Use UnsafeAsSpan while iterating over LogicalChildren

src/Avalonia.Base/Collections/AvaloniaList.cs

@@ -4,6 +4,7 @@ using System.Collections.Generic;
 using System.Collections.Specialized;
 using System.ComponentModel;
 using System.Linq;
+using System.Runtime.InteropServices;
 using Avalonia.Diagnostics;
 
 namespace Avalonia.Collections
@@ -742,6 +743,11 @@ namespace Avalonia.Collections
             NotifyCountChanged();
         }
 
+        internal Span<T> UnsafeAsSpan()
+        {
+            return CollectionsMarshal.AsSpan(_inner);
+        }
+        
         /// <summary>
         /// Enumerates the elements of a <see cref="AvaloniaList{T}"/>.
         /// </summary>

src/Avalonia.Base/Compatibility/CollectionCompatibilityExtensions.cs

@@ -1,32 +1,44 @@
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
-
-namespace System;
+using System.Runtime.CompilerServices;
 
 #if !NET6_0_OR_GREATER
-internal static class CollectionCompatibilityExtensions
+namespace System
 {
-    public static bool Remove<TKey, TValue>(
-        this Dictionary<TKey, TValue> o,
-        TKey key,
-        [MaybeNullWhen(false)] out TValue value)
-            where TKey : notnull
+    internal static class CollectionCompatibilityExtensions
     {
-        if (o.TryGetValue(key, out value))
-            return o.Remove(key);
-        return false;
-    }
+        public static bool Remove<TKey, TValue>(
+            this Dictionary<TKey, TValue> o,
+            TKey key,
+            [MaybeNullWhen(false)] out TValue value)
+            where TKey : notnull
+        {
+            if (o.TryGetValue(key, out value))
+                return o.Remove(key);
+            return false;
+        }
 
-    public static bool TryAdd<TKey, TValue>(this Dictionary<TKey, TValue> o, TKey key, TValue value)
-        where TKey : notnull
+        public static bool TryAdd<TKey, TValue>(this Dictionary<TKey, TValue> o, TKey key, TValue value)
+            where TKey : notnull
+        {
+            if (!o.ContainsKey(key))
+            {
+                o.Add(key, value);
+                return true;
+            }
+
+            return false;
+        }
+    }
+    
+    namespace Runtime.InteropServices
     {
-        if (!o.ContainsKey(key))
+        public static class CollectionsMarshal
         {
-            o.Add(key, value);
-            return true;
+            public static Span<T> AsSpan<T>(List<T>? list)
+                => list is null ? default : new Span<T>(Unsafe.As<StrongBox<T[]>>(list).Value, 0, list.Count);
         }
-
-        return false;
     }
 }
+
 #endif

src/Avalonia.Base/StyledElement.cs

@@ -5,8 +5,10 @@ using System.Collections.Specialized;
 using System.ComponentModel;
 using System.Diagnostics;
 using System.Linq;
+using System.Runtime.InteropServices;
 using Avalonia.Animation;
 using Avalonia.Collections;
+using Avalonia.Collections.Pooled;
 using Avalonia.Controls;
 using Avalonia.Data;
 using Avalonia.Diagnostics;
@@ -82,7 +84,7 @@ namespace Avalonia
         private string? _name;
         private Classes? _classes;
         private ILogicalRoot? _logicalRoot;
-        private IAvaloniaList<ILogical>? _logicalChildren;
+        private AvaloniaList<ILogical>? _logicalChildren;
         private IResourceDictionary? _resources;
         private Styles? _styles;
         private bool _stylesApplied;
@@ -558,18 +560,13 @@ namespace Avalonia
         /// <param name="e">The event args.</param>
         internal virtual void NotifyChildResourcesChanged(ResourcesChangedEventArgs e)
         {
-            if (_logicalChildren is object)
+            if (_logicalChildren is { Count: > 0 })
             {
-                var count = _logicalChildren.Count;
+                e ??= ResourcesChangedEventArgs.Empty;
 
-                if (count > 0)
+                foreach (var child in _logicalChildren.UnsafeAsSpan())
                 {
-                    e ??= ResourcesChangedEventArgs.Empty;
-
-                    for (var i = 0; i < count; ++i)
-                    {
-                        _logicalChildren[i].NotifyResourcesChanged(e);
-                    }
+                    child.NotifyResourcesChanged(e);
                 }
             }
         }
@@ -747,16 +744,16 @@ namespace Avalonia
                     element._dataContextUpdating = true;
                     element.OnDataContextBeginUpdate();
 
-                    var logicalChildren = element.LogicalChildren;
-                    var logicalChildrenCount = logicalChildren.Count;
-
-                    for (var i = 0; i < logicalChildrenCount; i++)
+                    if (element._logicalChildren is {} children)
                     {
-                        if (element.LogicalChildren[i] is StyledElement s &&
-                            s.InheritanceParent == element &&
-                            !s.IsSet(DataContextProperty))
+                        foreach (var child in children.UnsafeAsSpan())
                         {
-                            DataContextNotifying(s, updateStarted);
+                            if (child is StyledElement s &&
+                                s.InheritanceParent == element &&
+                                !s.IsSet(DataContextProperty))
+                            {
+                                DataContextNotifying(s, updateStarted);
+                            }
                         }
                     }
                 }
@@ -903,14 +900,14 @@ namespace Avalonia
                 AttachedToLogicalTree?.Invoke(this, e);
             }
 
-            var logicalChildren = LogicalChildren;
-            var logicalChildrenCount = logicalChildren.Count;
-
-            for (var i = 0; i < logicalChildrenCount; i++)
+            if (_logicalChildren is { } children)
             {
-                if (logicalChildren[i] is StyledElement child && child._logicalRoot != e.Root) // child may already have been attached within an event handler
+                foreach (var child in children.UnsafeAsSpan())
                 {
-                    child.OnAttachedToLogicalTreeCore(e);
+                    if (child is StyledElement s && s._logicalRoot != e.Root)
+                    {
+                        s.OnAttachedToLogicalTreeCore(e);
+                    }
                 }
             }
         }
@@ -924,14 +921,14 @@ namespace Avalonia
                 OnDetachedFromLogicalTree(e);
                 DetachedFromLogicalTree?.Invoke(this, e);
 
-                var logicalChildren = LogicalChildren;
-                var logicalChildrenCount = logicalChildren.Count;
-
-                for (var i = 0; i < logicalChildrenCount; i++)
+                if (_logicalChildren is { } children)
                 {
-                    if (logicalChildren[i] is StyledElement child)
+                    foreach (var child in children.UnsafeAsSpan())
                     {
-                        child.OnDetachedFromLogicalTreeCore(e);
+                        if (child is StyledElement s)
+                        {
+                            s.OnDetachedFromLogicalTreeCore(e);
+                        }
                     }
                 }
 
@@ -991,11 +988,9 @@ namespace Avalonia
 
             if (_logicalChildren is not null)
             {
-                var childCount = _logicalChildren.Count;
-
-                for (var i = 0; i < childCount; ++i)
+                foreach (var child in _logicalChildren.UnsafeAsSpan())
                 {
-                    (_logicalChildren[i] as StyledElement)?.DetachStyles(styles);
+                    (child as StyledElement)?.DetachStyles(styles);
                 }
             }
         }