experiment for CSD template

samples/Sandbox/DrawnDecorationsPresenter.cs

@@ -0,0 +1,161 @@
+using System;
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Metadata;
+using Avalonia.Controls.Primitives;
+using Avalonia.Controls.Templates;
+using Avalonia.LogicalTree;
+using Avalonia.Markup.Xaml.Templates;
+using Avalonia.Metadata;
+using Avalonia.Styling;
+
+[assembly: XmlnsDefinition("https://github.com/avaloniaui", "Sandbox.Controls")]
+namespace Sandbox.Controls;
+
+public class DrawnDecorationsPresenter : Control
+{
+    class Wrapper : Control
+    {
+        public Control? Inner
+        {
+            get=>field;
+            set
+            {
+                if (field == value)
+                    return;
+                if (field != null)
+                    VisualChildren.Remove(field);
+                field = value;
+                if (field != null)
+                    VisualChildren.Add(field);
+            }
+        }
+    }
+
+    private WindowDrawnDecorations _decorations = new WindowDrawnDecorations();
+    private Grid _grid = new Grid();
+    private Wrapper _overlay = new Wrapper(), _underlay = new Wrapper();
+    public DrawnDecorationsPresenter()
+    {
+        LogicalChildren.Add(_decorations);
+        ((ISetLogicalParent)_decorations).SetParent(this);
+        VisualChildren.Add(_grid);
+        _grid.Children.Add(_underlay);
+        _grid.Children.Add(_overlay);
+    }
+
+    public override void ApplyTemplate()
+    {
+        _decorations.ApplyTemplate();
+        _overlay.Inner = _decorations.Content?.Overlay;
+        _underlay.Inner = _decorations.Content?.Underlay;
+        
+        base.ApplyTemplate();
+    }
+}
+
+[ControlTemplateScope]
+public class WindowDrawnDecorationsTemplate : ITemplate
+{
+    [Content]
+    [TemplateContent(TemplateResultType = typeof(WindowDrawnDecorationsContent))]
+    public object? Content { get; set; }
+
+    public TemplateResult<WindowDrawnDecorationsContent> Build() =>
+        TemplateContent.Load<WindowDrawnDecorationsContent>(Content) ?? throw new InvalidOperationException();
+
+    object? ITemplate.Build() => Build();
+
+}
+
+public class WindowDrawnDecorationsContent : StyledElement
+{
+    public Control? Overlay
+    {
+        get => field;
+        set => HandleLogicalChild(ref field, value);
+    }
+    
+    public Control? Underlay
+    {
+        get => field;
+        set => HandleLogicalChild(ref field, value);
+    }
+
+    void HandleLogicalChild(ref Control? field, Control? value)
+    {
+        if (field == value)
+            return;
+        if (field != null)
+        {
+            LogicalChildren.Remove(field);
+            ((ISetLogicalParent)field).SetParent(null);
+        }
+
+        field = value;
+        if (field != null)
+        {
+            LogicalChildren.Add(field);
+            ((ISetLogicalParent)field).SetParent(this);
+        }
+    }
+}
+
+[PseudoClasses(":test")]
+public class WindowDrawnDecorations : StyledElement
+{
+    public WindowDrawnDecorations()
+    {
+        PseudoClasses.Add(":test");
+    }
+    
+    public static readonly StyledProperty<WindowDrawnDecorationsTemplate> TemplateProperty = AvaloniaProperty.Register<WindowDrawnDecorations, WindowDrawnDecorationsTemplate>(
+        "Template");
+
+    public WindowDrawnDecorationsTemplate Template
+    {
+        get => GetValue(TemplateProperty);
+        set => SetValue(TemplateProperty, value);
+    }
+
+    public static readonly StyledProperty<double> TitleBarHeightProperty = AvaloniaProperty.Register<WindowDrawnDecorations, double>(
+        "TitleBarHeight");
+
+    public double TitleBarHeight
+    {
+        get => GetValue(TitleBarHeightProperty);
+        set => SetValue(TitleBarHeightProperty, value);
+    }
+
+    public WindowDrawnDecorationsContent? Content { get; private set; }
+    
+    private WindowDrawnDecorationsTemplate? _appliedTemplate;
+    public void ApplyTemplate()
+    {
+        if (Template == _appliedTemplate)
+            return;
+        if (Content != null)
+        {
+            LogicalChildren.Remove(Content);
+            ((ISetLogicalParent)Content).SetParent(null);
+            Content = null;
+        }
+
+        var res = Template.Build();
+        Content = res.Result;
+        if (Content != null)
+        {
+            TemplatedControl.ApplyTemplatedParent(Content, this);
+            LogicalChildren.Add(Content);
+            ((ISetLogicalParent)Content).SetParent(this);
+        }
+    }
+}
+
+class TestTest : Border
+{
+    protected override void OnAttachedToLogicalTree(LogicalTreeAttachmentEventArgs e)
+    {
+        base.OnAttachedToLogicalTree(e);
+    }
+}

