Introduce abstract font manager

src/Avalonia.Visuals/Media/FontFamily.cs

@@ -8,11 +8,11 @@ using Avalonia.Media.Fonts;
 
 namespace Avalonia.Media
 {
-    public class FontFamily
+    public sealed class FontFamily
     {
         static FontFamily()
         {
-            Default = new FontFamily(FontManager.DefaultFontFamilyName);
+            Default = new FontFamily(FontManager.Default.DefaultFontFamilyName);
         }
 
         /// <inheritdoc />
@@ -64,7 +64,7 @@ namespace Avalonia.Media
         /// Consider using the new <see cref="FontManager"/> instead.
         /// </remarks>
         public static IEnumerable<FontFamily> SystemFontFamilies =>
-            FontManager.GetInstalledFontFamilyNames().Select(name => new FontFamily(name));
+            FontManager.Default.GetInstalledFontFamilyNames().Select(name => new FontFamily(name));
 
         /// <summary>
         /// Gets the primary family name of the font family.

src/Avalonia.Visuals/Media/FontManager.cs

@@ -7,50 +7,106 @@ using Avalonia.Platform;
 
 namespace Avalonia.Media
 {
-    public static class FontManager
+    /// <summary>
+    ///     The font manager is used to query the system's installed fonts and is responsible for caching loaded fonts.
+    ///     It is also responsible for the font fallback.
+    /// </summary>
+    public abstract class FontManager
     {
-        private static readonly IFontManagerImpl s_platformImpl = GetPlatformImpl();
+        public static readonly FontManager Default = CreateDefault();
 
-        /// <inheritdoc cref="IFontManagerImpl.DefaultFontFamilyName"/>
-        public static string DefaultFontFamilyName => s_platformImpl.DefaultFontFamilyName;
+        /// <summary>
+        ///     Gets the system's default font family's name.
+        /// </summary>
+        public string DefaultFontFamilyName
+        {
+            get;
+            protected set;
+        }
 
-        /// <inheritdoc cref="IFontManagerImpl.GetInstalledFontFamilyNames"/>
-        public static IEnumerable<string> GetInstalledFontFamilyNames(bool checkForUpdates = false) =>
-            s_platformImpl.GetInstalledFontFamilyNames(checkForUpdates);
+        /// <summary>
+        ///     Get all installed fonts in the system.
+        /// <param name="checkForUpdates">If <c>true</c> the font collection is updated.</param>
+        /// </summary>
+        public abstract IEnumerable<string> GetInstalledFontFamilyNames(bool checkForUpdates = false);
 
-        /// <inheritdoc cref="IFontManagerImpl.GetTypeface"/>
-        public static Typeface GetTypeface(FontFamily fontFamily, FontWeight fontWeight, FontStyle fontStyle) =>
-            s_platformImpl.GetTypeface(fontFamily, fontWeight, fontStyle);
+        /// <summary>
+        ///     Get a cached typeface from specified parameters.
+        /// </summary>
+        /// <param name="fontFamily">The font family.</param>
+        /// <param name="fontWeight">The font weight.</param>
+        /// <param name="fontStyle">The font style.</param>
+        /// <returns>
+        ///     The cached typeface.
+        /// </returns>
+        public abstract Typeface GetCachedTypeface(FontFamily fontFamily, FontWeight fontWeight, FontStyle fontStyle);
 
-        /// <inheritdoc cref="IFontManagerImpl.MatchCharacter"/>
-        public static Typeface MatchCharacter(int codepoint, FontWeight fontWeight = default,
+        /// <summary>
+        ///     Tries to match a specified character to a typeface that supports specified font properties.
+        ///     Returns <c>null</c> if no fallback was found.
+        /// </summary>
+        /// <param name="codepoint">The codepoint to match against.</param>
+        /// <param name="fontWeight">The font weight.</param>
+        /// <param name="fontStyle">The font style.</param>
+        /// <param name="fontFamily">The font family. This is optional and used for fallback lookup.</param>
+        /// <param name="culture">The culture.</param>
+        /// <returns>
+        ///     The matched typeface.
+        /// </returns>
+        public abstract Typeface MatchCharacter(int codepoint, FontWeight fontWeight = default,
             FontStyle fontStyle = default,
-            FontFamily fontFamily = null, CultureInfo culture = null) =>
-            s_platformImpl.MatchCharacter(codepoint, fontWeight, fontStyle, fontFamily, culture);
+            FontFamily fontFamily = null, CultureInfo culture = null);
 
-        private static IFontManagerImpl GetPlatformImpl()
+        public static FontManager CreateDefault()
         {
             var platformImpl = AvaloniaLocator.Current.GetService<IFontManagerImpl>();
 
-            return platformImpl ?? new EmptyFontManagerImpl();
+            if (platformImpl != null)
+            {
+                return new PlatformFontManager(platformImpl);
+            }
+
+            return new EmptyFontManager();
         }
 
