[layout] avoid IView[] creation in LayoutExtensions

Context: https://github.com/unoplatform/performance/tree/master/src/dopes/DopeTestMaui

Reviewing `dotnet trace` output of the "dopes" test sample, 7% of CPU
time is spent in `LayoutExtensions.OrderByZIndex()`:

    3.42s (16%)  microsoft.maui!Microsoft.Maui.Handlers.LayoutHandler.Add(Microsoft.Maui.IView)
    1.52s (7.3%) microsoft.maui!Microsoft.Maui.Handlers.LayoutExtensions.GetLayoutHandlerIndex(Microsoft.Maui.ILayout,Microsoft.Maui.IVie...
    1.50s (7.2%) microsoft.maui!Microsoft.Maui.Handlers.LayoutExtensions.OrderByZIndex(Microsoft.Maui.ILayout)

Reviewing the code in `OrderByZIndex()`:

* Makes a `record ViewAndIndex(IView View, int Index)`
  for each child
* Makes a `ViewAndIndex[]` the size of the number of children
* Call `Array.Sort()`
* Makes a `IView[]` the size of the number of children
* Iterating twice over the arrays in the process

Then if you look at the `GetLayoutHandlerIndex()` method, it does all
the same work to create an `IView[]`, get the index, then throws the
array away.

I removed the above code and used a System.Linq `OrderBy()` with
faster paths for 0 or 1 children. I also made an `internal`
`EnumerateByZIndex()` method for use by `foreach` loops. This avoids
creating arrays in those cases. The `ZIndexTests` still pass for me,
which proves out `OrderBy()`'s stable sort:

https://docs.microsoft.com/dotnet/api/system.linq.enumerable.orderby

After this change, the % time spent in these methods went down:

      1.84s (13%)    microsoft.maui!Microsoft.Maui.Handlers.LayoutHandler.Add(Microsoft.Maui.IView)
    352.24ms (2.50%) microsoft.maui!Microsoft.Maui.Handlers.LayoutExtensions.GetLayoutHandlerIndex(Microsoft.Maui.ILayout,Microsoft.Maui.IVie...
    181.27ms (1.30%) microsoft.maui!Microsoft.Maui.Handlers.LayoutExtensions.<>c.<EnumerateByZIndex>b__0_0(Microsoft.Maui.IView)
      2.78ms (0.02%) microsoft.maui!Microsoft.Maui.Handlers.LayoutExtensions.EnumerateByZIndex(Microsoft.Maui.ILayout)

This kind of goes against the guidance "avoid System.Linq".
But `OrderBy()` generally seems to be fine?

~~ Results ~~

A `Release` build on a Pixel 5 device, I was getting:

    Before:
    103.04 Dopes/s
    After:
    230.87 Dopes/s

I suspect that was a lot of `IView[]`'s before!

Author: jonathanpeppers

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

@@ -1,59 +1,39 @@
 using System;
-using System.Collections.Generic;
+using System.Linq;
 
 namespace Microsoft.Maui.Handlers
 {
 	internal static class LayoutExtensions
 	{
-		record ViewAndIndex(IView View, int Index);
-
-		class ZIndexComparer : IComparer<ViewAndIndex>
-		{
-			public int Compare(ViewAndIndex? x, ViewAndIndex? y)
-			{
-				if (x == null || y == null)
-				{
-					return 0;
-				}
-
-				var zIndexCompare = x.View.ZIndex.CompareTo(y.View.ZIndex);
-
-				if (zIndexCompare == 0)
-				{
-					return x.Index.CompareTo(y.Index);
-				}
-
-				return zIndexCompare;
-			}
-		}
-
-		static ZIndexComparer s_comparer = new();
+		// <summary>
+		// Use this overload if you want to `foreach` over it
+		// </summary>
+		internal static IOrderedEnumerable<IView> EnumerateByZIndex(this ILayout layout) => layout.OrderBy(v => v.ZIndex);
 
 		public static IView[] OrderByZIndex(this ILayout layout)
 		{
-			var count = layout.Count;
-			var indexedViews = new ViewAndIndex[count];
-
-			for (int n = 0; n < count; n++)
+			switch (layout.Count)
 			{
-				indexedViews[n] = new ViewAndIndex(layout[n], n);
+				case 0:
+					return Array.Empty<IView>();
+				case 1:
+					return new[] { layout[0] };
+				default:
+					return layout.EnumerateByZIndex().ToArray();
 			}
-
-			Array.Sort(indexedViews, s_comparer);
-
-			var ordered = new IView[count];
-
-			for (int n = 0; n < count; n++)
-			{
-				ordered[n] = indexedViews[n].View;
-			}
-
-			return ordered;
 		}
 
 		public static int GetLayoutHandlerIndex(this ILayout layout, IView view)
 		{
-			return layout.OrderByZIndex().IndexOf(view);
+			switch (layout.Count)
+			{
+				case 0:
+					return -1;
+				case 1:
+					return view == layout[0] ? 0 : -1;
+				default:
+					return layout.EnumerateByZIndex().IndexOf(view);
+			}
 		}
 	}
 }

src/Core/src/Handlers/Layout/LayoutHandler.Android.cs

@@ -38,7 +38,7 @@ public override void SetVirtualView(IView view)
 
 			PlatformView.RemoveAllViews();
 
-			foreach (var child in VirtualView.OrderByZIndex())
+			foreach (var child in VirtualView.EnumerateByZIndex())
 			{
 				PlatformView.AddView(child.ToPlatform(MauiContext));
 			}

src/Core/src/Handlers/Layout/LayoutHandler.Tizen.cs

@@ -45,7 +45,7 @@ public override void SetVirtualView(IView view)
 
 			PlatformView.Children.Clear();
 
-			foreach (var child in VirtualView.OrderByZIndex())
+			foreach (var child in VirtualView.EnumerateByZIndex())
 			{
 				PlatformView.Children.Add(child.ToPlatform(MauiContext));
 				if (child.Handler is IPlatformViewHandler thandler)

src/Core/src/Handlers/Layout/LayoutHandler.Windows.cs

@@ -29,7 +29,7 @@ public override void SetVirtualView(IView view)
 
 			PlatformView.Children.Clear();
 
-			foreach (var child in VirtualView.OrderByZIndex())
+			foreach (var child in VirtualView.EnumerateByZIndex())
 			{
 				PlatformView.Children.Add(child.ToPlatform(MauiContext));
 			}

src/Core/src/Handlers/Layout/LayoutHandler.iOS.cs

@@ -38,7 +38,7 @@ public override void SetVirtualView(IView view)
 			// Remove any previous children 
 			PlatformView.ClearSubviews();
 
-			foreach (var child in VirtualView.OrderByZIndex())
+			foreach (var child in VirtualView.EnumerateByZIndex())
 			{
 				PlatformView.AddSubview(child.ToPlatform(MauiContext));
 			}