Optimize Pen's subscriptions to IAffects render

src/Avalonia.Visuals/ApiCompatBaseline.txt

@@ -7,6 +7,9 @@ InterfacesShouldHaveSameMembers : Interface member 'public System.Threading.Task
 MembersMustExist : Member 'public System.Threading.Tasks.Task Avalonia.Animation.PageSlide.Start(Avalonia.Visual, Avalonia.Visual, System.Boolean)' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'public void Avalonia.Media.GlyphRun..ctor()' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'public void Avalonia.Media.GlyphRun.GlyphTypeface.set(Avalonia.Media.GlyphTypeface)' does not exist in the implementation but it does exist in the contract.
+CannotSealType : Type 'Avalonia.Media.Pen' is actually (has the sealed modifier) sealed in the implementation but not sealed in the contract.
+MembersMustExist : Member 'protected void Avalonia.Media.Pen.AffectsRender<T>(Avalonia.AvaloniaProperty[])' does not exist in the implementation but it does exist in the contract.
+MembersMustExist : Member 'protected void Avalonia.Media.Pen.RaiseInvalidated(System.EventArgs)' does not exist in the implementation but it does exist in the contract.
 TypeCannotChangeClassification : Type 'Avalonia.Media.Immutable.ImmutableSolidColorBrush' is a 'class' in the implementation but is a 'struct' in the contract.
 MembersMustExist : Member 'public void Avalonia.Media.TextFormatting.DrawableTextRun.Draw(Avalonia.Media.DrawingContext)' does not exist in the implementation but it does exist in the contract.
 CannotAddAbstractMembers : Member 'public void Avalonia.Media.TextFormatting.DrawableTextRun.Draw(Avalonia.Media.DrawingContext, Avalonia.Point)' is abstract in the implementation but is missing in the contract.
@@ -83,4 +86,4 @@ InterfacesShouldHaveSameMembers : Interface member 'public Avalonia.Size Avaloni
 InterfacesShouldHaveSameMembers : Interface member 'public System.TimeSpan Avalonia.Platform.IPlatformSettings.TouchDoubleClickTime' is present in the implementation but not in the contract.
 InterfacesShouldHaveSameMembers : Interface member 'public Avalonia.Size Avalonia.Platform.IPlatformSettings.TouchDoubleClickSize.get()' is present in the implementation but not in the contract.
 InterfacesShouldHaveSameMembers : Interface member 'public System.TimeSpan Avalonia.Platform.IPlatformSettings.TouchDoubleClickTime.get()' is present in the implementation but not in the contract.
-Total Issues: 84
+Total Issues: 87

src/Avalonia.Visuals/Media/Pen.cs

@@ -7,7 +7,7 @@ namespace Avalonia.Media
     /// <summary>
     /// Describes how a stroke is drawn.
     /// </summary>
-    public class Pen : AvaloniaObject, IPen
+    public sealed class Pen : AvaloniaObject, IPen, IWeakEventSubscriber<EventArgs>
     {
         /// <summary>
         /// Defines the <see cref="Brush"/> property.
@@ -45,6 +45,9 @@ namespace Avalonia.Media
         public static readonly StyledProperty<double> MiterLimitProperty =
             AvaloniaProperty.Register<Pen, double>(nameof(MiterLimit), 10.0);
 
+        private EventHandler? _invalidated;
+        private IAffectsRender? _subscribedTo;
+        
         /// <summary>
         /// Initializes a new instance of the <see cref="Pen"/> class.
         /// </summary>
@@ -96,17 +99,6 @@ namespace Avalonia.Media
             DashStyle = dashStyle;
         }
 
-        static Pen()
-        {
-            AffectsRender<Pen>(
-                BrushProperty,
-                ThicknessProperty,
-                DashStyleProperty,
-                LineCapProperty,
-                LineJoinProperty,
-                MiterLimitProperty);
-        }
-
         /// <summary>
         /// Gets or sets the brush used to draw the stroke.
         /// </summary>
@@ -116,6 +108,11 @@ namespace Avalonia.Media
             set => SetValue(BrushProperty, value);
         }
 
