Added PluggableTextInputMethod API

src/Avalonia.Base/Input/TextInput/InputMethodManager.cs

@@ -88,7 +88,7 @@ namespace Avalonia.Input.TextInput
         {
             if (ReferenceEquals(obj.Sender, _focusedElement))
             {
-                TryFindAndApplyClient();
+                RediscoverInputMethodAndClient();
             }
         }
 
@@ -132,42 +132,59 @@ namespace Avalonia.Input.TextInput
                 InputMethod.AddTextInputMethodClientRequeryRequestedHandler(_visualRoot,
                     TextInputMethodClientRequeryRequested);
             
-            var inputMethod = ((element as Visual)?.VisualRoot as ITextInputMethodRoot)?.InputMethod;
-
-            if (_im != inputMethod)
-            {
-                _im?.SetClient(null);
-            }
+            RediscoverInputMethodAndClient();
+        }
+        
+        void RediscoverInputMethodAndClient()
+        {
+            var (inputMethod, inputClient) = FindInputMethodAndClient();
+            
+            // Reset the previous input method and our state on input method change
+            if (_im != inputMethod) 
+                Client = null;
             
             _im = inputMethod;
             
-
-            TryFindAndApplyClient();
+            Client = inputClient;
         }
 
         private void TextInputMethodClientRequeryRequested(object? sender, RoutedEventArgs e)
         {
             if (_im != null)
-                TryFindAndApplyClient();
+                RediscoverInputMethodAndClient();
         }
-
-        private void TryFindAndApplyClient()
+        
+        private (ITextInputMethodImpl? im, TextInputMethodClient? client) FindInputMethodAndClient()
         {
             if (_focusedElement is not InputElement focused ||
                 _im == null ||
                 !InputMethod.GetIsInputMethodEnabled(focused))
             {
-                Client = null;
-                return;
+                // Input method system is disabled by focused element
+                return (null, null);
             }
 
+            // Attempt to get a user-provided input method
+            var imQuery = new PluggableTextInputMethodRequestedEventArgs()
+            {
+                RoutedEvent = PluggableTextInputMethod.TextInputMethodRequestedEvent
+            };
+            _focusedElement.RaiseEvent(imQuery);
+            
+            // Fall back to the system provided one, if any
+            var im = imQuery.InputMethod?.Adapter 
+                     ?? (focused.VisualRoot as ITextInputMethodRoot)?.InputMethod;
+
+            if (im == null)
+                return (null, null);
+
             var clientQuery = new TextInputMethodClientRequestedEventArgs
             {
                 RoutedEvent = InputElement.TextInputMethodClientRequestedEvent
             };
 
             _focusedElement.RaiseEvent(clientQuery);
-            Client = clientQuery.Client;
+            return (im, clientQuery.Client);
         }
     }
 }

src/Avalonia.Base/Input/TextInput/PluggableTextInputMethod.cs

@@ -0,0 +1,57 @@
+using Avalonia.Interactivity;
+
+namespace Avalonia.Input.TextInput;
+
+public abstract class PluggableTextInputMethod
+{
+    internal TextInputMethodAdapter Adapter { get; }
+    
+    public PluggableTextInputMethod()
+    {
+        Adapter = new(this);
+    }
+
+    public virtual void SetClient(TextInputMethodClient? client)
+    {
+    }
+
+    public virtual void SetOptions(TextInputOptions options)
+    {
+    }
+    
+    internal class TextInputMethodAdapter(PluggableTextInputMethod method) : ITextInputMethodImpl
+    {
+        public void SetClient(TextInputMethodClient? client)
+        {
+            method.SetClient(client);
+        }
+
+        public void SetCursorRect(Rect rect)
+        {
+            // No-op
+        }
+
+        public void SetOptions(TextInputOptions options) => method.SetOptions(options);
+
+        public void Reset()
+        {
+            // Implementations should be subscribing to reset event manually
+        }
+    }
+    
+    /// <summary>
+    /// Defines the <see cref="TextInputMethodRequestedEvent"/> event.
+    /// </summary>
+    public static readonly RoutedEvent<TextInputMethodClientRequestedEventArgs> TextInputMethodRequestedEvent =
+        RoutedEvent.Register<InputElement, TextInputMethodClientRequestedEventArgs>(
+            nameof(PluggableTextInputMethodRequestedEventArgs),
+            RoutingStrategies.Tunnel | RoutingStrategies.Bubble);
+}
+
+public class PluggableTextInputMethodRequestedEventArgs : RoutedEventArgs
+{
+    /// <summary>
+    /// Set this property to a valid pluggable text input method to enable its usage with the input system
+    /// </summary>
+    public PluggableTextInputMethod? InputMethod { get; set; }
+}