Added some ControlTheme benchmarks.

tests/Avalonia.Benchmarks/Styling/ControlTheme_Apply.cs

@@ -0,0 +1,147 @@
+using System.Collections.Generic;
+using System.Runtime.CompilerServices;
+using Avalonia.Controls;
+using Avalonia.Controls.Primitives;
+using Avalonia.Controls.Templates;
+using Avalonia.Media;
+using Avalonia.Styling;
+using BenchmarkDotNet.Attributes;
+
+#nullable enable
+
+namespace Avalonia.Benchmarks.Styling
+{
+    [MemoryDiagnoser]
+    public class ControlTheme_Apply
+    {
+        private ControlTheme _theme;
+        private ControlTheme _otherTheme;
+        private List<Style> _styles = new();
+
+        public ControlTheme_Apply()
+        {
+            RuntimeHelpers.RunClassConstructor(typeof(TestControl).TypeHandle);
+
+            _theme = CreateControlTheme(Brushes.Red);
+            _otherTheme = CreateControlTheme(Brushes.Orange);
+
+            for (var i = 0; i < 100; ++i)
+            {
+                _styles.Add(new Style(x => x.OfType<TestControl>())
+                {
+                    Setters = { new Setter(TestControl.BackgroundProperty, Brushes.Yellow) }
+                });
+            }
+        }
+
+        [Benchmark]
+        public void Apply_Control_Theme()
+        {
+            var target = new TestControl();
+
+            target.BeginBatchUpdate();
+
+            _theme.TryAttach(target, null);
+            target.ApplyTemplate();
+            _theme.TryAttach(target.VisualChild, null);
+
+            target.EndBatchUpdate();
+        }
+
+
+        [Benchmark]
+        public void Apply_Remove_Control_Theme()
+        {
+            var target = new TestControl();
+
+            target.BeginBatchUpdate();
+
+            _theme.TryAttach(target, null);
+            target.ApplyTemplate();
+            _theme.TryAttach(target.VisualChild, null);
+
+            target.EndBatchUpdate();
+
+            // Switching to another theme will cause the current theme to be removed but won't
+            // immediately apply the new theme, so for the benefit of the benchmark it has the
+            // effect of simply removing the theme.
+            target.Theme = _otherTheme;
+        }
+
+        [Benchmark]
+        public void Apply_Control_Theme_With_Styles()
+        {
+            var target = new TestControl();
+
+            target.BeginBatchUpdate();
+
+            _theme.TryAttach(target, null);
+            target.ApplyTemplate();
+            _theme.TryAttach(target.VisualChild, null);
+
+            foreach (var style in _styles)
+                style.TryAttach(target, null);
+
+            target.EndBatchUpdate();
+        }
+
+        [Benchmark]
+        public void Apply_Remove_Control_Theme_With_Styles()
+        {
+            var target = new TestControl();
+
+            target.BeginBatchUpdate();
+
+            _theme.TryAttach(target, null);
+            target.ApplyTemplate();
+            _theme.TryAttach(target.VisualChild, null);
+
+            foreach (var style in _styles)
+                style.TryAttach(target, null);
+
+            target.EndBatchUpdate();
+
+            // Switching to another theme will cause the current theme to be removed but won't
+            // immediately apply the new theme, so for the benefit of the benchmark it has the
+            // effect of simply removing the theme.
+            target.Theme = _otherTheme;
+        }
+
+        private static ControlTheme CreateControlTheme(IBrush background)
+        {
+            return new ControlTheme(typeof(TestControl))
+            {
+                Setters =
+                {
+                    new Setter(TestControl.BackgroundProperty, Brushes.Transparent),
+                    new Setter(TestControl.TemplateProperty, new FuncControlTemplate<TestControl>((_, x) =>
+                        new Border())),
+                },
+                Children =
+                {
+                    new Style(x => x.Nesting().Template().OfType<Border>())
+                    {
+                        Setters = { new Setter(TestControl.BackgroundProperty, Brushes.Red), }
+                    },
+                    new Style(x => x.Nesting().Class(":pointerover").Template().OfType<Border>())
+                    {
+                        Setters = { new Setter(TestControl.BackgroundProperty, Brushes.Green), }
+                    },
+                    new Style(x => x.Nesting().Class(":pressed").Template().OfType<Border>())
+                    {
+                        Setters = { new Setter(TestControl.BackgroundProperty, Brushes.Blue), }
+                    },
+                }
+            };
+        }
+
+        private class TestControl : TemplatedControl
+        {
+            public IStyleable VisualChild => (IStyleable)VisualChildren[0];
+        }
+
+        private class TestClass2 : Control
+        {
+        }
+    }
+}