Font size and font family Optimizations

src/Core/src/Fonts/FontManager.Windows.cs

@@ -1,12 +1,13 @@
-#nullable enable
-using System;
+using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.IO;
 using Microsoft.Extensions.Logging;
 using Microsoft.Graphics.Canvas.Text;
 using Microsoft.Maui.ApplicationModel;
 using Microsoft.Maui.Storage;
+using Microsoft.UI.Xaml;
+using Microsoft.UI.Xaml.Controls;
 using Microsoft.UI.Xaml.Media;
 
 namespace Microsoft.Maui
@@ -27,6 +28,9 @@ public class FontManager : IFontManager
 		readonly IFontRegistrar _fontRegistrar;
 		readonly IServiceProvider? _serviceProvider;
 
+		FontFamily? _defaultFontFamily;
+		double? _defaultFontSize;
+
 		/// <summary>
 		/// Creates a new <see cref="EmbeddedFontLoader"/> instance.
 		/// </summary>
@@ -37,15 +41,42 @@ public FontManager(IFontRegistrar fontRegistrar, IServiceProvider? serviceProvid
 		{
 			_fontRegistrar = fontRegistrar;
 			_serviceProvider = serviceProvider;
+
+			Application.Current.Resources.RegisterPropertyChangedCallback(Control.FontFamilyProperty, OnPropertyChanged);
+			Application.Current.Resources.RegisterPropertyChangedCallback(Control.FontSizeProperty, OnPropertyChanged);
+		}
+
+		private void OnPropertyChanged(DependencyObject sender, DependencyProperty dp)
+		{
+			if (dp == Control.FontFamilyProperty)
+			{
+				_defaultFontFamily = (FontFamily)Application.Current.Resources[SystemFontFamily];
+			}
+			else if (dp == Control.FontSizeProperty)
+			{
+				_defaultFontSize = (double)Application.Current.Resources[SystemFontSize];
+			}
 		}
 
 		/// <inheritdoc/>
-		public FontFamily DefaultFontFamily =>
-			(FontFamily)UI.Xaml.Application.Current.Resources[SystemFontFamily];
+		public FontFamily DefaultFontFamily
+		{
+			get
+			{
+				_defaultFontFamily ??= (FontFamily)Application.Current.Resources[SystemFontFamily];
+				return _defaultFontFamily;
+			}
+		}
 
 		/// <inheritdoc/>
-		public double DefaultFontSize =>
-			(double)UI.Xaml.Application.Current.Resources[SystemFontSize];
+		public double DefaultFontSize
+		{
+			get
+			{
+				_defaultFontSize ??= (double)Application.Current.Resources[SystemFontSize];
+				return _defaultFontSize.Value;
+			}
+		}
 
 		/// <inheritdoc/>
 		public FontFamily GetFontFamily(Font font)