[iOS] Protect against SetNativeControl when no Element is present (#6415) fixes #6368 fixes #6328

* [Controls] Add repo for issue #6368 and #6328

* [iOS] Protect against no Element from CustomRenderers bad usage

* [Controls] Drop private on sample code

Xamarin.Forms.ControlGallery.iOS/CustomRenderers/CustomRenderer.cs

@@ -0,0 +1,41 @@
+using System;
+using UIKit;
+using Xamarin.Forms;
+using Xamarin.Forms.ControlGallery.iOS;
+using Xamarin.Forms.Platform.iOS;
+using static Xamarin.Forms.Controls.Issues.Issue6368;
+
+[assembly: ExportRenderer(typeof(CustomView), typeof(CustomRenderer))]
+namespace Xamarin.Forms.ControlGallery.iOS
+{
+	public class CustomRenderer : ViewRenderer<CustomView, UIView>
+	{
+		protected override void OnElementChanged(ElementChangedEventArgs<CustomView> e)
+		{
+			base.OnElementChanged(e);
+
+			//
+			// --- Important --- //
+			//
+			// This is a WRONG Pattern!
+			//Pattern taken from: https://docs.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/custom-renderer/view
+			if (this.Control == null)
+			{
+				// Instantiate the native control and assign it to the Control property with
+				// the SetNativeControl method
+				UIView myView = new UIView();
+				this.SetNativeControl(myView);
+			}
+
+			if (e.OldElement != null)
+			{
+				// Unsubscribe from event handlers and cleanup any resources
+			}
+
+			if (e.NewElement != null)
+			{
+				// Configure the control and subscribe to event handlers
+			}
+		}
+	}
+}

Xamarin.Forms.ControlGallery.iOS/CustomRenderers/RoundedLabelRenderer.cs

@@ -0,0 +1,45 @@
+using System;
+using UIKit;
+using Xamarin.Forms;
+using Xamarin.Forms.ControlGallery.iOS.CustomRenderers;
+using Xamarin.Forms.Platform.iOS;
+using static Xamarin.Forms.Controls.Issues.Issue6368;
+
+[assembly: ExportRenderer(typeof(RoundedLabel), typeof(RoundedLabelRenderer))]
+namespace Xamarin.Forms.ControlGallery.iOS.CustomRenderers
+{
+	public class RoundedLabelRenderer : LabelRenderer
+	{
+		protected override void OnElementChanged(ElementChangedEventArgs<Label> e)
+		{
+			if (Control == null)
+			{
+				SetNativeControl(new TagUiLabel());
+			}
+			base.OnElementChanged(e);
+			if (e.NewElement != null)
+			{
+				this.Layer.CornerRadius = 10;
+				this.Layer.BorderColor = ColorExtensions.ToCGColor(ColorExtensions.ToColor(UIColor.FromRGB(3, 169, 244)));
+				this.Layer.BackgroundColor = ColorExtensions.ToCGColor(Color.GhostWhite);
+				this.Layer.BorderWidth = 1;
+			}
+		}
+	}
+
+	class TagUiLabel : UILabel
+	{
+		UIEdgeInsets _edgeInsets = new UIEdgeInsets(10, 5, 10, 5);
+		UIEdgeInsets _inverseEdgeInsets = new UIEdgeInsets(-10, -5, -10, -5);
+
+		public override CoreGraphics.CGRect TextRectForBounds(CoreGraphics.CGRect bounds, nint numberOfLines)
+		{
+			var textRect = base.TextRectForBounds(_edgeInsets.InsetRect(bounds), numberOfLines);
+			return _inverseEdgeInsets.InsetRect(textRect);
+		}
+		public override void DrawText(CoreGraphics.CGRect rect)
+		{
+			base.DrawText(_edgeInsets.InsetRect(rect));
+		}
+	}
+}

Xamarin.Forms.ControlGallery.iOS/Xamarin.Forms.ControlGallery.iOS.csproj

@@ -117,6 +117,8 @@
     <Compile Include="CustomRenderers.cs" />
     <Compile Include="CustomRendererBugzila38731.cs" />
     <Compile Include="CustomApplication.cs" />
+    <Compile Include="CustomRenderers\CustomRenderer.cs" />
+    <Compile Include="CustomRenderers\RoundedLabelRenderer.cs" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\Xamarin.Forms.Controls\Xamarin.Forms.Controls.csproj">
@@ -386,6 +388,7 @@
   </ItemGroup>
   <ItemGroup>
     <Folder Include="Resources\Fonts\" />
+    <Folder Include="CustomRenderers\" />
   </ItemGroup>
   <Import Project="$(MSBuildExtensionsPath)\Xamarin\iOS\Xamarin.iOS.CSharp.targets" />
 </Project>

