Using AvalonEdit (WPF Text Editor)

// Copyright (c) AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
// This code is distributed under the GNU LGPL (for details please see \doc\license.txt)

using System;
using System.ComponentModel;
using System.Diagnostics;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Security;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Interop;
using System.Windows.Media;
using System.Windows.Media.TextFormatting;
using ICSharpCode.AvalonEdit;
using ICSharpCode.AvalonEdit.Document;
using ICSharpCode.AvalonEdit.Rendering;

namespace ICSharpCode.AvalonEdit.Editing
{
	class ImeSupport
	{
		readonly TextArea textArea;
		IntPtr currentContext;
		IntPtr previousContext;
		IntPtr defaultImeWnd;
		HwndSource hwndSource;
		EventHandler requerySuggestedHandler; // we need to keep the event handler instance alive because CommandManager.RequerySuggested uses weak references
		bool isReadOnly;
		
		public ImeSupport(TextArea textArea)
		{
			if (textArea == null)
				throw new ArgumentNullException("textArea");
			this.textArea = textArea;
			InputMethod.SetIsInputMethodSuspended(this.textArea, textArea.Options.EnableImeSupport);
			// We listen to CommandManager.RequerySuggested for both caret offset changes and changes to the set of read-only sections.
			// This is because there's no dedicated event for read-only section changes; but RequerySuggested needs to be raised anyways
			// to invalidate the Paste command.
			requerySuggestedHandler = OnRequerySuggested;
			CommandManager.RequerySuggested += requerySuggestedHandler;
			textArea.OptionChanged += TextAreaOptionChanged;
		}

		void OnRequerySuggested(object sender, EventArgs e)
		{
			UpdateImeEnabled();
		}
		
		void TextAreaOptionChanged(object sender, PropertyChangedEventArgs e)
		{
			if (e.PropertyName == "EnableImeSupport") {
				InputMethod.SetIsInputMethodSuspended(this.textArea, textArea.Options.EnableImeSupport);
				UpdateImeEnabled();
			}
		}
		
		public void OnGotKeyboardFocus(KeyboardFocusChangedEventArgs e)
		{
			UpdateImeEnabled();
		}
		
		public void OnLostKeyboardFocus(KeyboardFocusChangedEventArgs e)
		{
			if (e.OldFocus == textArea && currentContext != IntPtr.Zero)
				ImeNativeWrapper.NotifyIme(currentContext);
			ClearContext();
		}
		
		void UpdateImeEnabled()
		{
			if (textArea.Options.EnableImeSupport && textArea.IsKeyboardFocused) {
				bool newReadOnly = !textArea.ReadOnlySectionProvider.CanInsert(textArea.Caret.Offset);
				if (hwndSource == null || isReadOnly != newReadOnly) {
					ClearContext(); // clear existing context (on read-only change)
					isReadOnly = newReadOnly;
					CreateContext();
				}
			} else {
				ClearContext();
			}
		}
		
		void ClearContext()
		{
			if (hwndSource != null) {
				ImeNativeWrapper.ImmAssociateContext(hwndSource.Handle, previousContext);
				ImeNativeWrapper.ImmReleaseContext(defaultImeWnd, currentContext);
				currentContext = IntPtr.Zero;
				defaultImeWnd = IntPtr.Zero;
				hwndSource.RemoveHook(WndProc);
				hwndSource = null;
			}
		}
		
		void CreateContext()
		{
			hwndSource = (HwndSource)PresentationSource.FromVisual(this.textArea);
			if (hwndSource != null) {
				if (isReadOnly) {
					defaultImeWnd = IntPtr.Zero;
					currentContext = IntPtr.Zero;
				} else {
					defaultImeWnd = ImeNativeWrapper.ImmGetDefaultIMEWnd(IntPtr.Zero);
					currentContext = ImeNativeWrapper.ImmGetContext(defaultImeWnd);
				}
				previousContext = ImeNativeWrapper.ImmAssociateContext(hwndSource.Handle, currentContext);
				hwndSource.AddHook(WndProc);
				// UpdateCompositionWindow() will be called by the caret becoming visible
				
				var threadMgr = ImeNativeWrapper.GetTextFrameworkThreadManager();
				if (threadMgr != null) {
					// Even though the docu says passing null is invalid, this seems to help
					// activating the IME on the default input context that is shared with WPF
					threadMgr.SetFocus(IntPtr.Zero);
				}
			}
		}
		
		IntPtr WndProc(IntPtr hWnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
		{
			switch (msg) {
				case ImeNativeWrapper.WM_INPUTLANGCHANGE:
					// Don't mark the message as handled; other windows
					// might want to handle it as well.
					
					// If we have a context, recreate it
					if (hwndSource != null) {
						ClearContext();
						CreateContext();
					}
					break;
				case ImeNativeWrapper.WM_IME_COMPOSITION:
					UpdateCompositionWindow();
					break;
			}
			return IntPtr.Zero;
		}
		
		public void UpdateCompositionWindow()
		{
			if (currentContext != IntPtr.Zero) {
				ImeNativeWrapper.SetCompositionFont(hwndSource, currentContext, textArea);
				ImeNativeWrapper.SetCompositionWindow(hwndSource, currentContext, textArea);
			}
		}
	}
}