[iOS/Critical] Fix ListView memory leaks (#524)

* fix memory leaks

added sample code

changed page name

Changed page title

add screen instructions

fix copy paste error

change subview dispose logic

Fix context action memory leak

add sample code

change custom page names

Revert "change custom page names"

This reverts commit 7aaab2625d9526080ca5c51c02e909f09ee3f610.

changed class names

changes

UI test for 32206

update ui test

fix whitespace

UITests done

* run UI tests on iOS only

* fix code style

* dispose prototype

* fix child renderer issue

* add null check

3 files changed, 134 insertions, 49 deletions

diff --git a/Xamarin.Forms.Platform.iOS/Cells/CellTableViewCell.cs b/Xamarin.Forms.Platform.iOS/Cells/CellTableViewCell.cs
index e32e86cd..b42f69f4 100644
--- a/Xamarin.Forms.Platform.iOS/Cells/CellTableViewCell.cs
+++ b/Xamarin.Forms.Platform.iOS/Cells/CellTableViewCell.cs
@@ -7,8 +7,8 @@ namespace Xamarin.Forms.Platform.iOS
 	public class CellTableViewCell : UITableViewCell, INativeElementView
 	{
 		Cell _cell;
-
 		public Action<object, PropertyChangedEventArgs> PropertyChanged;
+		bool _disposed;
 
 		public CellTableViewCell(UITableViewCellStyle style, string key) : base(style, key)
 		{
@@ -95,5 +95,21 @@ namespace Xamarin.Forms.Platform.iOS
 
 			return nativeCell;
 		}
+
+		protected override void Dispose(bool disposing)
+		{
+			if (_disposed)
+				return;
+
+			if (disposing)
+			{
+				PropertyChanged = null;
+				_cell = null;
+			}
+
+			_disposed = true;
+
+			base.Dispose(disposing);
+		}
 	}
 }
\ No newline at end of file
diff --git a/Xamarin.Forms.Platform.iOS/Cells/ViewCellRenderer.cs b/Xamarin.Forms.Platform.iOS/Cells/ViewCellRenderer.cs
index 8f8a92ce..30c17234 100644
--- a/Xamarin.Forms.Platform.iOS/Cells/ViewCellRenderer.cs
+++ b/Xamarin.Forms.Platform.iOS/Cells/ViewCellRenderer.cs
@@ -48,9 +48,12 @@ namespace Xamarin.Forms.Platform.iOS
 		internal class ViewTableCell : UITableViewCell, INativeElementView
 		{
 			WeakReference<IVisualElementRenderer> _rendererRef;
-
 			ViewCell _viewCell;
 
+			Element INativeElementView.Element => ViewCell;
+			internal bool SupressSeparator { get; set; }
+			bool _disposed;
+
 			public ViewTableCell(string key) : base(UITableViewCellStyle.Default, key)
 			{
 			}
@@ -66,10 +69,6 @@ namespace Xamarin.Forms.Platform.iOS
 				}
 			}
 
-			Element INativeElementView.Element => ViewCell;
-
-			internal bool SupressSeparator { get; set; }
-
 			public override void LayoutSubviews()
 			{
 				//This sets the content views frame.
@@ -115,6 +114,9 @@ namespace Xamarin.Forms.Platform.iOS
 
 			protected override void Dispose(bool disposing)
 			{
+				if (_disposed)
+					return;
+
 				if (disposing)
 				{
 					IVisualElementRenderer renderer;
@@ -126,8 +128,12 @@ namespace Xamarin.Forms.Platform.iOS
 
 						_rendererRef = null;
 					}
+
+					_viewCell = null;
 				}
 
+				_disposed = true;
+
 				base.Dispose(disposing);
 			}
 
diff --git a/Xamarin.Forms.Platform.iOS/Renderers/ListViewRenderer.cs b/Xamarin.Forms.Platform.iOS/Renderers/ListViewRenderer.cs
index 3c235806..fd0f15f6 100644
--- a/Xamarin.Forms.Platform.iOS/Renderers/ListViewRenderer.cs
+++ b/Xamarin.Forms.Platform.iOS/Renderers/ListViewRenderer.cs
@@ -29,6 +29,7 @@ namespace Xamarin.Forms.Platform.iOS
 		IListViewController Controller => Element;
 		ITemplatedItemsView<Cell> TemplatedItemsView => Element;
 		public override UIViewController ViewController => _tableViewController;
+		bool _disposed;
 
 		public override SizeRequest GetDesiredSize(double widthConstraint, double heightConstraint)
 		{
@@ -90,28 +91,31 @@ namespace Xamarin.Forms.Platform.iOS
 			}
 		}
 