-        private class EmptyFontManagerImpl : IFontManagerImpl
+        private class PlatformFontManager : FontManager
         {
-            public string DefaultFontFamilyName => "Arial";
-
-            public IEnumerable<string> GetInstalledFontFamilyNames(bool checkForUpdates = false) => new[] { "Arial" };
+            private readonly IFontManagerImpl _platformImpl;
 
-            public Typeface GetTypeface(FontFamily fontFamily, FontWeight fontWeight, FontStyle fontStyle)
+            public PlatformFontManager(IFontManagerImpl platformImpl)
             {
-                return new Typeface(fontFamily, fontWeight, fontStyle);
+                _platformImpl = platformImpl;
+
+                DefaultFontFamilyName = _platformImpl.DefaultFontFamilyName;
             }
 
-            public Typeface MatchCharacter(int codepoint, FontWeight fontWeight = default, FontStyle fontStyle = default,
-                FontFamily fontFamily = null, CultureInfo culture = null)
+            public override IEnumerable<string> GetInstalledFontFamilyNames(bool checkForUpdates = false) =>
+                _platformImpl.GetInstalledFontFamilyNames(checkForUpdates);
+
+            public override Typeface GetCachedTypeface(FontFamily fontFamily, FontWeight fontWeight, FontStyle fontStyle) =>
+                _platformImpl.GetTypeface(fontFamily, fontWeight, fontStyle);
+
+            public override Typeface MatchCharacter(int codepoint, FontWeight fontWeight = default,
+                FontStyle fontStyle = default,
+                FontFamily fontFamily = null, CultureInfo culture = null) =>
+                _platformImpl.MatchCharacter(codepoint, fontWeight, fontStyle, fontFamily, culture);
+        }
+
+        private class EmptyFontManager : FontManager
+        {
+            public EmptyFontManager()
             {
-                return null;
+                DefaultFontFamilyName = "Empty";
             }
+
+            public override IEnumerable<string> GetInstalledFontFamilyNames(bool checkForUpdates = false) =>
+                new[] { DefaultFontFamilyName };
+
+            public override Typeface GetCachedTypeface(FontFamily fontFamily, FontWeight fontWeight, FontStyle fontStyle) => new Typeface(fontFamily, fontWeight, fontStyle);
+
+            public override Typeface MatchCharacter(int codepoint, FontWeight fontWeight = default,
+                FontStyle fontStyle = default,
+                FontFamily fontFamily = null, CultureInfo culture = null) => null;
         }
     }
 }

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

@@ -9,7 +9,7 @@ using System.Text;
 
 namespace Avalonia.Media.Fonts
 {
-    public class FamilyNameCollection : IReadOnlyList<string>
+    public sealed class FamilyNameCollection : IReadOnlyList<string>
     {
         /// <summary>
         /// Initializes a new instance of the <see cref="FamilyNameCollection"/> class.

src/Avalonia.Visuals/Media/GlyphTypeface.cs

@@ -7,7 +7,7 @@ using Avalonia.Platform;
 
 namespace Avalonia.Media
 {
-    public class GlyphTypeface : IDisposable
+    public sealed class GlyphTypeface : IDisposable
     {
         private static readonly IPlatformRenderInterface s_platformRenderInterface =
             AvaloniaLocator.Current.GetService<IPlatformRenderInterface>();

src/Avalonia.Visuals/Media/Typeface.cs

@@ -10,7 +10,7 @@ namespace Avalonia.Media
     /// <summary>
     /// Represents a typeface.
     /// </summary>
-    [DebuggerDisplay("Name = {FontFamily.Name}, Style = {Style}, Weight = {Weight}")]
+    [DebuggerDisplay("Name = {FontFamily.Name}, Weight = {Weight}, Style = {Style}")]
     public class Typeface : IEquatable<Typeface>
     {
         public static readonly Typeface Default = new Typeface(FontFamily.Default);

src/Skia/Avalonia.Skia/GlyphTypefaceImpl.cs

@@ -10,7 +10,7 @@ using SkiaSharp;
 
 namespace Avalonia.Skia
 {
-    internal class GlyphTypefaceImpl : IGlyphTypefaceImpl
+    public class GlyphTypefaceImpl : IGlyphTypefaceImpl
     {
         private bool _isDisposed;
 

src/Windows/Avalonia.Direct2D1/Media/GlyphTypefaceImpl.cs

@@ -9,7 +9,7 @@ using SharpDX.DirectWrite;
 
 namespace Avalonia.Direct2D1.Media
 {
-    internal class GlyphTypefaceImpl : IGlyphTypefaceImpl
+    public class GlyphTypefaceImpl : IGlyphTypefaceImpl
     {
         private bool _isDisposed;