Ordering children while adding

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue8129.cs

@@ -0,0 +1,81 @@
+using System.Linq;
+using System.Threading.Tasks;
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+using System;
+
+#if UITEST
+using Xamarin.Forms.Core.UITests;
+using Xamarin.UITest;
+using NUnit.Framework;
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Github, 8129, "[Bug] Adding children to iOS VisualElementPackager has O(N^2) performance and thrashes the native layer", PlatformAffected.iOS)]
+	public class Issue8129 : TestContentPage
+	{
+		protected override void Init()
+		{
+			Title = "Page with too many elements (2000) Tests";
+
+			var grid = new Grid();
+			var stackLayout = new StackLayout();
+
+			var addChildrenCommand = new Command((parameter) =>
+			{
+				Enumerable
+				.Range(0, 2000)
+				.Select(_ => new Label() { HeightRequest = 200, Text = $"I am Label #{_}" })
+				.ForEach(x => stackLayout.Children.Add(x));
+			});
+
+			var addChildrenButton = new Button
+			{
+				Text = "Add 2000 Labels",
+				Command = addChildrenCommand
+			};
+
+			var addZChildrenButton = new Button
+			{
+				Text = "Add BoxView on Top",
+				Command = new Command((parameter) =>
+				{
+					grid.AddChild(new BoxView
+					{
+						HeightRequest = 300,
+						WidthRequest = 300,
+						BackgroundColor = Color.Green
+					}, 0, 0, 1, 3);
+				})
+			};
+			grid.AddChild(addChildrenButton, 0, 0);
+			grid.AddChild(addZChildrenButton, 0, 1);
+			grid.AddChild(stackLayout, 0, 2);
+
+			Content = grid;
+		}
+#if UITEST
+		[Test]
+		public void AddTooManyContentsTest()
+		{
+			RunningApp.WaitForElement(q => q.Button("Add 2000 Labels"));
+			RunningApp.Screenshot("Before adding 2000 Labels");
+			RunningApp.Tap(q => q.Button("Add 2000 Labels"));
+			RunningApp.WaitForElement(q => q.Marked("I am Label #1"));
+			RunningApp.Screenshot("After adding 2000 Labels");
+		}
+
+		[Test]
+		public void ZIndexAfterAddingContentsTest()
+		{
+			RunningApp.WaitForElement(q => q.Button("Add BoxView on Top"));
+			RunningApp.Screenshot("Before adding BoxView on Top");
+			RunningApp.Tap(q => q.Button("Add BoxView on Top"));
+			RunningApp.WaitForNoElement(q => q.Button("Add BoxView on Top"));
+			RunningApp.Screenshot("After adding BoxView on Top");
+		}
+#endif
+	}
+}

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems

@@ -1761,6 +1761,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Issue13616.xaml.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue13670.xaml.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue13684.xaml.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Issue8129.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue12300.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue12150.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue12590.xaml.cs" />

Xamarin.Forms.Platform.iOS/VisualElementPackager.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using Xamarin.Forms.Internals;
 
 #if __MOBILE__
@@ -46,7 +47,10 @@ public void Load()
 			{
 				var child = ElementController.LogicalChildren[i] as VisualElement;
 				if (child != null)
+				{
 					OnChildAdded(child);
+					OrderElement(child, i);
+				}
 			}
 		}
 
@@ -120,8 +124,6 @@ protected virtual void OnChildAdded(VisualElement view)
 
 				if (Renderer.ViewController != null && viewRenderer.ViewController != null)
 					Renderer.ViewController.AddChildViewController(viewRenderer.ViewController);
-
-				EnsureChildrenOrder();
 			}
 
 			Performance.Stop(reference);
@@ -141,34 +143,59 @@ protected virtual void OnChildRemoved(VisualElement view)
 			viewRenderer.Dispose();
 		}
 
-		void EnsureChildrenOrder()
+		void EnsureChildrenOrder(VisualElement element = null)
 		{
 			if (ElementController.LogicalChildren.Count == 0)
 				return;
 
-			for (var z = 0; z < ElementController.LogicalChildren.Count; z++)
+			var effectedChildIndex = 0;
+			if (element != null)
+				effectedChildIndex = ElementController.LogicalChildren.IndexOf(element);
+
+			for (var z = effectedChildIndex; z < ElementController.LogicalChildren.Count; z++)
 			{
 				var child = ElementController.LogicalChildren[z] as VisualElement;
 				if (child == null)
 					continue;
-				var childRenderer = Platform.GetRenderer(child);
+				OrderElement(child, z);
+			}
+		}
 
-				if (childRenderer == null)
-					continue;
+		void OrderElement(VisualElement element, int order)
+		{
+			Performance.Start(out string reference);
+			if (CompressedLayout.GetIsHeadless(element))
+			{
+				Performance.Stop(reference);
 
-				var nativeControl = childRenderer.NativeView;
+				return;
+			}
+
+			var childRenderer = Platform.GetRenderer(element);
+
+			if (childRenderer == null)
+			{
+				Performance.Stop(reference);
+
+				return;
+			}
+
+			var nativeControl = childRenderer.NativeView;
 #if __MOBILE__
-				Renderer.NativeView.BringSubviewToFront(nativeControl);
+			Renderer.NativeView.BringSubviewToFront(nativeControl);
 #endif
-				nativeControl.Layer.ZPosition = z * 1000;
-			}
+			nativeControl.Layer.ZPosition = order * 1000;
+			Performance.Stop(reference);
 		}
 
 		void OnChildAdded(object sender, ElementEventArgs e)
 		{
 			var view = e.Element as VisualElement;
 			if (view != null)
+			{
 				OnChildAdded(view);
+				EnsureChildrenOrder(view);
+			}
 		}
 
 		void OnChildRemoved(object sender, ElementEventArgs e)