CB6-A68E-9598E1CB57BB")]
+	internal interface IWindowNative
+	{
+		IntPtr WindowHandle { get; }
+	}
+}

src/Core/src/Platform/Windows/MauiWinUIWindow.cs

@@ -1,14 +1,25 @@
-using Microsoft.Maui.LifecycleEvents;
+using System;
+using System.Runtime.InteropServices;
+using Microsoft.Maui.LifecycleEvents;
+using WinRT;
 
 namespace Microsoft.Maui
 {
 	public class MauiWinUIWindow : UI.Xaml.Window
 	{
 		public MauiWinUIWindow()
 		{
+			NativeMessage += OnNativeMessage;
 			Activated += OnActivated;
 			Closed += OnClosed;
 			VisibilityChanged += OnVisibilityChanged;
+
+			SubClassingWin32();
+		}
+
+		protected virtual void OnNativeMessage(object? sender, WindowsNativeMessageEventArgs args)
+		{
+			MauiWinUIApplication.Current.Services?.InvokeLifecycleEvents<WindowsLifecycle.OnNativeMessage>(m => m(this, args));
 		}
 
 		protected virtual void OnActivated(object sender, UI.Xaml.WindowActivatedEventArgs args)
@@ -25,5 +36,39 @@ protected virtual void OnVisibilityChanged(object sender, UI.Xaml.WindowVisibili
 		{
 			MauiWinUIApplication.Current.Services?.InvokeLifecycleEvents<WindowsLifecycle.OnVisibilityChanged>(del => del(this, args));
 		}
+
+		public event EventHandler<WindowsNativeMessageEventArgs> NativeMessage;
+
+		#region Native Window
+		IntPtr _hwnd = IntPtr.Zero;
+		delegate IntPtr WinProc(IntPtr hWnd, uint msg, IntPtr wParam, IntPtr lParam);
+		WinProc? newWndProc = null;
+		IntPtr oldWndProc = IntPtr.Zero;
+
+		[DllImport("user32")]
+		static extern IntPtr SetWindowLongPtr(IntPtr hWnd, int nIndex, WinProc newProc);
+		[DllImport("user32.dll")]
+		static extern IntPtr CallWindowProc(IntPtr lpPrevWndFunc, IntPtr hWnd, uint msg, IntPtr wParam, IntPtr lParam);
+
+		void SubClassingWin32()
+		{
+			//Get the Window's HWND
+			_hwnd = this.As<IWindowNative>().WindowHandle;
+			if (_hwnd == IntPtr.Zero)
+				throw new NullReferenceException("The Window Handle is null.");
+
+			newWndProc = new WinProc(NewWindowProc);
+			oldWndProc = SetWindowLongPtr(_hwnd, /* GWL_WNDPROC */ -4, newWndProc);
+		}
+
+		IntPtr NewWindowProc(IntPtr hWnd, uint msg, IntPtr wParam, IntPtr lParam)
+		{
+			var args = new WindowsNativeMessageEventArgs(hWnd, msg, wParam, lParam);
+
+			NativeMessage?.Invoke(this, args);
+
+			return CallWindowProc(oldWndProc, hWnd, msg, wParam, lParam);
+		}
+		#endregion
 	}
 }

src/Core/src/Platform/Windows/WindowsNativeMessageEventArgs.cs

@@ -0,0 +1,20 @@
+using System;
+
+namespace Microsoft.Maui
+{
+	public class WindowsNativeMessageEventArgs : EventArgs
+	{
+		public WindowsNativeMessageEventArgs(IntPtr hwnd, uint messageId, IntPtr wParam, IntPtr lParam)
+		{
+			Hwnd = hwnd;
+			MessageId = messageId;
+			WParam = wParam;
+			LParam = lParam;
+		}
+
+		public IntPtr Hwnd { get; private set; }
+		public uint MessageId { get; private set; }
+		public IntPtr WParam { get; private set; }
+		public IntPtr LParam { get; private set; }
+	}
+}

src/Core/src/Platform/Windows/WindowsNativeMessageIds.cs

@@ -0,0 +1,10 @@
+namespace Microsoft.Maui
+{
+	internal static class WindowsNativeMessageIds
+	{
+		public const int WM_DPICHANGED = 0x02E0;
+		public const int WM_DISPLAYCHANGE = 0x007E;
+		public const int WM_SETTINGCHANGE = 0x001A;
+		public const int WM_THEMECHANGE = 0x031A;
+	}
+}