feat: added AvaloniaEdit for the text editor with basic highlight an line count, better "about" dialog and adde textmate

GsaEditor/Views/MainWindow.axaml

@@ -3,6 +3,7 @@
         xmlns:vm="using:GsaEditor.ViewModels"
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+        xmlns:aedit="https://github.com/avaloniaui/avaloniaedit"
         mc:Ignorable="d" d:DesignWidth="1100" d:DesignHeight="700"
         x:Class="GsaEditor.Views.MainWindow"
         x:CompileBindings="False"
@@ -199,13 +200,12 @@
                             <Button Content="Cancel" Command="{Binding CancelEditCommand}"
                                     IsVisible="{Binding IsEditingText}"/>
                         </StackPanel>
-                        <TextBox Text="{Binding PreviewText, Mode=TwoWay}"
-                                 IsReadOnly="{Binding !IsEditingText}"
-                                 FontFamily="Consolas, Courier New, monospace"
-                                 FontSize="13"
-                                 AcceptsReturn="True"
-                                 TextWrapping="NoWrap"
-                                 Margin="10,0,10,10"/>
+                        <aedit:TextEditor Name="TextEditor"
+                                          IsReadOnly="{Binding !IsEditingText}"
+                                          FontFamily="Consolas, Courier New, monospace"
+                                          FontSize="13"
+                                          ShowLineNumbers="True"
+                                          Margin="10,0,10,10"/>
                     </DockPanel>
 
                     <!-- Image Preview -->

GsaEditor/Views/MainWindow.axaml.cs

@@ -1,16 +1,31 @@
+using System.ComponentModel;
 using Avalonia.Controls;
 using Avalonia.Interactivity;
+using AvaloniaEdit;
+using AvaloniaEdit.TextMate;
 using GsaEditor.ViewModels;
+using TextMateSharp.Grammars;
 
 namespace GsaEditor.Views;
 
 public partial class MainWindow : Window
 {
     private bool _forceClose;
+    private TextEditor? _textEditor;
+    private TextMate.Installation? _textMateInstall;
+    private readonly RegistryOptions _registryOptions;
 
     public MainWindow()
     {
         InitializeComponent();
+
+        _registryOptions = new RegistryOptions(ThemeName.DarkPlus);
+
+        _textEditor = this.FindControl<TextEditor>("TextEditor");
+        if (_textEditor != null)
+        {
+            _textMateInstall = _textEditor.InstallTextMate(_registryOptions);
+        }
     }
 
     protected override void OnLoaded(RoutedEventArgs e)
@@ -20,9 +35,59 @@ public partial class MainWindow : Window
         if (DataContext is MainWindowViewModel vm)
         {
             vm.SetWindow(this);
+            vm.PropertyChanged += ViewModel_PropertyChanged;
         }
     }
 
+    private void ViewModel_PropertyChanged(object? sender, PropertyChangedEventArgs e)
+    {
+        if (sender is not MainWindowViewModel vm) return;
+
+        if (e.PropertyName == nameof(MainWindowViewModel.PreviewText))
+        {
+            if (_textEditor != null && vm.PreviewText != null)
+            {
+                _textEditor.Text = vm.PreviewText;
+            }
+            else if (_textEditor != null)
+            {
+                _textEditor.Text = string.Empty;
+            }
+        }
+
+        if (e.PropertyName == nameof(MainWindowViewModel.SelectedEntry))
+        {
+            ApplySyntaxHighlighting(vm);
+        }
+    }
+
+    private void ApplySyntaxHighlighting(MainWindowViewModel vm)
+    {
+        if (_textMateInstall == null || vm.SelectedEntry == null) return;
+
+        var alias = vm.SelectedEntry.Alias;
+        var ext = System.IO.Path.GetExtension(alias).ToLowerInvariant();
+
+        // Map file extension to a TextMate language scope
+        var language = _registryOptions.GetLanguageByExtension(ext);
+        if (language != null)
+        {
+            _textMateInstall.SetGrammar(_registryOptions.GetScopeByLanguageId(language.Id));
+        }
+        else
+        {
+            _textMateInstall.SetGrammar(null);
+        }
+    }
+
+    /// <summary>
+    /// Called by the ViewModel to retrieve the current text from the editor.
+    /// </summary>
+    public string? GetEditorText()
+    {
+        return _textEditor?.Text;
+    }
+
     private void TreeView_SelectionChanged(object? sender, SelectionChangedEventArgs e)
     {
         if (DataContext is MainWindowViewModel vm && sender is TreeView tree)