samples/Sandbox/MainWindow.axaml

@@ -1,4 +1,44 @@
 <Window xmlns="https://github.com/avaloniaui"
-        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
+  xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
-        x:Class="Sandbox.MainWindow">
+  xmlns:sandbox="clr-namespace:Sandbox"
+  x:Class="Sandbox.MainWindow">
+  
+  <Window.Resources>
+    <ResourceDictionary>
+      <ControlTheme x:Key="{x:Type WindowDrawnDecorations}" TargetType="WindowDrawnDecorations">
+        <Setter Property="TitleBarHeight" Value="32"/>
+        <Setter Property="Template">
+          <WindowDrawnDecorationsTemplate>
+            <WindowDrawnDecorationsContent>
+              <WindowDrawnDecorationsContent.Overlay>
+                <StackPanel x:Name="PART_TitleBar" VerticalAlignment="Top" Orientation="Horizontal" Height="{TemplateBinding TitleBarHeight}">
+                  <TextBlock Foreground="Green" FontSize="20">TITLEBAR</TextBlock>
+                  <TextBlock Foreground="Green" Text="{TemplateBinding TitleBarHeight}"/>
+                </StackPanel>
+              </WindowDrawnDecorationsContent.Overlay>
+              
+              <WindowDrawnDecorationsContent.Underlay>
+                <Border Background="Azure" BorderThickness="1" BorderBrush="Gray"/>
+              </WindowDrawnDecorationsContent.Underlay>
+              
+            </WindowDrawnDecorationsContent>
+          </WindowDrawnDecorationsTemplate>
+        </Setter>
+        <Style Selector="^ /template/ StackPanel">
+          <Setter Property="Background" Value="Red"/>
+        </Style>
+        <Style Selector="^ /template/ TestTest">
+          <Setter Property="Background" Value="Red"/>
+        </Style>
+        <Style Selector="^:test /template/ StackPanel#PART_TitleBar">
+          <Setter Property="Background" Value="DarkBlue"/>
+        </Style>
+      </ControlTheme>
+    </ResourceDictionary>
+    
+  </Window.Resources>
+  
+
+  <DrawnDecorationsPresenter/>
+  
 </Window>

src/Avalonia.Base/Avalonia.Base.csproj

@@ -25,6 +25,7 @@
   </ItemGroup>
 
   <ItemGroup Label="InternalsVisibleTo">
+    <InternalsVisibleTo Include="Sandbox, PublicKey=$(AvaloniaPublicKey)" />
     <InternalsVisibleTo Include="Avalonia.Base.UnitTests, PublicKey=$(AvaloniaPublicKey)" />
     <InternalsVisibleTo Include="Avalonia.Desktop, PublicKey=$(AvaloniaPublicKey)" />
     <InternalsVisibleTo Include="Avalonia.Benchmarks, PublicKey=$(AvaloniaPublicKey)" />

src/Avalonia.Controls/Avalonia.Controls.csproj

@@ -11,6 +11,7 @@
   <Import Project="..\..\build\DevAnalyzers.props" />
 
   <ItemGroup Label="InternalsVisibleTo">
+    <InternalsVisibleTo Include="Sandbox, PublicKey=$(AvaloniaPublicKey)" />
     <InternalsVisibleTo Include="Avalonia.Controls.ItemsRepeater, PublicKey=$(AvaloniaPublicKey)" />
     <InternalsVisibleTo Include="Avalonia.UnitTests, PublicKey=$(AvaloniaPublicKey)" />
     <InternalsVisibleTo Include="Avalonia.Skia.RenderTests, PublicKey=$(AvaloniaPublicKey)" />

src/Avalonia.Controls/Primitives/TemplatedControl.cs

@@ -422,7 +422,7 @@ namespace Avalonia.Controls.Primitives
         /// </summary>
         /// <param name="control">The control.</param>
         /// <param name="templatedParent">The templated parent to apply.</param>
-        internal static void ApplyTemplatedParent(StyledElement control, AvaloniaObject? templatedParent)
+        public static void ApplyTemplatedParent(StyledElement control, AvaloniaObject? templatedParent)
         {
             control.TemplatedParent = templatedParent;