Fixes memory leak from grouped ListView with HasUnevenRows set (#12447)

fixes #6742 fixes #10820

* Fixes #6742
The Item created to measure was never removed, causing memory leak until the page is closed.

* cleanup

* cleanup, remove UITEST code

* remove commented code

* remove unused variable

* Fix bad merge of project file.

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue6742.xaml

@@ -0,0 +1,31 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<controls:TestContentPage xmlns:controls="clr-namespace:Xamarin.Forms.Controls"
+             xmlns="http://xamarin.com/schemas/2014/forms"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             xmlns:d="http://xamarin.com/schemas/2014/forms/design"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" mc:Ignorable="d"
+             x:Class="Xamarin.Forms.Controls.Issues.Issue6742">
+    <StackLayout>
+        <Button AutomationId="Issue6742Refresh" Clicked="RefreshItems" Text="Refresh" BackgroundColor="Cyan"/>
+
+        <Label Text="Tap Refresh button a few times." />
+        <Label AutomationId="Issue6742Label" Text="{Binding DisposeString}"/>
+        <Label AutomationId="Issue6742Mem" Text="{Binding TotalMemory}"/>
+
+        <ListView
+            HasUnevenRows="True"
+            ItemsSource="{Binding ItemGroups}"
+            IsGroupingEnabled="True"
+            GroupDisplayBinding="{Binding GroupName}">
+            <ListView.ItemTemplate>
+                <DataTemplate>
+                    <ViewCell>
+                        <StackLayout Margin="20">
+                            <Label Text="{Binding Name}" />
+                        </StackLayout>
+                    </ViewCell>
+                </DataTemplate>
+            </ListView.ItemTemplate>
+        </ListView>
+    </StackLayout>
+</controls:TestContentPage>

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue6742.xaml.cs

@@ -0,0 +1,159 @@
+using System;
+using System.Collections.ObjectModel;
+using System.Runtime.CompilerServices;
+using System.ComponentModel;
+using System.Collections.Generic;
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+
+namespace Xamarin.Forms.Controls.Issues
+{
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Github, 6742, "Memory leak in ListView with uneven rows on IOS", PlatformAffected.Default)]
+	public partial class Issue6742 : TestContentPage
+	{
+		public Issue6742()
+		{
+#if APP
+			InitializeComponent();
+#endif
+			BindingContext = viewModel = new ViewModelIssue6742();
+
+		}
+		protected override void Init()
+		{
+
+		}
+		ViewModelIssue6742 viewModel;
+		IList<WeakReference<ModelIssue6742>> _weakList = new List<WeakReference<ModelIssue6742>>();
+		private void RefreshItems(object sender, EventArgs e)
+		{
+			foreach (var item in this.viewModel.ItemGroups)
+			{
+				item.Clear();
+			}
+			viewModel.ItemGroups.Clear();
+
+			GC.Collect();
+			GC.WaitForPendingFinalizers();
+			GC.Collect();
+
+			CleanWeakList(_weakList);
+			viewModel.DisposeString = $"{_weakList.Count} object(s) alive";
+
+			string report = $"MEMORY:{GC.GetTotalMemory(true)}";
+			viewModel.TotalMemory = report;
+
+			var g1 = new ModelIssue6742Group("Group 1")
+			{
+				new ModelIssue6742("first", viewModel),
+				new ModelIssue6742("second", viewModel),
+			};
+
+			_weakList.Add(new WeakReference<ModelIssue6742>(g1[0]));
+			_weakList.Add(new WeakReference<ModelIssue6742>(g1[1]));
+
+			viewModel.ItemGroups.Add(g1);
+
+			var g2 = new ModelIssue6742Group("Group 2")
+			{
+				new ModelIssue6742("third", viewModel),
+				new ModelIssue6742("fourth", viewModel),
+			};
+
+			_weakList.Add(new WeakReference<ModelIssue6742>(g2[0]));
+			_weakList.Add(new WeakReference<ModelIssue6742>(g2[1]));
+
+			viewModel.ItemGroups.Add(g2);
+
+			var g3 = new ModelIssue6742Group("Group 3")
+			{
+				new ModelIssue6742("fifth", viewModel),
+				new ModelIssue6742("sixth", viewModel),
+				new ModelIssue6742("seventh", viewModel),
+			};
+
+			_weakList.Add(new WeakReference<ModelIssue6742>(g3[0]));
+			_weakList.Add(new WeakReference<ModelIssue6742>(g3[1]));
+			_weakList.Add(new WeakReference<ModelIssue6742>(g3[2]));
+
+			viewModel.ItemGroups.Add(g3);
+		}
+
+		private void CleanWeakList(IList<WeakReference<ModelIssue6742>> weakList)
+		{
+			ModelIssue6742 item;
+			for (int i = weakList.Count-1; i >= 0; i--)
+			{
+				if (!weakList[i].TryGetTarget(out item))
+				{
+					weakList.RemoveAt(i);
+				}
+			}
+		}
+	}
+
+	[Preserve(AllMembers = true)]
+	public class ViewModelIssue6742 : INotifyPropertyChanged
+	{
+		public ObservableCollection<ModelIssue6742Group> ItemGroups { get; set; } = new ObservableCollection<ModelIssue6742Group>();
+		public event System.ComponentModel.PropertyChangedEventHandler PropertyChanged;
+
+		private void NotifyPropertyChanged([CallerMemberName] string propertyName = null)
+		{
+			PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+		}
+
+
+		string _DisposeString;
+		public string DisposeString
+		{
+			get { return _DisposeString; } 
+			set
+			{
+				_DisposeString = value;
+				NotifyPropertyChanged();
+			}
+		}
+
+		string _totalMemory;
+		public string TotalMemory 
+		{
+			get => _totalMemory;
+			set
+			{
+				_totalMemory = value;
+				NotifyPropertyChanged();
+			}
+		}
+
+		public ViewModelIssue6742()
+		{
+
+		}
+	}
+
+	[Preserve(AllMembers = true)]
+	public class ModelIssue6742Group : ObservableCollection<ModelIssue6742>
+	{
+		public ModelIssue6742Group(string name)
+		{
+			GroupName = name;
+		}
+		public string GroupName { get; set; }
+
+	}
+
+	[Preserve(AllMembers = true)]
+	public class ModelIssue6742
+	{
+		ViewModelIssue6742 _model;
+
+		public ModelIssue6742(string name, ViewModelIssue6742 model)
+		{
+			_model = model;
+			Name = name;
+		}
+		public string Name { get; set; }
+	}
+}

