[controls] fix memory leak in CollectionView (#14329)

Fixes: #10578
Context: https://github.com/Vroomer/MAUI-navigation-memory-leak.git

After testing the above sample, I found that adding a `CollectionView`
to a `Page`, makes it and the entire page live forever.

Android & Windows:

* `MauiRecyclerView` and `StructuredItemsViewHandler` respectively
  subscribed to `ItemsLayout.PropertyChanged`. This kept the
  `CollectionView` alive -> all the way up to the `Page`.

* Switched to using `WeakNotifyPropertyChangedProxy` solved the issue
  for these two platforms.

iOS:

* Generally had a small "nest" of circular references. Initially, I saw
  `CollectionView`, `UICollectionView`, and various helper classes that
  would live forever.

* `ItemsViewController : UICollectionViewController` ->
  * `ObservableItemsSource` ->
    * `UICollectionViewController` and `UICollectionView`

* `UICollectionView` ->
  * `ItemsViewLayout : UICollectionViewFlowLayout` ->
    * `Func<UICollectionViewCell>` ->
      * `ItemsViewController : UICollectionViewController` ->
        * `UICollectionView`

I switched to using `WeakReference<T>` to break the circular references.
This required several null checks, where `null` references were not
possible before.

After these changes my tests pass, yay!

src/Controls/src/Core/Handlers/Items/Android/MauiRecyclerView.cs

@@ -42,6 +42,10 @@ public class MauiRecyclerView<TItemsView, TAdapter, TItemsViewSource> : Recycler
 
  ItemTouchHelper _itemTouchHelper;
  SimpleItemTouchHelperCallback _itemTouchHelperCallback;
+ WeakNotifyPropertyChangedProxy _layoutPropertyChangedProxy;
+ PropertyChangedEventHandler _layoutPropertyChanged;
+
+ ~MauiRecyclerView() => _layoutPropertyChangedProxy?.Unsubscribe();
 
  public MauiRecyclerView(Context context, Func<IItemsLayout> getItemsLayout, Func<TAdapter> getAdapter) : base(context)
  {
@@ -58,9 +62,10 @@ public MauiRecyclerView(Context context, Func<IItemsLayout> getItemsLayout, Func
  public virtual void TearDownOldElement(TItemsView oldElement)
  {
  // Stop listening for layout property changes
- if (ItemsLayout != null)
+ if (_layoutPropertyChangedProxy is not null)
  {
- ItemsLayout.PropertyChanged -= LayoutPropertyChanged;
+ _layoutPropertyChangedProxy.Unsubscribe();
+ _layoutPropertyChanged = null;
  }
 
  // Stop listening for ScrollTo requests
@@ -283,14 +288,16 @@ public virtual void UpdateCanReorderItems()
 
  public virtual void UpdateLayoutManager()
  {
- if (ItemsLayout != null)
- ItemsLayout.PropertyChanged -= LayoutPropertyChanged;
+ _layoutPropertyChangedProxy?.Unsubscribe();
 
  ItemsLayout = _getItemsLayout();
 
  // Keep track of the ItemsLayout's property changes
  if (ItemsLayout != null)
- ItemsLayout.PropertyChanged += LayoutPropertyChanged;
+ {
+ _layoutPropertyChanged ??= LayoutPropertyChanged;
+ _layoutPropertyChangedProxy = new WeakNotifyPropertyChangedProxy(ItemsLayout, _layoutPropertyChanged);
+ }
 
  SetLayoutManager(SelectLayoutManager(ItemsLayout));
 

src/Controls/src/Core/Handlers/Items/StructuredItemsViewHandler.Windows.cs

@@ -16,6 +16,10 @@ public partial class StructuredItemsViewHandler<TItemsView> : ItemsViewHandler<T
  {
  View _currentHeader;
  View _currentFooter;
+ WeakNotifyPropertyChangedProxy _layoutPropertyChangedProxy;
+ PropertyChangedEventHandler _layoutPropertyChanged;
+
+ ~StructuredItemsViewHandler() => _layoutPropertyChangedProxy?.Unsubscribe();
 
  protected override IItemsLayout Layout { get => ItemsView?.ItemsLayout; }
 
@@ -24,15 +28,26 @@ protected override void ConnectHandler(ListViewBase platformView)
  base.ConnectHandler(platformView);
 
