Avalonia/src/Avalonia.Base/Media/Fonts/SystemFontCollection.cs

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Diagnostics.CodeAnalysis;
using Avalonia.Platform;

namespace Avalonia.Media.Fonts
{
    internal class SystemFontCollection : FontCollectionBase
    {
        private readonly FontManager _fontManager;
        private readonly string[] _familyNames;

        public SystemFontCollection(FontManager fontManager)
        {
            _fontManager = fontManager;
            _familyNames = fontManager.PlatformImpl.GetInstalledFontFamilyNames();
        }

        public override Uri Key => FontManager.SystemFontsKey;

        public override FontFamily this[int index]
        {
            get
            {
                var familyName = _familyNames[index];

                return new FontFamily(familyName);
            }
        }

        public override int Count => _familyNames.Length;

        public override bool TryGetGlyphTypeface(string familyName, FontStyle style, FontWeight weight,
            FontStretch stretch, [NotNullWhen(true)] out IGlyphTypeface? glyphTypeface)
        {
            glyphTypeface = null;

            var key = new FontCollectionKey(style, weight, stretch);

            var glyphTypefaces = _glyphTypefaceCache.GetOrAdd(familyName, (key) => new ConcurrentDictionary<FontCollectionKey, IGlyphTypeface?>());

            if (!glyphTypefaces.TryGetValue(key, out glyphTypeface))
            {
                _fontManager.PlatformImpl.TryCreateGlyphTypeface(familyName, style, weight, stretch, out glyphTypeface);

                if (!glyphTypefaces.TryAdd(key, glyphTypeface))
                {
                    return false;
                }
            }

            return glyphTypeface != null;
        }

        public override void Initialize(IFontManagerImpl fontManager)
        {
            //We initialize the system font collection during construction.
        }

        public override IEnumerator<FontFamily> GetEnumerator()
        {
            foreach (var familyName in _familyNames)
            {
                yield return new FontFamily(familyName);
            }
        }
    }
}