[iOS] Fix NavigationPage memory leak when back button title is set (#523)

* fix memory leak

* fix parent

* added comment

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Bugzilla44047.cs

@@ -0,0 +1,61 @@
+using System;
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+
+#if UITEST
+using Xamarin.UITest;
+using NUnit.Framework;
+#endif
+
+namespace Xamarin.Forms.Controls
+{
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Bugzilla, 44047, "Memory leak when using SetBackButtonTitle on iOS", PlatformAffected.iOS)]
+	public class Bugzilla44047 : TestMasterDetailPage
+	{
+		protected override void Init()
+		{
+			Master = new ContentPage
+			{
+				Title = "Menu"
+			};
+
+			Detail = new NavigationPage(new Page1());
+		}
+	}
+
+	public class Page1 : ContentPage
+	{
+		public Page1()
+		{
+			Title = "Page1";
+			Content = new Button
+			{
+				Text = "Open Page2",
+				Command = new Command(async o =>
+				{
+					await (Parent as NavigationPage).PushAsync(new Page2());
+				})
+			};
+		}
+	}
+
+	public class Page2 : ContentPage
+	{
+		public Page2()
+		{
+			GC.Collect();
+			GC.WaitForPendingFinalizers();
+			GC.Collect();
+
+			Title = "Page2";
+			System.Diagnostics.Debug.WriteLine("Constructor");
+			NavigationPage.SetBackButtonTitle(this, "Custom");
+		}
+
+		~Page2()
+		{
+			System.Diagnostics.Debug.WriteLine("Finalizer");
+		}
+	}
+}

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

@@ -143,6 +143,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla43313.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla43469.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla43516.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Bugzilla44047.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla43941.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla43663.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla43867.cs" />

Xamarin.Forms.Platform.iOS/Renderers/NavigationRenderer.cs

@@ -366,8 +366,8 @@ namespace Xamarin.Forms.Platform.iOS
 			var titleText = NavigationPage.GetBackButtonTitle(page);
 			if (titleText != null)
 			{
-				pack.NavigationItem.BackBarButtonItem = 
-					new UIBarButtonItem(titleText, UIBarButtonItemStyle.Plain, async (o, e) => await PopViewAsync(page));
+				// adding a custom event handler to UIBarButtonItem for navigating back seems to be ignored.
+				pack.NavigationItem.BackBarButtonItem = new UIBarButtonItem { Title = titleText, Style = UIBarButtonItemStyle.Plain };
 			}
 
 			var pageRenderer = Platform.GetRenderer(page);