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue6368.cs

@@ -0,0 +1,67 @@
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+
+#if UITEST
+using Xamarin.Forms.Core.UITests;
+using Xamarin.UITest;
+using NUnit.Framework;
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+
+#if UITEST
+	[Category(UITestCategories.CustomRenderers)]
+#endif
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Github, 6368, "[CustomRenderer]Crash when navigating back from page with custom renderer control", PlatformAffected.iOS)]
+	public class Issue6368 : TestNavigationPage 
+	{
+		public class CustomView : View
+		{
+		}
+
+		public class RoundedLabel : Label
+		{
+		}
+
+		protected override void Init()
+		{
+			var rootPage = new ContentPage();
+			var button = new Button()
+			{
+				AutomationId = "btnGo",
+				Text = "Click me to go to the next page",
+				Command = new Command(() => PushAsync(new ContentPage()
+				{
+					Content = GetContent()
+				}))
+			};
+			var content = GetContent();
+			content.Children.Add(button);
+			rootPage.Content = content;
+			PushAsync(rootPage);
+		}
+
+		static StackLayout GetContent()
+		{
+			var content2 = new StackLayout();
+			content2.Children.Add(new RoundedLabel { Text = "Go to next Page" });
+			content2.Children.Add(new RoundedLabel { Text = "then navigate back" });
+			content2.Children.Add(new RoundedLabel { Text = "If test doesn't crash it passed" });
+			content2.Children.Add(new CustomView());
+			return content2;
+		}
+
+#if UITEST && __IOS__
+		[Test]
+		public void Issue6368Test() 
+		{
+			RunningApp.WaitForElement (q => q.Marked ("btnGo"));
+			RunningApp.Tap(q => q.Marked("btnGo"));
+			RunningApp.WaitForElement(q => q.Marked("Go to next Page"));
+			RunningApp.NavigateBack();
+		}
+#endif
+	}
+}

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

@@ -897,6 +897,7 @@
       <DependentUpon>Issue4356.xaml</DependentUpon>
     </Compile>
     <Compile Include="$(MSBuildThisFileDirectory)Issue5888.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Issue6368.cs" />
   </ItemGroup>
   <ItemGroup>
     <EmbeddedResource Include="$(MSBuildThisFileDirectory)Bugzilla22229.xaml">

Xamarin.Forms.Core.UITests.Shared/UITestCategories.cs

@@ -47,5 +47,7 @@
 		public const string Performance = "Performance";
 		public const string Visual = "Visual";
 		public const string AppLinks = "AppLinks";
+		public const string Shell = "Shell";
+		public const string CustomRenderers = "CustomRenderers";
 	}
 }

Xamarin.Forms.Platform.iOS/ViewRenderer.cs

@@ -40,7 +40,7 @@ namespace Xamarin.Forms.Platform.MacOS
 		event EventHandler _controlChanging;
 		event EventHandler _controlChanged;
 
-
+		bool IsElementOrControlEmpty => Element == null || Control == null;
 
 		protected virtual TNativeView CreateNativeControl()
 		{
@@ -188,7 +188,7 @@ namespace Xamarin.Forms.Platform.MacOS
 
 		protected override void SetBackgroundColor(Color color)
 		{
-			if (Control == null)
+			if (IsElementOrControlEmpty)
 				return;
 #if __MOBILE__
 			if (color == Color.Default)
@@ -217,8 +217,7 @@ namespace Xamarin.Forms.Platform.MacOS
 #endif
 			Control = uiview;
 
-			if (Element.BackgroundColor != Color.Default)
-				SetBackgroundColor(Element.BackgroundColor);
+			UpdateBackgroundColor();
 
 			UpdateIsEnabled();
 
@@ -236,9 +235,18 @@ namespace Xamarin.Forms.Platform.MacOS
 		}
 #endif
 
+		void UpdateBackgroundColor()
+		{
+			if (IsElementOrControlEmpty)
+				return;
+
+			if (Element.BackgroundColor != Color.Default)
+				SetBackgroundColor(Element.BackgroundColor);
+		}
+
 		void UpdateIsEnabled()
 		{
-			if (Element == null || Control == null)
+			if (IsElementOrControlEmpty)
 				return;
 
 			var uiControl = Control as NativeControl;
@@ -249,6 +257,9 @@ namespace Xamarin.Forms.Platform.MacOS
 
 		void UpdateFlowDirection()
 		{
+			if (IsElementOrControlEmpty)
+				return;
+
 			Control.UpdateFlowDirection(Element);
 		}
 
@@ -260,4 +271,4 @@ namespace Xamarin.Forms.Platform.MacOS
 			focusRequestArgs.Result = focusRequestArgs.Focus ? Control.BecomeFirstResponder() : Control.ResignFirstResponder();
 		}
 	}
-}
+}