+        private static readonly WeakEvent<IAffectsRender, EventArgs> InvalidatedWeakEvent =
+            WeakEvent.Register<IAffectsRender>(
+                (s, h) => s.Invalidated += h,
+                (s, h) => s.Invalidated -= h);
+
         /// <summary>
         /// Gets or sets the stroke thickness.
         /// </summary>
@@ -165,7 +162,19 @@ namespace Avalonia.Media
         /// <summary>
         /// Raised when the pen changes.
         /// </summary>
-        public event EventHandler? Invalidated;
+        public event EventHandler? Invalidated
+        {
+            add
+            {
+                _invalidated += value;
+                UpdateBrushSubscription();
+            }
+            remove
+            {
+                _invalidated -= value; 
+                UpdateBrushSubscription();
+            }
+        }
 
         /// <summary>
         /// Creates an immutable clone of the brush.
@@ -182,68 +191,33 @@ namespace Avalonia.Media
                 MiterLimit);
         }
 
-        /// <summary>
-        /// Marks a property as affecting the pen's visual representation.
-        /// </summary>
-        /// <param name="properties">The properties.</param>
-        /// <remarks>
-        /// After a call to this method in a pen's static constructor, any change to the
-        /// property will cause the <see cref="Invalidated"/> event to be raised on the pen.
-        /// </remarks>
-        protected static void AffectsRender<T>(params AvaloniaProperty[] properties)
-            where T : Pen
+        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
         {
-            static void Invalidate(AvaloniaPropertyChangedEventArgs e)
-            {
-                if (e.Sender is T sender)
-                {
-                    sender.RaiseInvalidated(EventArgs.Empty);
-                }
-            }
+            _invalidated?.Invoke(this, EventArgs.Empty);
+            if(change.Property == BrushProperty)
+                UpdateBrushSubscription();
+            base.OnPropertyChanged(change);
+        }
 
-            static void InvalidateAndSubscribe(AvaloniaPropertyChangedEventArgs e)
+        void UpdateBrushSubscription()
+        {
+            if ((_invalidated == null || _subscribedTo != Brush) && _subscribedTo != null)
             {
-                if (e.Sender is T sender)
-                {
-                    if (e.OldValue is IAffectsRender oldValue)
-                    {
-                        WeakEventHandlerManager.Unsubscribe<EventArgs, T>(
-                            oldValue,
-                            nameof(oldValue.Invalidated),
-                            sender.AffectsRenderInvalidated);
-                    }
-
-                    if (e.NewValue is IAffectsRender newValue)
-                    {
-                        WeakEventHandlerManager.Subscribe<IAffectsRender, EventArgs, T>(
-                            newValue,
-                            nameof(newValue.Invalidated),
-                            sender.AffectsRenderInvalidated);
-                    }
-
-                    sender.RaiseInvalidated(EventArgs.Empty);
-                }
+                InvalidatedWeakEvent.Unsubscribe(_subscribedTo, this);
+                _subscribedTo = null;
             }
 
-            foreach (var property in properties)
+            if (_invalidated != null && _subscribedTo != Brush && Brush is IAffectsRender affectsRender)
             {
-                if (property.CanValueAffectRender())
-                {
-                    property.Changed.Subscribe(e => InvalidateAndSubscribe(e));
-                }
-                else
-                {
-                    property.Changed.Subscribe(e => Invalidate(e));
-                }
+                InvalidatedWeakEvent.Subscribe(affectsRender, this);
+                _subscribedTo = affectsRender;
             }
         }
-
-        /// <summary>
-        /// Raises the <see cref="Invalidated"/> event.
-        /// </summary>
-        /// <param name="e">The event args.</param>
-        protected void RaiseInvalidated(EventArgs e) => Invalidated?.Invoke(this, e);
-
-        private void AffectsRenderInvalidated(object? sender, EventArgs e) => RaiseInvalidated(EventArgs.Empty);
+        
+        void IWeakEventSubscriber<EventArgs>.OnEvent(object? sender, WeakEvent ev, EventArgs e)
+        {
+            if (ev == InvalidatedWeakEvent) 
+                _invalidated?.Invoke(this, EventArgs.Empty);
+        }
     }
 }