Fixed SelectedContent and SelectedContentTemplate only updating when the selected tab changes

Flipped ContentTemplate priority to match WPF, where TabItem's template wins over TabControl's
Made various properties read-only DirectProperty

src/Avalonia.Controls/TabControl.cs

@@ -10,6 +10,7 @@ using Avalonia.LogicalTree;
 using Avalonia.VisualTree;
 using Avalonia.Automation;
 using Avalonia.Controls.Metadata;
+using Avalonia.Reactive;
 
 namespace Avalonia.Controls
 {
@@ -19,6 +20,10 @@ namespace Avalonia.Controls
     [TemplatePart("PART_ItemsPresenter", typeof(ItemsPresenter))]
     public class TabControl : SelectingItemsControl, IContentPresenterHost
     {
+        private object? _selectedContent;
+        private IDataTemplate? _selectedContentTemplate;
+        private CompositeDisposable? _selectedItemSubscriptions;
+
         /// <summary>
         /// Defines the <see cref="TabStripPlacement"/> property.
         /// </summary>
@@ -46,14 +51,14 @@ namespace Avalonia.Controls
         /// <summary>
         /// The selected content property
         /// </summary>
-        public static readonly StyledProperty<object?> SelectedContentProperty =
-            AvaloniaProperty.Register<TabControl, object?>(nameof(SelectedContent));
+        public static readonly DirectProperty<TabControl, object?> SelectedContentProperty =
+            AvaloniaProperty.RegisterDirect<TabControl, object?>(nameof(SelectedContent), o => o.SelectedContent);
 
         /// <summary>
         /// The selected content template property
         /// </summary>
-        public static readonly StyledProperty<IDataTemplate?> SelectedContentTemplateProperty =
-            AvaloniaProperty.Register<TabControl, IDataTemplate?>(nameof(SelectedContentTemplate));
+        public static readonly DirectProperty<TabControl, IDataTemplate?> SelectedContentTemplateProperty =
+            AvaloniaProperty.RegisterDirect<TabControl, IDataTemplate?>(nameof(SelectedContentTemplate), o => o.SelectedContentTemplate);
         
         /// <summary>
         /// The default value for the <see cref="ItemsControl.ItemsPanel"/> property.
@@ -115,11 +120,10 @@ namespace Avalonia.Controls
         /// <value>
         /// The content of the selected tab.
         /// </value>
-        [System.Diagnostics.CodeAnalysis.SuppressMessage("AvaloniaProperty", "AVP1032", Justification = "This property is supposed to be a styled readonly property.")]
         public object? SelectedContent
         {
-            get { return GetValue(SelectedContentProperty); }
-            internal set { SetValue(SelectedContentProperty, value); }
+            get => _selectedContent;
+            internal set => SetAndRaise(SelectedContentProperty, ref _selectedContent, value);
         }
 
         /// <summary>
@@ -128,11 +132,10 @@ namespace Avalonia.Controls
         /// <value>
         /// The content template of the selected tab.
         /// </value>
-        [System.Diagnostics.CodeAnalysis.SuppressMessage("AvaloniaProperty", "AVP1032", Justification = "This property is supposed to be a styled readonly property.")]
         public IDataTemplate? SelectedContentTemplate
         {
-            get { return GetValue(SelectedContentTemplateProperty); }
-            internal set { SetValue(SelectedContentTemplateProperty, value); }
+            get => _selectedContentTemplate;
+            internal set => SetAndRaise(SelectedContentTemplateProperty, ref _selectedContentTemplate, value);
         }
 
         internal ItemsPresenter? ItemsPresenterPart { get; private set; }
@@ -161,18 +164,10 @@ namespace Avalonia.Controls
         protected internal override void PrepareContainerForItemOverride(Control element, object? item, int index)
         {
             base.PrepareContainerForItemOverride(element, item, index);
-            
-            if (element is TabItem tabItem)
-            {
-                if (ContentTemplate is { } ct)
-                    tabItem.ContentTemplate = ct;
-                tabItem.SetValue(TabStripPlacementProperty, TabStripPlacement);
-            }
 
-            if (index == SelectedIndex && element is ContentControl container)
+            if (index == SelectedIndex)
             {
-                SelectedContentTemplate = container.ContentTemplate;
-                SelectedContent = container.Content;
+                UpdateSelectedContent(element);
             }
         }
 
@@ -192,18 +187,25 @@ namespace Avalonia.Controls
             UpdateSelectedContent();
         }
 
