Add native Windows IME support

osu.Framework/Platform/SDL2/SDL2Extensions.cs

@@ -1011,37 +1011,5 @@ public static unsafe bool TryGetStringFromBytePointer(byte* bytePointer, out str
             str = Marshal.PtrToStringUTF8(ptr) ?? string.Empty;
             return true;
         }
-
-        // ReSharper disable InconsistentNaming (mimics SDL and SDL2-CS naming)
-        // ReSharper disable IdentifierTypo
-
-        [StructLayout(LayoutKind.Sequential)]
-        internal struct INTERNAL_windows_wmmsg
-        {
-            public IntPtr hwnd;
-            public uint msg;
-            public ulong wParam;
-            public long lParam;
-        }
-
-        [StructLayout(LayoutKind.Explicit)]
-        internal struct INTERNAL_SysWMmsgUnion
-        {
-            [FieldOffset(0)]
-            public INTERNAL_windows_wmmsg win;
-
-            // could add more native events here if required
-        }
-
-        [StructLayout(LayoutKind.Sequential)]
-        internal struct SDL_SysWMmsg
-        {
-            public SDL.SDL_version version;
-            public SDL.SDL_SYSWM_TYPE subsystem;
-            public INTERNAL_SysWMmsgUnion msg;
-        }
-
-        // ReSharper restore InconsistentNaming
-        // ReSharper restore IdentifierTypo
     }
 }

osu.Framework/Platform/SDL2/SDL2Structs.cs

@@ -0,0 +1,48 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using System.Runtime.InteropServices;
+using SDL2;
+
+// ReSharper disable MemberCanBePrivate.Global
+// (Some members not currently used)
+
+// ReSharper disable InconsistentNaming
+// ReSharper disable IdentifierTypo
+// (Mimics SDL and SDL2-CS naming)
+
+namespace osu.Framework.Platform.SDL2
+{
+    internal static class SDL2Structs
+    {
+        [StructLayout(LayoutKind.Sequential)]
+        internal struct INTERNAL_windows_wmmsg
+        {
+            public IntPtr hwnd;
+            public uint msg;
+            public ulong wParam;
+            public long lParam;
+        }
+
+        [StructLayout(LayoutKind.Explicit)]
+        internal struct INTERNAL_SysWMmsgUnion
+        {
+            [FieldOffset(0)]
+            public INTERNAL_windows_wmmsg win;
+
+            // could add more native events here if required
+        }
+
+        /// <summary>
+        /// Member <c>msg</c> of <see cref="SDL.SDL_SysWMEvent"/>.
+        /// </summary>
+        [StructLayout(LayoutKind.Sequential)]
+        public struct SDL_SysWMmsg
+        {
+            public SDL.SDL_version version;
+            public SDL.SDL_SYSWM_TYPE subsystem;
+            public INTERNAL_SysWMmsgUnion msg;
+        }
+    }
+}

osu.Framework/Platform/SDL2DesktopWindow.cs

@@ -427,19 +427,22 @@ public virtual void Create()
         /// </summary>
         public void Run()
         {
-            // polling via SDL_PollEvent blocks on resizes (https://stackoverflow.com/a/50858339)
             SDL.SDL_SetEventFilter(eventFilterDelegate = (_, eventPtr) =>
             {
-                // ReSharper disable once PossibleNullReferenceException
-                var e = (SDL.SDL_Event)Marshal.PtrToStructure(eventPtr, typeof(SDL.SDL_Event));
+                var e = Marshal.PtrToStructure<SDL.SDL_Event>(eventPtr);
+                OnSDLEvent?.Invoke(e);
 
+                return 1;
+            }, IntPtr.Zero);
+
+            // polling via SDL_PollEvent blocks on resizes (https://stackoverflow.com/a/50858339)
+            OnSDLEvent += e =>
+            {
                 if (e.type == SDL.SDL_EventType.SDL_WINDOWEVENT && e.window.windowEvent == SDL.SDL_WindowEventID.SDL_WINDOWEVENT_RESIZED)
                 {
                     updateWindowSize();
                 }
-
-                return 1;
-            }, IntPtr.Zero);
+            };
 
             while (Exists)
             {
@@ -718,10 +721,6 @@ private void handleSDLEvent(SDL.SDL_Event e)
                 case SDL.SDL_EventType.SDL_DROPCOMPLETE:
                     handleDropEvent(e.drop);
                     break;
-
-                case SDL.SDL_EventType.SDL_SYSWMEVENT:
-                    HandleSysWMEvent(e.syswm);
-                    break;
             }
         }
 
@@ -990,19 +989,6 @@ private void handleWindowEvent(SDL.SDL_WindowEvent evtWindow)
             }
         }
 
-        /// <summary>
-        /// Override if platform requires special handling of native events.
-        /// </summary>
-        /// <remarks>
-        /// Make sure to enable the events with
-        ///   <code>
-        ///     SDL.SDL_EventState(SDL.SDL_EventType.SDL_SYSWMEVENT, SDL.SDL_ENABLE);
-        ///   </code>
-        /// </remarks>
-        protected virtual void HandleSysWMEvent(SDL.SDL_SysWMEvent sysWM)
-        {
-        }
-
         /// <summary>
         /// Should be run on a regular basis to check for external window state changes.
         /// </summary>
@@ -1495,6 +1481,11 @@ private static DisplayMode displayModeFromSDL(SDL.SDL_DisplayMode mode, int disp
         /// </summary>
         public event Action<string> DragDrop;
 
+        /// <summary>
+        /// Invoked on every SDL event before it's posted to the event queue.
+        /// </summary>
+        protected event Action<SDL.SDL_Event> OnSDLEvent;
+
         #endregion
 
         public void Dispose()