Improves performance of GetLayoutHandlerIndex (#18499)

src/Core/src/Handlers/Layout/LayoutExtensions.cs

@@ -9,14 +9,34 @@ internal static class LayoutExtensions
 
 		public static int GetLayoutHandlerIndex(this ILayout layout, IView view)
 		{
-			switch (layout.Count)
+			var count = layout.Count;
+			switch (count)
 			{
 				case 0:
 					return -1;
 				case 1:
 					return view == layout[0] ? 0 : -1;
 				default:
-					return layout.OrderByZIndex().IndexOf(view);
+					var found = false;
+					var zIndex = view.ZIndex;
+					var lowerViews = 0;
+
+					for (int i = 0; i < count; i++)
+					{
+						var child = layout[i];
+						var childZIndex = child.ZIndex;
+
+						if (child == view) {
+							found = true;
+						}
+
+						if (childZIndex < zIndex || !found && childZIndex == zIndex)
+						{
+							++lowerViews;
+						}
+					}
+
+					return found ? lowerViews : -1;
 			}
 		}
 	}

src/Core/src/Properties/AssemblyInfo.cs

@@ -25,6 +25,7 @@
 [assembly: InternalsVisibleTo("Microsoft.Maui.Controls.DeviceTests")]
 [assembly: InternalsVisibleTo("Microsoft.Maui.Controls.Core.UnitTests")]
 [assembly: InternalsVisibleTo("Microsoft.Maui.Benchmarks.Droid")]
+[assembly: InternalsVisibleTo("Core.Benchmarks")]
 [assembly: InternalsVisibleTo("Comet")]
 [assembly: InternalsVisibleTo("Comet.Tests")]
 [assembly: InternalsVisibleTo("CommunityToolkit.Maui")]

src/Core/tests/Benchmarks/Benchmarks/LayoutBenchmarker.cs

@@ -0,0 +1,31 @@
+using BenchmarkDotNet.Attributes;
+using Microsoft.Maui.Controls;
+using Microsoft.Maui.Handlers;
+
+namespace Microsoft.Maui.Benchmarks
+{
+	[MemoryDiagnoser]
+	public class LayoutBenchmarker
+	{
+		static readonly Border[] Views = new[]
+		{
+			new Border(), new Border(), new Border(), new Border(), new Border(), new Border(), new Border(),
+			new Border(), new Border(), new Border(), new Border(), new Border(), new Border(), new Border()
+		};
+
+		static readonly int Iterations = Views.Length;
+
+		[Benchmark]
+		public void GetLayoutHandlerIndex()
+		{
+			var layout = new VerticalStackLayout();
+
+			for (int i = 0; i < Iterations; i++)
+			{
+				var view = Views[i];
+				layout.Add(view);
+				layout.GetLayoutHandlerIndex(view);
+			}
+		}
+	}
+}

src/Core/tests/UnitTests/Layouts/ZIndexTests.cs

@@ -200,6 +200,41 @@ public void LayoutHandlerIndexFollowsAddOrderWhenZIndexesAreEqual()
 			Assert.Equal(2, layout.GetLayoutHandlerIndex(view2));
 			Assert.Equal(3, layout.GetLayoutHandlerIndex(view3));
 		}
+
+		[Fact]
+		public void LayoutHandlerIndexIsNegativeWhenChildIsNotFound()
+		{
+			var layout = new FakeLayout();
+			var view0 = CreateTestView(zIndex: 0);
+
+			Assert.Equal(-1, layout.GetLayoutHandlerIndex(view0));
+
+			layout.Add(CreateTestView(zIndex: 0));
+			Assert.Equal(-1, layout.GetLayoutHandlerIndex(view0));
+
+
+			layout.Add(CreateTestView(zIndex: 0));
+			Assert.Equal(-1, layout.GetLayoutHandlerIndex(view0));
+		}
+
+		[Fact]
+		public void LayoutHandlerIndexPreservesAddOrderForEqualZIndexes()
+		{
+			var layout = new FakeLayout();
+			var view0 = CreateTestView(zIndex: 10);
+			var view1 = CreateTestView(zIndex: 10);
+			var view2 = CreateTestView(zIndex: 10);
+			var view3 = CreateTestView(zIndex: 5);
+			layout.Add(view0);
+			layout.Add(view1);
+			layout.Add(view2);
+			layout.Add(view3);
+
+			Assert.Equal(1, layout.GetLayoutHandlerIndex(view0));
+			Assert.Equal(2, layout.GetLayoutHandlerIndex(view1));
+			Assert.Equal(3, layout.GetLayoutHandlerIndex(view2));
+			Assert.Equal(0, layout.GetLayoutHandlerIndex(view3));
+		}
 
 		[Fact]
 		public void ItemsOrderByZIndex()