-        private void UpdateSelectedContent()
+        private void UpdateSelectedContent(Control? container = null)
         {
+            _selectedItemSubscriptions?.Dispose();
+            _selectedItemSubscriptions = null;
+
             if (SelectedIndex == -1)
             {
                 SelectedContent = SelectedContentTemplate = null;
             }
             else
             {
-                var container = SelectedItem as IContentControl ??
-                    ContainerFromIndex(SelectedIndex) as IContentControl;
-                SelectedContentTemplate = container?.ContentTemplate;
-                SelectedContent = container?.Content;
+                container ??= ContainerFromIndex(SelectedIndex);
+                if (container != null)
+                {
+                    _selectedItemSubscriptions = new CompositeDisposable(
+                        container.GetObservable(ContentControl.ContentProperty).Subscribe(v => SelectedContent = v),
+                        // Note how we fall back to our own ContentTemplate if the container doesn't specify one
+                        container.GetObservable(ContentControl.ContentTemplateProperty).Subscribe(v => SelectedContentTemplate = v ?? ContentTemplate));
+                }
             }
         }
 
@@ -285,6 +287,16 @@ namespace Avalonia.Controls
             {
                 RefreshContainers();
             }
+            else if (change.Property == ContentTemplateProperty)
+            {
+                var newTemplate = change.GetNewValue<IDataTemplate?>();
+                if (SelectedContentTemplate != newTemplate &&
+                    ContainerFromIndex(SelectedIndex) is { } container && 
+                    container.GetValue(ContentControl.ContentTemplateProperty) == null)
+                {
+                    SelectedContentTemplate = newTemplate; // See also UpdateSelectedContent
+                }
+            }
             else if (change.Property == KeyboardNavigation.TabOnceActiveElementProperty &&
                 ItemsPresenterPart?.Panel is { } panel)
             {

src/Avalonia.Controls/TabItem.cs

@@ -1,22 +1,28 @@
+using System;
 using Avalonia.Automation;
 using Avalonia.Automation.Peers;
 using Avalonia.Controls.Metadata;
 using Avalonia.Controls.Mixins;
 using Avalonia.Controls.Primitives;
+using Avalonia.Reactive;
+using Avalonia.VisualTree;
 
 namespace Avalonia.Controls
 {
     /// <summary>
-    /// An item in  a <see cref="TabStrip"/> or <see cref="TabControl"/>.
+    /// An item in a <see cref="TabControl"/>.
     /// </summary>
     [PseudoClasses(":pressed", ":selected")]
     public class TabItem : HeaderedContentControl, ISelectable
     {
+        private Dock? _tabStripPlacement;
+        private IDisposable? _ownerSubscriptions;
+
         /// <summary>
         /// Defines the <see cref="TabStripPlacement"/> property.
         /// </summary>
-        public static readonly StyledProperty<Dock> TabStripPlacementProperty =
-            TabControl.TabStripPlacementProperty.AddOwner<TabItem>();
+        public static readonly DirectProperty<TabItem, Dock?> TabStripPlacementProperty =
+            AvaloniaProperty.RegisterDirect<TabItem, Dock?>(nameof(TabStripPlacement), o => o.TabStripPlacement);
 
         /// <summary>
         /// Defines the <see cref="IsSelected"/> property.
@@ -37,16 +43,12 @@ namespace Avalonia.Controls
         }
 
         /// <summary>
-        /// Gets the tab strip placement.
+        /// Gets the placement of this tab relative to the outer <see cref="TabControl"/>, if there is one.
         /// </summary>
-        /// <value>
-        /// The tab strip placement.
-        /// </value>
-        [System.Diagnostics.CodeAnalysis.SuppressMessage("AvaloniaProperty", "AVP1031",
-            Justification = "This property is supposed to be inherited only and settable on parent TabControl.")]
-        public Dock TabStripPlacement
+        public Dock? TabStripPlacement
         {
-            get { return GetValue(TabStripPlacementProperty); }
+            get => _tabStripPlacement;
+            private set => SetAndRaise(TabStripPlacementProperty, ref _tabStripPlacement, value);
         }
 
         /// <summary>
@@ -60,6 +62,24 @@ namespace Avalonia.Controls
 
         protected override AutomationPeer OnCreateAutomationPeer() => new ListItemAutomationPeer(this);
 
+        protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
+        {
+            base.OnAttachedToVisualTree(e);
+
+            _ownerSubscriptions?.Dispose();
+            _ownerSubscriptions = null;
+
+            if (this.FindAncestorOfType<TabControl>() is { } owner && owner.IndexFromContainer(this) != -1)
+            {
+                SubscribeToOwnerProperties(owner);
+            }
+        }
+
+        protected void SubscribeToOwnerProperties(AvaloniaObject owner)
+        {
+            _ownerSubscriptions = owner.GetObservable(TabControl.TabStripPlacementProperty).Subscribe(v => TabStripPlacement = v);
+        }
+
         private void UpdateHeader(AvaloniaPropertyChangedEventArgs obj)
         {
             if (Header == null)