feat(templates,ui): add TemplateRegistry and dynamic template selecto…

src/TextForge.Core/Engine/DocumentEngine.cs

@@ -1,4 +1,3 @@
-using System.Linq;
 using TextForge.Core.Documents;
 using TextForge.Core.Modules;
 using TextForge.Core.Templates;
@@ -7,36 +6,42 @@ namespace TextForge.Core.Engine;
 
 public class DocumentEngine : IDocumentEngine
 {
-    private readonly DefaultTemplate _defaultTemplate = new();
+    private readonly TemplateRegistry _registry;
 
-    public RenderTree Evaluate(Document document, DefaultTemplate template)
+    public DocumentEngine(TemplateRegistry? registry = null)
+    {
+        _registry = registry ?? TemplateRegistry.Default;
+    }
+
+    public RenderTree Evaluate(Document document, IDocumentTemplate? template = null)
     {
         ArgumentNullException.ThrowIfNull(document);
-        ArgumentNullException.ThrowIfNull(template);
+
+        var activeTemplate = template ?? _registry.Get(document.TemplateName);
 
         var rootNodes = document.Modules
-            .Where(module => module.ConnectionState == ModuleConnectionState.Connected) // Skip detached root modules
-            .Select(module => EvaluateModule(module, template))
+            .Where(module => module.ConnectionState == ModuleConnectionState.Connected)
+            .Select(module => EvaluateModule(module, activeTemplate))
             .ToList();
 
         return new RenderTree(document.Metadata, rootNodes);
     }
 
-    public TOutput Render<TOutput>(Document document, DefaultTemplate template, IRenderTarget<TOutput> target)
+    public TOutput Render<TOutput>(Document document, IDocumentTemplate? template, IRenderTarget<TOutput> target)
     {
         ArgumentNullException.ThrowIfNull(target);
 
         var tree = Evaluate(document, template);
         return target.Render(tree);
     }
 
-    private RenderNode EvaluateModule(Module module, DefaultTemplate template)
+    private RenderNode EvaluateModule(Module module, IDocumentTemplate template)
     {
         var resolvedFeatures = template.ResolveFeatures(module);
         var resolvedLayout = template.ResolveLayout(module);
 
         var children = module.SubModules
-            .Where(child => child.ConnectionState == ModuleConnectionState.Connected) // Skip detached child modules
+            .Where(child => child.ConnectionState == ModuleConnectionState.Connected)
             .Select(child => EvaluateModule(child, template))
             .ToList();
 

src/TextForge.Core/Engine/IDocumentEngine.cs

@@ -16,7 +16,7 @@ public interface IRenderTarget<out TOutput>
 /// </summary>
 public interface IDocumentEngine
 {
-    RenderTree Evaluate(Document document, DefaultTemplate template);
+    RenderTree Evaluate(Document document, IDocumentTemplate? template = null);
 
-    TOutput Render<TOutput>(Document document, DefaultTemplate template, IRenderTarget<TOutput> target);
+    TOutput Render<TOutput>(Document document, IDocumentTemplate? template, IRenderTarget<TOutput> target);
 }

src/TextForge.Core/Templates/DefaultTemblate.cs

@@ -5,9 +5,11 @@
 
 namespace TextForge.Core.Templates;
 
-public class DefaultTemplate
+public class DefaultTemplate : IDocumentTemplate
 {
     public string Name { get; init; } = "Default";
+    public string DisplayName => "Default";
+    public string Description => "Standard document layout and styling.";
 
     /// <summary>
     /// Global template baseline defaults. Default text color is standard blue.

src/TextForge.Core/Templates/IDocumentTemplate.cs

@@ -0,0 +1,17 @@
+using TextForge.Core.Engine;
+using TextForge.Core.Modules;
+
+namespace TextForge.Core.Templates;
+
+/// <summary>
+/// Defines the contract for document styling and layout resolution templates.
+/// </summary>
+public interface IDocumentTemplate
+{
+    string Name { get; }
+    string DisplayName { get; }
+    string Description { get; }
+
+    ModuleFeatures ResolveFeatures(Module module);
+    LayoutProperties ResolveLayout(Module module);
+}

src/TextForge.Core/Templates/TemplateRegistry.cs

@@ -0,0 +1,62 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace TextForge.Core.Templates;
+
+/// <summary>
+/// Central registry providing access to all defined document templates.
+/// </summary>
+public class TemplateRegistry
+{
+    private readonly Dictionary<string, IDocumentTemplate> _templates = new(StringComparer.OrdinalIgnoreCase);
+
+    /// <summary>
+    /// Global shared registry instance pre-loaded with default templates.
+    /// </summary>
+    public static TemplateRegistry Default { get; } = CreateDefault();
+
+    /// <summary>
+    /// Registers a template into the registry.
+    /// </summary>
+    public void Register(IDocumentTemplate template)
+    {
+        ArgumentNullException.ThrowIfNull(template);
+        _templates[template.Name] = template;
+    }
+
+    /// <summary>
+    /// Retrieves a template by its unique name, falling back to "Default" if not found.
+    /// </summary>
+    public IDocumentTemplate Get(string name)
+    {
+        if (!string.IsNullOrWhiteSpace(name) && _templates.TryGetValue(name, out var template))
+        {
+            return template;
+        }
+
+        if (_templates.TryGetValue("Default", out var defaultTemplate))
+        {
+            return defaultTemplate;
+        }
+
+        throw new InvalidOperationException($"Template '{name}' was not found and no Default template is registered.");
+    }
+
+    /// <summary>
+    /// Retrieves all registered templates.
+    /// </summary>
+    public IEnumerable<IDocumentTemplate> GetAll() => _templates.Values;
+
+    /// <summary>
+    /// Retrieves all registered template identifiers.
+    /// </summary>
+    public IEnumerable<string> GetAvailableNames() => _templates.Values.Select(t => t.Name);
+
+    private static TemplateRegistry CreateDefault()
+    {
+        var registry = new TemplateRegistry();
+        registry.Register(new DefaultTemplate());
+        return registry;
+    }
+}

