Font fallbacks.

Voile/Source/Resources/FontSet.cs

@@ -0,0 +1,45 @@
+using System.Diagnostics.CodeAnalysis;
+
+namespace Voile.Resources;
+
+/// <summary>
+/// Contains a set of multiple fonts. Used to fetch fonts based on availability of glyphs.
+/// </summary>
+public class FontSet
+{
+    public FontSet() { }
+
+    public FontSet(IEnumerable<ResourceRef<Font>> fonts)
+    {
+        _fonts = fonts.ToList();
+    }
+
+    public void AddFont(ResourceRef<Font> font) => _fonts.Add(font);
+
+    /// <summary>
+    /// Tries to get a suitable font that has a given character.
+    /// </summary>
+    /// <param name="c">Character to get a suitable font for.</param>
+    /// <param name="result">Font that contains this character.</param>
+    /// <returns><c>true</c> if a font that contains this character exists, <c>false</c> otherwise.</returns>
+    public bool TryGetFontFor(char c, [NotNullWhen(true)] out ResourceRef<Font>? result)
+    {
+        result = ResourceRef<Font>.Empty();
+        foreach (var fontRef in _fonts)
+        {
+            if (!fontRef.TryGetValue(out var font))
+            {
+                return false;
+            }
+
+            if (font.HasGlyph(c))
+            {
+                result = fontRef;
+                return true;
+            }
+        }
+        return false;
+    }
+
+    private List<ResourceRef<Font>> _fonts = new();
+}

Voile/Source/Resources/Resource.cs

@@ -52,6 +52,28 @@ namespace Voile
         {
             Guid = guid;
         }
+
+        public override bool Equals(object? obj)
+        {
+            return obj is ResourceRef<T> other && Guid.Equals(other.Guid);
+        }
+
+        public override int GetHashCode()
+        {
+            return Guid.GetHashCode();
+        }
+
+        public static bool operator ==(ResourceRef<T>? left, ResourceRef<T>? right)
+        {
+            if (ReferenceEquals(left, right)) return true;
+            if (left is null || right is null) return false;
+            return left.Guid == right.Guid;
+        }
+
+        public static bool operator !=(ResourceRef<T>? left, ResourceRef<T>? right)
+        {
+            return !(left == right);
+        }
     }
 
     /// <summary>

Voile/Source/Resources/ResourceManager.cs

@@ -231,7 +231,7 @@ namespace Voile.Resources
         /// </summary>
         /// <param name="path">Path to the resource.</param>
         /// <returns>True if a resource at the specified path is loaded, otherwise false.</returns>
-        public bool IsResourceLoaded(string path) => _resourcePathMap.ContainsKey(path);
+        public static bool IsResourceLoaded(string path) => _resourcePathMap.ContainsKey(path);
 
         /// <summary>
         /// Adds a resource loader associated with a resource type.

Voile/Source/UI/Widgets/Label.cs

@@ -1,6 +1,7 @@
 using System.Numerics;
 using Voile.Input;
 using Voile.Rendering;
+using Voile.Resources;
 
 namespace Voile.UI.Widgets;
 
@@ -17,10 +18,26 @@ public class Label : Widget
         }
     }
 
+    /// <summary>
+    /// <see cref="FontSet"/> to use with this label.
+    /// </summary>
+    public FontSet FontSet { get; set; } = new();
+
     public Label(string text, ResourceRef<Font> fontOverride)
     {
         _text = text;
-        _fontOverride = fontOverride;
+
+        FontSet.AddFont(fontOverride);
+
+        MarkDirty();
+        Update();
+    }
+
+    public Label(string text, FontSet fontSet)
+    {
+        _text = text;
+
+        FontSet = fontSet;
 
         MarkDirty();
         Update();
@@ -34,20 +51,34 @@ public class Label : Widget
     public override void Render(RenderSystem renderer, Style style)
     {
         // TODO: use style here.
-        if (!_fontOverride.HasValue) return;
+
         renderer.SetTransform(GlobalPosition, Vector2.Zero);
-        renderer.DrawText(_fontOverride, _text, Color.White);
+        renderer.DrawText(_suitableFont, _text, Color.White);
     }
 
     protected override void OnUpdate()
     {
-        var font = _fontOverride.Value;
+        ResourceRef<Font> fontRef = ResourceRef<Font>.Empty();
+        foreach (var c in _text)
+        {
+            if (FontSet.TryGetFontFor(c, out var fallbackFont))
+            {
+                if (fallbackFont != fontRef)
+                {
+                    fontRef = fallbackFont;
+                }
+            }
+        }
+
+        _suitableFont = fontRef;
+
+        var font = _suitableFont.Value;
         _textSize = font.Measure(_text);
 
         Size = _textSize;
     }
 
-    private ResourceRef<Font> _fontOverride = ResourceRef<Font>.Empty();
+    private ResourceRef<Font> _suitableFont = ResourceRef<Font>.Empty();
 
     private string _text = "Hello, World!";
     private Rect _textSize = Rect.Zero;