Theme loading benchmark

tests/Avalonia.Benchmarks/Themes/ThemeBenchmark.cs

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+using System;
+
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml.Styling;
+using Avalonia.Shared.PlatformSupport;
+using Avalonia.Styling;
+using Avalonia.UnitTests;
+
+using BenchmarkDotNet.Attributes;
+
+namespace Avalonia.Benchmarks.Themes
+{
+    [MemoryDiagnoser]
+    public class ThemeBenchmark : IDisposable
+    {
+        private IDisposable _app;
+
+        public ThemeBenchmark()
+        {
+            AssetLoader.RegisterResUriParsers();
+
+            _app = UnitTestApplication.Start(TestServices.StyledWindow.With(theme: () => null));
+            // Add empty style to override it later
+            UnitTestApplication.Current.Styles.Add(new Style());
+        }
+
+        [Benchmark]
+        [Arguments("avares://Avalonia.Themes.Fluent/FluentDark.xaml")]
+        [Arguments("avares://Avalonia.Themes.Fluent/FluentLight.xaml")]
+        public bool InitFluentTheme(string themeUri)
+        {
+            UnitTestApplication.Current.Styles[0] = new StyleInclude(new Uri("resm:Styles?assembly=Avalonia.Benchmarks"))
+            {
+                Source = new Uri(themeUri)
+            };
+            return ((IResourceHost)UnitTestApplication.Current).TryGetResource("SystemAccentColor", out _);
+        }
+
+        [Benchmark]
+        [Arguments("avares://Avalonia.Themes.Default/Accents/BaseLight.xaml")]
+        [Arguments("avares://Avalonia.Themes.Default/Accents/BaseDark.xaml")]
+        public bool InitDefaultTheme(string themeUri)
+        {
+            UnitTestApplication.Current.Styles[0] = new Styles
+            {
+                new StyleInclude(new Uri("resm:Styles?assembly=Avalonia.Benchmarks"))
+                {
+                    Source = new Uri(themeUri)
+                },
+                new StyleInclude(new Uri("resm:Styles?assembly=Avalonia.Benchmarks"))
+                {
+                    Source = new Uri("avares://Avalonia.Themes.Default/DefaultTheme.xaml")
+                }
+            };
+            return ((IResourceHost)UnitTestApplication.Current).TryGetResource("ThemeAccentColor", out _);
+        }
+
+        public void Dispose()
+        {
+            _app.Dispose();
+        }
+    }
+}