src/TextForge.Desktop/Views/Components/ModuleEditorView.axaml.cs

@@ -15,7 +15,7 @@ public partial class ModuleEditorView : UserControl
     public event EventHandler<Module>? ModuleDeleteRequested;
     public event EventHandler<Module>? ModuleDuplicateRequested;
     public event EventHandler<Module>? ModuleDetachRequested;
-    public event EventHandler<Module>? ModuleReconnectRequested;
+    // public event EventHandler<Module>? ModuleReconnectRequested;
 
     public ModuleEditorView()
     {

src/TextForge.Desktop/Views/Components/PreviewHostView.axaml

@@ -1,13 +1,45 @@
 <UserControl xmlns="https://github.com/avaloniaui"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:templates="using:TextForge.Core.Templates"
              x:Class="TextForge.Desktop.Views.Components.PreviewHostView">
     <!-- Live Preview Viewport (Uniform A4 Canvas) -->
     <Border Padding="20" Background="#E5E7EB">
         <DockPanel>
-            <TextBlock DockPanel.Dock="Top"
-                       Text="Document Preview"
-                       FontWeight="SemiBold"
-                       Margin="0,0,0,8" />
+            <!-- Header Toolbar: Title & Template Dropdown -->
+            <Grid DockPanel.Dock="Top" Margin="0,0,0,8">
+                <Grid.ColumnDefinitions>
+                    <ColumnDefinition Width="*" />
+                    <ColumnDefinition Width="Auto" />
+                </Grid.ColumnDefinitions>
+
+                <TextBlock Text="Document Preview"
+                           FontWeight="SemiBold"
+                           VerticalAlignment="Center" />
+
+                <!-- Template Selector Dropdown -->
+                <StackPanel Grid.Column="1"
+                            Orientation="Horizontal"
+                            Spacing="6"
+                            VerticalAlignment="Center">
+                    <TextBlock Text="Template:"
+                               FontSize="12"
+                               Foreground="#6B7280"
+                               VerticalAlignment="Center" />
+
+                    <ComboBox Name="TemplateSelector"
+                              Width="140"
+                              Height="28"
+                              Padding="8,2"
+                              FontSize="12"
+                              ToolTip.Tip="Select Document Template">
+                        <ComboBox.ItemTemplate>
+                            <DataTemplate x:DataType="templates:IDocumentTemplate">
+                                <TextBlock Text="{Binding DisplayName}" VerticalAlignment="Center" />
+                            </DataTemplate>
+                        </ComboBox.ItemTemplate>
+                    </ComboBox>
+                </StackPanel>
+            </Grid>
 
             <ScrollViewer HorizontalScrollBarVisibility="Auto"
                           VerticalScrollBarVisibility="Auto">

src/TextForge.Desktop/Views/Components/PreviewHostView.axaml.cs

@@ -1,14 +1,70 @@
+using System;
+using System.Linq;
 using Avalonia.Controls;
+using TextForge.Core.Documents;
+using TextForge.Core.Templates;
 
 namespace TextForge.Desktop.Views.Components;
 
-/// <summary>
-/// Container viewport rendering the compiled Avalonia document canvas preview.
-/// </summary>
 public partial class PreviewHostView : UserControl
 {
+    private Document? _currentDocument;
+    private bool _isSyncingSelection;
+
+    public event EventHandler<string>? TemplateChanged;
+
     public PreviewHostView()
     {
         InitializeComponent();
+        InitializeTemplateSelector();
+    }
+
+    private void InitializeTemplateSelector()
+    {
+        // 1. Populate items directly from Core TemplateRegistry
+        var templates = TemplateRegistry.Default.GetAll().ToList();
+        TemplateSelector.ItemsSource = templates;
+
+        // 2. Default to the primary template
+        TemplateSelector.SelectedItem = templates.FirstOrDefault();
+
+        // 3. Hook event
+        TemplateSelector.SelectionChanged += TemplateSelector_SelectionChanged;
+    }
+
+    public void BindDocument(Document document)
+    {
+        _currentDocument = document;
+
+        _isSyncingSelection = true;
+        try
+        {
+            var matchingTemplate = TemplateRegistry.Default
+                .GetAll()
+                .FirstOrDefault(t => string.Equals(t.Name, document.TemplateName, StringComparison.OrdinalIgnoreCase))
+                ?? TemplateRegistry.Default.Get("Default");
+
+            TemplateSelector.SelectedItem = matchingTemplate;
+        }
+        finally
+        {
+            _isSyncingSelection = false;
+        }
+    }
+
+    private void TemplateSelector_SelectionChanged(object? sender, SelectionChangedEventArgs e)
+    {
+        if (_isSyncingSelection) return;
+
+        if (TemplateSelector.SelectedItem is IDocumentTemplate selectedTemplate)
+        {
+            if (_currentDocument is not null && _currentDocument.TemplateName != selectedTemplate.Name)
+            {
+                _currentDocument.TemplateName = selectedTemplate.Name;
+                _currentDocument.NotifyChanged();
+            }
+
+            TemplateChanged?.Invoke(this, selectedTemplate.Name);
+        }
     }
 }