Added style caching to Style children.

Extracted out the caching code from `Styles` and reuse for `Style.Children`.

src/Avalonia.Base/Styling/Style.cs

@@ -16,6 +16,7 @@ namespace Avalonia.Styling
         private IResourceDictionary? _resources;
         private List<ISetter>? _setters;
         private List<IAnimation>? _animations;
+        private StyleCache? _childCache;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="Style"/> class.
@@ -124,12 +125,10 @@ namespace Avalonia.Styling
 
             if (_children is not null)
             {
-                foreach (var child in _children)
-                {
-                    var childResult = child.TryAttach(target, host);
-                    if (childResult > result)
-                        result = childResult;
-                }
+                _childCache ??= new StyleCache();
+                var childResult = _childCache.TryAttach(_children, target, host);
+                if (childResult > result)
+                    result = childResult;
             }
 
             return result;

src/Avalonia.Base/Styling/StyleCache.cs

@@ -0,0 +1,58 @@
+using System;
+using System.Collections.Generic;
+
+namespace Avalonia.Styling
+{
+    /// <summary>
+    /// Simple cache for improving performance of applying styles.
+    /// </summary>
+    /// <remarks>
+    /// Maps <see cref="IStyleable.StyleKey"/> to a list of styles that are known be be possible
+    /// matches.
+    /// </remarks>
+    internal class StyleCache : Dictionary<Type, List<IStyle>?>
+    {
+        public SelectorMatchResult TryAttach(IList<IStyle> styles, IStyleable target, IStyleHost? host)
+        {
+            if (TryGetValue(target.StyleKey, out var cached))
+            {
+                if (cached is object)
+                {
+                    var result = SelectorMatchResult.NeverThisType;
+
+                    foreach (var style in cached)
+                    {
+                        var childResult = style.TryAttach(target, host);
+                        if (childResult > result)
+                            result = childResult;
+                    }
+
+                    return result;
+                }
+                else
+                {
+                    return SelectorMatchResult.NeverThisType;
+                }
+            }
+            else
+            {
+                List<IStyle>? matches = null;
+
+                foreach (var child in styles)
+                {
+                    if (child.TryAttach(target, host) != SelectorMatchResult.NeverThisType)
+                    {
+                        matches ??= new List<IStyle>();
+                        matches.Add(child);
+                    }
+                }
+
+                Add(target.StyleKey, matches);
+
+                return matches is null ?
+                    SelectorMatchResult.NeverThisType :
+                    SelectorMatchResult.AlwaysThisType;
+            }
+        }
+    }
+}

src/Avalonia.Base/Styling/Styles.cs

@@ -20,7 +20,7 @@ namespace Avalonia.Styling
         private readonly AvaloniaList<IStyle> _styles = new AvaloniaList<IStyle>();
         private IResourceHost? _owner;
         private IResourceDictionary? _resources;
-        private Dictionary<Type, List<IStyle>?>? _cache;
+        private StyleCache? _cache;
 
         public Styles()
         {
@@ -111,43 +111,8 @@ namespace Avalonia.Styling
 
         public SelectorMatchResult TryAttach(IStyleable target, IStyleHost? host)
         {
-            _cache ??= new Dictionary<Type, List<IStyle>?>();
-
-            if (_cache.TryGetValue(target.StyleKey, out var cached))
-            {
-                if (cached is object)
-                {
-                    foreach (var style in cached)
-                    {
-                        style.TryAttach(target, host);
-                    }
-
-                    return SelectorMatchResult.AlwaysThisType;
-                }
-                else
-                {
-                    return SelectorMatchResult.NeverThisType;
-                }
-            }
-            else
-            {
-                List<IStyle>? matches = null;
-
-                foreach (var child in this)
-                {
-                    if (child.TryAttach(target, host) != SelectorMatchResult.NeverThisType)
-                    {
-                        matches ??= new List<IStyle>();
-                        matches.Add(child);
-                    }
-                }
-
-                _cache.Add(target.StyleKey, matches);
-                
-                return matches is null ?
-                    SelectorMatchResult.NeverThisType :
-                    SelectorMatchResult.AlwaysThisType;
-            }
+            _cache ??= new StyleCache();
+            return _cache.TryAttach(this, target, host);
         }
 
         /// <inheritdoc/>