Introduce a struct enumerator implementation for ReadOnlyList that can be reused

src/Avalonia.Base/Utilities/ImmutableReadOnlyListStructEnumerator.cs

@@ -0,0 +1,48 @@
+using System.Collections;
+using System.Collections.Generic;
+
+namespace Avalonia.Utilities
+{
+    public struct ImmutableReadOnlyListStructEnumerator<T> : IEnumerator, IEnumerator<T>
+    {
+        private readonly IReadOnlyList<T> _readOnlyList;
+        private int _pos;
+
+        public ImmutableReadOnlyListStructEnumerator(IReadOnlyList<T> readOnlyList)
+        {
+            _readOnlyList = readOnlyList;
+            _pos = -1;
+            Current = default;
+        }
+
+        public T Current
+        {
+            get;
+            private set;
+        }
+
+        object IEnumerator.Current => Current;
+
+        public void Dispose() { }
+
+        public bool MoveNext()
+        {
+            if (_pos >= _readOnlyList.Count - 1)
+            {
+                return false;
+            }
+
+            Current = _readOnlyList[++_pos];
+
+            return true;
+
+        }
+
+        public void Reset()
+        {
+            _pos = -1;
+
+            Current = default;
+        }
+    }
+}

src/Avalonia.Visuals/Media/FontManager.cs

@@ -70,9 +70,9 @@ namespace Avalonia.Media
         }
 
         /// <summary>
-        ///     Get all installed fonts.
-        /// <param name="checkForUpdates">If <c>true</c> the font collection is updated.</param>
+        ///     Get all installed font family names.
         /// </summary>
+        /// <param name="checkForUpdates">If <c>true</c> the font collection is updated.</param>
         public IEnumerable<string> GetInstalledFontFamilyNames(bool checkForUpdates = false) =>
             PlatformImpl.GetInstalledFontFamilyNames(checkForUpdates);
 

src/Avalonia.Visuals/Media/Fonts/FamilyNameCollection.cs

@@ -6,6 +6,7 @@ using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
+using Avalonia.Utilities;
 
 namespace Avalonia.Media.Fonts
 {
@@ -57,9 +58,9 @@ namespace Avalonia.Media.Fonts
         /// <summary>
         /// Returns an enumerator for the name collection.
         /// </summary>
-        public Enumerator GetEnumerator()
+        public ImmutableReadOnlyListStructEnumerator<string> GetEnumerator()
         {
-            return new Enumerator(this);
+            return new ImmutableReadOnlyListStructEnumerator<string>(this);
         }
 
         IEnumerator<string> IEnumerable<string>.GetEnumerator()
@@ -72,49 +73,6 @@ namespace Avalonia.Media.Fonts
             return GetEnumerator();
         }
 
-        public struct Enumerator : IEnumerator, IEnumerator<string>
-        {
-            private readonly IReadOnlyList<string> _names;
-            private int _pos;
-
-            public Enumerator(IReadOnlyList<string> names)
-            {
-                _names = names;
-                _pos = -1;
-                Current = default;
-            }
-
-            public string Current
-            {
-                get;
-                private set;
-            }
-
-            object IEnumerator.Current => Current;
-
-            public void Dispose() { }
-
-            public bool MoveNext()
-            {
-                if (_pos >= _names.Count - 1)
-                {
-                    return false;
-                }
-
-                Current = _names[++_pos];
-
-                return true;
-
-            }
-
-            public void Reset()
-            {
-                _pos = -1;
-
-                Current = default;
-            }
-        }
-
         /// <summary>
         /// Returns a <see cref="string" /> that represents this instance.
         /// </summary>

src/Skia/Avalonia.Skia/FontManagerImpl.cs

@@ -30,7 +30,7 @@ namespace Avalonia.Skia
             return _skFontManager.FontFamilies;
         }
 
-        [ThreadStatic] private static string[] s_languageTagBuffer;
+        [ThreadStatic] private static string[] t_languageTagBuffer;
 
         public bool TryMatchCharacter(int codepoint, FontWeight fontWeight, FontStyle fontStyle,
             FontFamily fontFamily, CultureInfo culture, out FontKey fontKey)
@@ -40,20 +40,20 @@ namespace Avalonia.Skia
                 culture = CultureInfo.CurrentUICulture;
             }
 
-            if (s_languageTagBuffer == null)
+            if (t_languageTagBuffer == null)
             {
-                s_languageTagBuffer = new string[2];
+                t_languageTagBuffer = new string[2];
             }
 
-            s_languageTagBuffer[0] = culture.TwoLetterISOLanguageName;
-            s_languageTagBuffer[1] = culture.ThreeLetterISOLanguageName;
+            t_languageTagBuffer[0] = culture.TwoLetterISOLanguageName;
+            t_languageTagBuffer[1] = culture.ThreeLetterISOLanguageName;
 
             if (fontFamily != null)
             {
                 foreach (var familyName in fontFamily.FamilyNames)
                 {
                     var skTypeface = _skFontManager.MatchCharacter(familyName, (SKFontStyleWeight)fontWeight,
-                        SKFontStyleWidth.Normal, (SKFontStyleSlant)fontStyle, s_languageTagBuffer, codepoint);
+                        SKFontStyleWidth.Normal, (SKFontStyleSlant)fontStyle, t_languageTagBuffer, codepoint);
 
                     if (skTypeface == null)
                     {
@@ -68,7 +68,7 @@ namespace Avalonia.Skia
             else
             {
                 var skTypeface = _skFontManager.MatchCharacter(null, (SKFontStyleWeight)fontWeight,
-                    SKFontStyleWidth.Normal, (SKFontStyleSlant)fontStyle, s_languageTagBuffer, codepoint);
+                    SKFontStyleWidth.Normal, (SKFontStyleSlant)fontStyle, t_languageTagBuffer, codepoint);
 
                 if (skTypeface != null)
                 {

tests/Avalonia.Visuals.UnitTests/VisualTree/MockRenderInterface.cs

@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.IO;
 using Avalonia.Media;
 using Avalonia.Platform;
+using Avalonia.UnitTests;
 
 namespace Avalonia.Visuals.UnitTests.VisualTree
 {
@@ -57,7 +58,7 @@ namespace Avalonia.Visuals.UnitTests.VisualTree
 
         public IFontManagerImpl CreateFontManager()
         {
-            throw new NotImplementedException();
+            return new MockFontManagerImpl();
         }
 
         public IWriteableBitmapImpl CreateWriteableBitmap(PixelSize size, Vector dpi, PixelFormat? fmt)