Add control cache support

src/Dock.Avalonia/Controls/DocumentControl.axaml

@@ -44,7 +44,11 @@
               <ContentControl x:Name="PART_ContentPresenter"
                               Content="{Binding}"
                               HorizontalAlignment="Stretch"
-                              VerticalAlignment="Stretch" />
+                              VerticalAlignment="Stretch">
+                <ContentControl.ContentTemplate>
+                  <ControlRecyclingDataTemplate Parent="{Binding #PART_ContentPresenter}" />
+                </ContentControl.ContentTemplate>
+              </ContentControl>
             </DockableControl>
           </Border>
         </DockPanel>

src/Dock.Avalonia/Controls/HostWindow.axaml.cs

@@ -267,7 +267,7 @@ public void Present(bool isDialog)
                     Window.Factory?.OnWindowOpened(Window);
                 }
 
-                ShowDialog(null); // FIXME: Set correct parent window.
+                ShowDialog(null!); // FIXME: Set correct parent window.
             }
         }
         else

src/Dock.Avalonia/Controls/Recycling/ControlRecycling.cs

@@ -0,0 +1,67 @@
+using System;
+using System.Collections.Generic;
+using Avalonia.Controls;
+using Avalonia.Controls.Templates;
+
+namespace Dock.Avalonia.Controls.Recycling;
+
+/// <summary>
+/// 
+/// </summary>
+public class ControlRecycling
+{
+    private readonly Dictionary<object, Control> _cache = new();
+
+    private bool TryGetValue(object? data, out Control? control)
+    {
+        if (data is null)
+        {
+            control = null;
+            return false;
+        }
+
+        return _cache.TryGetValue(data, out control);
+    }
+
+    private void Add(object data, Control control)
+    {
+        _cache[data] = control;
+    }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="data"></param>
+    /// <param name="existing"></param>
+    /// <param name="parent"></param>
+    /// <returns></returns>
+    public Control? Build(object? data, Control? existing, Control? parent)
+    {
+        if (data is null)
+        {
+            return null;
+        }
+
+        if (TryGetValue(data, out var control))
+        {
+#if DEBUG
+            Console.WriteLine($"[Cached] {data}, {control}");
+#endif
+            return control;
+        }
+
+        var dataTemplate = parent?.FindDataTemplate(data, null);
+
+        control = dataTemplate?.Build(data);
+        if (control is null)
+        {
+            return null;
+        }
+
+        Add(data, control);
+#if DEBUG
+        Console.WriteLine($"[Added] {data}, {control}");
+#endif
+        return control;
+    }
+}

src/Dock.Avalonia/Controls/Recycling/RecylingDataTemplate.cs

@@ -0,0 +1,59 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Templates;
+
+namespace Dock.Avalonia.Controls.Recycling;
+
+/// <summary>
+/// 
+/// </summary>
+public class ControlRecyclingDataTemplate : AvaloniaObject, IRecyclingDataTemplate
+{
+    private static readonly ControlRecycling s_controlRecycling = new();
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public static readonly StyledProperty<Control?> ParentProperty =
+        AvaloniaProperty.Register<ControlRecyclingDataTemplate, Control?>("Parent");
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public Control? Parent
+    {
+        get => GetValue(ParentProperty);
+        set => SetValue(ParentProperty, value);
+    }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="param"></param>
+    /// <returns></returns>
+    public Control? Build(object? param)
+    {
+        return null;
+    }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="data"></param>
+    /// <returns></returns>
+    public bool Match(object? data)
+    {
+        return true;
+    }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="data"></param>
+    /// <param name="existing"></param>
+    /// <returns></returns>
+    public Control? Build(object? data, Control? existing)
+    {
+        return s_controlRecycling.Build(data, existing, Parent);
+    }
+}

src/Dock.Avalonia/Controls/ToolControl.axaml

@@ -32,7 +32,11 @@
               <ContentControl x:Name="PART_ContentPresenter"
                               Content="{Binding}"
                               HorizontalAlignment="Stretch"
-                              VerticalAlignment="Stretch" />
+                              VerticalAlignment="Stretch">
+                <ContentControl.ContentTemplate>
+                  <ControlRecyclingDataTemplate Parent="{Binding #PART_ContentPresenter}" />
+                </ContentControl.ContentTemplate>
+              </ContentControl>
             </DockableControl>
           </Border>
         </DockPanel>

src/Dock.Avalonia/Properties/AssemblyInfo.cs

@@ -2,6 +2,7 @@
 
 [assembly: XmlnsDefinition("https://github.com/avaloniaui", "Dock.Avalonia")]
 [assembly: XmlnsDefinition("https://github.com/avaloniaui", "Dock.Avalonia.Controls")]
+[assembly: XmlnsDefinition("https://github.com/avaloniaui", "Dock.Avalonia.Controls.Recycling")]
 [assembly: XmlnsDefinition("https://github.com/avaloniaui", "Dock.Avalonia.Converters")]
 [assembly: XmlnsDefinition("https://github.com/avaloniaui", "Dock.Avalonia.MarkupExtension")]
 [assembly: XmlnsDefinition("https://github.com/avaloniaui", "Dock.Avalonia.Themes")]

src/Dock.Model.Avalonia/Controls/Document.cs

@@ -24,6 +24,8 @@ public class Document : DockableBase, IDocument, IDocumentContent, ITemplate<Con
     public static readonly StyledProperty<object?> ContentProperty =
         AvaloniaProperty.Register<Document, object?>(nameof(Content));
 
+    private Control? _cached;
+
     /// <summary>
     /// Initializes new instance of the <see cref="Document"/> class.
     /// </summary>
@@ -97,7 +99,16 @@ public bool Match(object? data)
     /// <returns></returns>
     public Control? Build(object? data, Control? existing)
     {
-        return existing ?? TemplateContent.Load(Content)?.Result;
+        if (_cached is not null)
+        {
+            return _cached;
+        }
+        var control = TemplateContent.Load(Content)?.Result;
+        if (control is not null)
+        {
+            _cached = control;
+        }
+        return control;
     }
 
     private static TemplateResult<Control>? Load(object? templateContent)

src/Dock.Model.Avalonia/Controls/Tool.cs

@@ -24,6 +24,8 @@ public class Tool : DockableBase, ITool, IDocument, IToolContent, ITemplate<Cont
     public static readonly StyledProperty<object?> ContentProperty =
         AvaloniaProperty.Register<Tool, object?>(nameof(Content));
 
+    private Control? _cached;
+
     /// <summary>
     /// Initializes new instance of the <see cref="Tool"/> class.
     /// </summary>
@@ -99,7 +101,16 @@ public bool Match(object? data)
     /// <returns></returns>
     public Control? Build(object? data, Control? existing)
     {
-        return existing ?? TemplateContent.Load(Content)?.Result;
+        if (_cached is not null)
+        {
+            return _cached;
+        }
+        var control = TemplateContent.Load(Content)?.Result;
+        if (control is not null)
+        {
+            _cached = control;
+        }
+        return control;
     }
 
     private static TemplateResult<Control>? Load(object? templateContent)