+		void DisposeSubviews(UIView view)
+		{
+			foreach (UIView subView in view.Subviews)
+				DisposeSubviews(subView);
+
+			view.RemoveFromSuperview();
+			view.Dispose();
+		}
+
 		protected override void Dispose(bool disposing)
 		{
-			// check inset tracker for null to 
-			if (disposing && _insetTracker != null)
-			{
-				_insetTracker.Dispose();
-				_insetTracker = null;
+			if (_disposed)
+				return;
 
-				var viewsToLookAt = new Stack<UIView>(Subviews);
-				while (viewsToLookAt.Count > 0)
+			if (disposing)
+			{
+				if (_insetTracker != null)
 				{
-					var view = viewsToLookAt.Pop();
-					var viewCellRenderer = view as ViewCellRenderer.ViewTableCell;
-					if (viewCellRenderer != null)
-						viewCellRenderer.Dispose();
-					else
-					{
-						foreach (var child in view.Subviews)
-							viewsToLookAt.Push(child);
-					}
+					_insetTracker.Dispose();
+					_insetTracker = null;
 				}
 
+				foreach (UIView subview in Subviews)
+					DisposeSubviews(subview);
+
 				if (Element != null)
 				{
 					var templatedItems = TemplatedItemsView.TemplatedItems;
@@ -119,27 +123,28 @@ namespace Xamarin.Forms.Platform.iOS
 					templatedItems.GroupedCollectionChanged -= OnGroupedCollectionChanged;
 				}
 
+				if (_dataSource != null)
+				{
+					_dataSource.Dispose();
+					_dataSource = null;
+				}
+
 				if (_tableViewController != null)
 				{
 					_tableViewController.Dispose();
 					_tableViewController = null;
 				}
-			}
 
-			if (disposing)
-			{
 				if (_headerRenderer != null)
 				{
-					var platform = _headerRenderer.Element.Platform as Platform;
-					if (platform != null)
-						platform.DisposeModelAndChildrenRenderers(_headerRenderer.Element);
+					var platform = _headerRenderer.Element?.Platform as Platform;
+					platform?.DisposeModelAndChildrenRenderers(_headerRenderer.Element);
 					_headerRenderer = null;
 				}
 				if (_footerRenderer != null)
 				{
-					var platform = _footerRenderer.Element.Platform as Platform;
-					if (platform != null)
-						platform.DisposeModelAndChildrenRenderers(_footerRenderer.Element);
+					var platform = _footerRenderer.Element?.Platform as Platform;
+					platform?.DisposeModelAndChildrenRenderers(_footerRenderer.Element);
 					_footerRenderer = null;
 				}
 
@@ -154,6 +159,8 @@ namespace Xamarin.Forms.Platform.iOS
 				Control?.TableFooterView?.Dispose();
 			}
 
+			_disposed = true;
+
 			base.Dispose(disposing);
 		}
 
@@ -602,6 +609,7 @@ namespace Xamarin.Forms.Platform.iOS
 		internal class UnevenListViewDataSource : ListViewDataSource
 		{
 			IVisualElementRenderer _prototype;
+			bool _disposed;
 
 			public UnevenListViewDataSource(ListView list, FormsUITableViewController uiTableViewController) : base(list, uiTableViewController)
 			{
@@ -661,27 +669,47 @@ namespace Xamarin.Forms.Platform.iOS
 				{
 					var target = viewCell.View;
 					if (_prototype == null)
-					{
 						_prototype = Platform.CreateRenderer(target);
-						Platform.SetRenderer(target, _prototype);
-					}
 					else
-					{
 						_prototype.SetElement(target);
-						Platform.SetRenderer(target, _prototype);
-					}
+
+					Platform.SetRenderer(target, _prototype);
 
