Added benchmark for changing control theme.

tests/Avalonia.Benchmarks/Styling/ControlTheme_Change.cs

@@ -0,0 +1,149 @@
+using System;
+using System.Collections.Generic;
+using System.Runtime.CompilerServices;
+using Avalonia.Controls;
+using Avalonia.Media;
+using Avalonia.Styling;
+using Avalonia.UnitTests;
+using BenchmarkDotNet.Attributes;
+
+namespace Avalonia.Benchmarks.Styling
+{
+    [MemoryDiagnoser]
+    public class ControlTheme_Change : IDisposable
+    {
+        private readonly IDisposable _app;
+        private readonly TestRoot _root;
+        private readonly TextBox _control;
+        private readonly ControlTheme _theme1;
+        private readonly ControlTheme _theme2;
+
+        public ControlTheme_Change()
+        {
+            _app = UnitTestApplication.Start(
+                TestServices.StyledWindow.With(
+                    renderInterface: new NullRenderingPlatform(),
+                    threadingInterface: new NullThreadingPlatform()));
+
+            // Simulate an application with a lot of styles by creating a tree of nested panels,
+            // each with a bunch of styles applied.
+            var (rootPanel, leafPanel) = CreateNestedPanels(10);
+
+            // We're benchmarking how long it takes to switch control theme on a TextBox in this
+            // situation.
+            var baseTheme = (ControlTheme)Application.Current.FindResource(typeof(TextBox)) ??
+                throw new Exception("Base TextBox theme not found.");
+
+            _theme1 = new ControlTheme(typeof(TextBox))
+            {
+                BasedOn = baseTheme,
+                Setters = { new Setter(TextBox.BackgroundProperty, Brushes.Red) },
+            };
+
+            _theme2 = new ControlTheme(typeof(TextBox))
+            {
+                BasedOn = baseTheme,
+                Setters = { new Setter(TextBox.BackgroundProperty, Brushes.Green) },
+            };
+
+            _control = new TextBox { Theme = _theme1 };
+            leafPanel.Children.Add(_control);
+
+            _root = new TestRoot(true, rootPanel)
+            {
+                Renderer = new NullRenderer(),
+            };
+
+            _root.LayoutManager.ExecuteInitialLayoutPass();
+        }
+
+        [Benchmark]
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        public void Change_ControlTheme()
+        {
+            if (_control.Background != Brushes.Red)
+                throw new Exception("Invalid benchmark state");
+
+            _control.Theme = _theme2;
+            _root.LayoutManager.ExecuteLayoutPass();
+
+            if (_control.Background != Brushes.Green)
+                throw new Exception("Invalid benchmark state");
+
+            _control.Theme = _theme1;
+            _root.LayoutManager.ExecuteLayoutPass();
+
+            if (_control.Background != Brushes.Red)
+                throw new Exception("Invalid benchmark state");
+        }
+
+        public void Dispose()
+        {
+            _app.Dispose();
+        }
+
+        private static (Panel, Panel) CreateNestedPanels(int count)
+        {
+            var root = new Panel();
+            var last = root;
+
+            for (var i = 0; i < count; ++i)
+            {
+                var panel = new Panel();
+                panel.Styles.AddRange(CreateStyles());
+                last.Children.Add(panel);
+                last = panel;
+            }
+
+            return (root, last);
+        }
+
+        private static IEnumerable<IStyle> CreateStyles()
+        {
+            var types = new[]
+            {
+                typeof(Border),
+                typeof(Button),
+                typeof(ButtonSpinner),
+                typeof(Carousel),
+                typeof(CheckBox),
+                typeof(ComboBox),
+                typeof(ContentControl),
+                typeof(Expander),
+                typeof(ItemsControl),
+                typeof(Label),
+                typeof(ListBox),
+                typeof(ProgressBar),
+                typeof(RadioButton),
+                typeof(RepeatButton),
+                typeof(ScrollViewer),
+                typeof(Slider),
+                typeof(Spinner),
+                typeof(SplitView),
+                typeof(TextBox),
+                typeof(ToggleSwitch),
+                typeof(TreeView),
+                typeof(Viewbox),
+                typeof(Window),
+            };
+
+            foreach (var type in types)
+            {
+                yield return new Style(x => x.OfType(type))
+                {
+                    Setters = { new Setter(Control.TagProperty, type.Name) }
+                };
+
+                yield return new Style(x => x.OfType(type).Class("foo"))
+                {
+                    Setters = { new Setter(Control.TagProperty, type.Name + " foo") }
+                };
+
+                yield return new Style(x => x.OfType(type).Class("bar"))
+                {
+                    Setters = { new Setter(Control.TagProperty, type.Name + " bar") }
+                };
+            }
+        }
+    }
+}