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

@@ -65,6 +65,9 @@
       <DependentUpon>Issue11794.xaml</DependentUpon>
     </Compile>
     <Compile Include="$(MSBuildThisFileDirectory)Issue11769.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Issue6742.xaml.cs">
+      <DependentUpon>Issue6742.xaml</DependentUpon>
+    </Compile>
     <Compile Include="$(MSBuildThisFileDirectory)Issue12079.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue12060.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue12344.xaml.cs">
@@ -2670,6 +2673,10 @@
     </EmbeddedResource>
   </ItemGroup>
   <ItemGroup>
+    <EmbeddedResource Include="$(MSBuildThisFileDirectory)Issue6742.xaml">
+        <SubType>Designer</SubType>
+        <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
+    </EmbeddedResource>
     <EmbeddedResource Include="$(MSBuildThisFileDirectory)Issue10897.xaml">
       <SubType>Designer</SubType>
       <Generator>MSBuild:UpdateDesignTimeXaml</Generator>

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

@@ -858,6 +858,7 @@ nfloat GetEstimatedRowHeight(UITableView table)
 
 				var firstCell = templatedItems.ActivateContent(0, item);
 
+				nfloat returnValue;
 				// Let's skip this optimization for grouped lists. It will likely cause more trouble than it's worth.
 				if (firstCell?.Height > 0 && !isGroupingEnabled)
 				{
@@ -867,10 +868,15 @@ nfloat GetEstimatedRowHeight(UITableView table)
 					// In this case, we will cache the specified cell heights asynchronously, which will be returned one time on
 					// table load by EstimatedHeight.
 
-					return 0;
+					returnValue= 0;
+				}
+				else
+				{
+					returnValue = CalculateHeightForCell(table, firstCell);
 				}
 
-				return CalculateHeightForCell(table, firstCell);
+				TemplatedItemsView.UnhookContent(firstCell);
+				return returnValue;
 			}
 
 			internal override void InvalidatingPrototypicalCellCache()