 					var req = target.Measure(tableView.Frame.Width, double.PositiveInfinity, MeasureFlags.IncludeMargins);
 
 					target.ClearValue(Platform.RendererProperty);
-					foreach (var descendant in target.Descendants())
+					foreach (Element descendant in target.Descendants())
+					{
+						IVisualElementRenderer renderer = Platform.GetRenderer(descendant as VisualElement);
 						descendant.ClearValue(Platform.RendererProperty);
+						renderer?.Dispose();
+					}
 
 					return (nfloat)req.Request.Height;
 				}
+
 				var renderHeight = cell.RenderHeight;
 				return renderHeight > 0 ? (nfloat)renderHeight : DefaultRowHeight;
 			}
+
+			protected override void Dispose(bool disposing)
+			{
+				if (_disposed)
+					return;
+
+				_disposed = true;
+
+				if (disposing)
+				{
+					if (_prototype != null)
+					{
+						_prototype.Dispose();
+						_prototype = null;
+					}
+				}
+
+				base.Dispose(disposing);
+			}
 		}
 
 		internal class ListViewDataSource : UITableViewSource
@@ -689,14 +717,15 @@ namespace Xamarin.Forms.Platform.iOS
 			const int DefaultItemTemplateId = 1;
 			static int s_dataTemplateIncrementer = 2; // lets start at not 0 because
 			readonly nfloat _defaultSectionHeight;
-			readonly Dictionary<DataTemplate, int> _templateToId = new Dictionary<DataTemplate, int>();
-			readonly UITableView _uiTableView;
-			readonly FormsUITableViewController _uiTableViewController;
-			protected readonly ListView List;
+			Dictionary<DataTemplate, int> _templateToId = new Dictionary<DataTemplate, int>();
+			UITableView _uiTableView;
+			FormsUITableViewController _uiTableViewController;
+			protected ListView List;
 			IListViewController Controller => List;
 			protected ITemplatedItemsView<Cell> TemplatedItemsView => List;
 			bool _isDragging;
 			bool _selectionFromNative;
+			bool _disposed;
 
 			public ListViewDataSource(ListViewDataSource source)
 			{
@@ -1016,6 +1045,29 @@ namespace Xamarin.Forms.Platform.iOS
 						((INotifyCollectionChanged)templatedList.ShortNames).CollectionChanged -= OnShortNamesCollectionChanged;
 				}
 			}
+
+			protected override void Dispose(bool disposing)
+			{
+				if (_disposed)
+					return;
+
+				if (disposing)
+				{
+					if (List != null)
+					{
+						List.ItemSelected -= OnItemSelected;
+						List = null;
+					}
+
+					_templateToId = null;
+					_uiTableView = null;
+					_uiTableViewController = null;
+				}
+
+				_disposed = true;
+
+				base.Dispose(disposing);
+			}
 		}
 	}
 
@@ -1031,11 +1083,12 @@ namespace Xamarin.Forms.Platform.iOS
 
 	internal class FormsUITableViewController : UITableViewController
 	{
-		readonly ListView _list;
+		ListView _list;
 		IListViewController Controller => _list;
 		UIRefreshControl _refresh;
 
 		bool _refreshAdded;
+		bool _disposed;
 
 		public FormsUITableViewController(ListView element)
 		{
@@ -1117,15 +1170,25 @@ namespace Xamarin.Forms.Platform.iOS
 
 		protected override void Dispose(bool disposing)
 		{
-			base.Dispose(disposing);
+			if (_disposed)
+				return;
 
-			if (disposing && _refresh != null)
+			if (disposing)
 			{
-				_refresh.ValueChanged -= OnRefreshingChanged;
-				_refresh.EndRefreshing();
-				_refresh.Dispose();
-				_refresh = null;
+				if (_refresh != null)
+				{
+					_refresh.ValueChanged -= OnRefreshingChanged;
+					_refresh.EndRefreshing();
+					_refresh.Dispose();
+					_refresh = null;
+				}
+
+				_list = null;
 			}
+
+			_disposed = true;
+
+			base.Dispose(disposing);
 		}
 
 		void CheckContentSize()