Merge pull request #3668 from MarchingCube/style-class-activator-delegate-alloc

Avoid allocating extra delegates when initializing and deinitializing StyleClassActivator

src/Avalonia.Styling/Styling/Activators/StyleClassActivator.cs

@@ -14,6 +14,7 @@ namespace Avalonia.Styling.Activators
     {
         private readonly IList<string> _match;
         private readonly IAvaloniaReadOnlyList<string> _classes;
+        private NotifyCollectionChangedEventHandler? _classesChangedHandler;
 
         public StyleClassActivator(IAvaloniaReadOnlyList<string> classes, IList<string> match)
         {
@@ -21,6 +22,9 @@ namespace Avalonia.Styling.Activators
             _match = match;
         }
 
+        private NotifyCollectionChangedEventHandler ClassesChangedHandler =>
+            _classesChangedHandler ??= ClassesChanged;
+
         public static bool AreClassesMatching(IReadOnlyList<string> classes, IList<string> toMatch)
         {
             int remainingMatches = toMatch.Count;
@@ -51,16 +55,15 @@ namespace Avalonia.Styling.Activators
             return remainingMatches == 0;
         }
 
-
         protected override void Initialize()
         {
             PublishNext(IsMatching());
-            _classes.CollectionChanged += ClassesChanged;
+            _classes.CollectionChanged += ClassesChangedHandler;
         }
 
         protected override void Deinitialize()
         {
-            _classes.CollectionChanged -= ClassesChanged;
+            _classes.CollectionChanged -= ClassesChangedHandler;
         }
 
         private void ClassesChanged(object sender, NotifyCollectionChangedEventArgs e)