  if (Layout is not null)
- Layout.PropertyChanged += LayoutPropertyChanged;
+ {
+ _layoutPropertyChanged ??= LayoutPropertyChanged;
+ _layoutPropertyChangedProxy = new WeakNotifyPropertyChangedProxy(Layout, _layoutPropertyChanged);
+ }
+ else if (_layoutPropertyChangedProxy is not null)
+ {
+ _layoutPropertyChangedProxy.Unsubscribe();
+ _layoutPropertyChangedProxy = null;
+ }
  }
 
  protected override void DisconnectHandler(ListViewBase platformView)
  {
  base.DisconnectHandler(platformView);
 
- if (Layout is not null)
- Layout.PropertyChanged -= LayoutPropertyChanged;
+ if (_layoutPropertyChangedProxy is not null)
+ {
+ _layoutPropertyChangedProxy.Unsubscribe();
+ _layoutPropertyChangedProxy = null;
+ }
  }
 
  void LayoutPropertyChanged(object sender, PropertyChangedEventArgs e)

src/Controls/src/Core/Handlers/Items/iOS/CarouselViewDelegator.cs

@@ -49,11 +49,11 @@ protected override (bool VisibleItems, int First, int Center, int Last) GetVisib
  {
  var (VisibleItems, First, Center, Last) = GetVisibleItemsIndexPath();
  int firstVisibleItemIndex = -1, centerItemIndex = -1, lastVisibleItemIndex = -1;
- if (VisibleItems)
+ if (VisibleItems && ViewController is CarouselViewController vc)
  {
- firstVisibleItemIndex = ViewController.GetIndexFromIndexPath(First);
- centerItemIndex = ViewController.GetIndexFromIndexPath(Center);
- lastVisibleItemIndex = ViewController.GetIndexFromIndexPath(Last);
+ firstVisibleItemIndex = vc.GetIndexFromIndexPath(First);
+ centerItemIndex = vc.GetIndexFromIndexPath(Center);
+ lastVisibleItemIndex = vc.GetIndexFromIndexPath(Last);
  }
  return (VisibleItems, firstVisibleItemIndex, centerItemIndex, lastVisibleItemIndex);
  }

src/Controls/src/Core/Handlers/Items/iOS/GroupableItemsViewDelegator.cs

@@ -17,12 +17,12 @@ public GroupableItemsViewDelegator(ItemsViewLayout itemsViewLayout, TViewControl
 
  public override CGSize GetReferenceSizeForHeader(UICollectionView collectionView, UICollectionViewLayout layout, nint section)
  {
- return ViewController.GetReferenceSizeForHeader(collectionView, layout, section);
+ return ViewController?.GetReferenceSizeForHeader(collectionView, layout, section) ?? CGSize.Empty;
  }
 
  public override CGSize GetReferenceSizeForFooter(UICollectionView collectionView, UICollectionViewLayout layout, nint section)
  {
- return ViewController.GetReferenceSizeForFooter(collectionView, layout, section);
+ return ViewController?.GetReferenceSizeForFooter(collectionView, layout, section) ?? CGSize.Empty;
  }
 
  public override void ScrollAnimationEnded(UIScrollView scrollView)
@@ -37,7 +37,7 @@ public override UIEdgeInsets GetInsetForSection(UICollectionView collectionView,
  return default;
  }
 
- return ViewController.GetInsetForSection(ItemsViewLayout, collectionView, section);
+ return ViewController?.GetInsetForSection(ItemsViewLayout, collectionView, section) ?? UIEdgeInsets.Zero;
  }
  }
 }

src/Controls/src/Core/Handlers/Items/iOS/ItemsViewController.cs

@@ -30,6 +30,7 @@ public abstract class ItemsViewController<TItemsView> : UICollectionViewControll
  bool _emptyViewDisplayed;
  bool _disposed;
 
+ Func<UICollectionViewCell> _getPrototype;
  UIView _emptyUIView;
  VisualElement _emptyViewFormsElement;
  Dictionary<object, TemplatedCell> _measurementCells = new Dictionary<object, TemplatedCell>();
@@ -200,7 +201,8 @@ void EnsureLayoutInitialized()
 
  _initialized = true;
 
- ItemsViewLayout.GetPrototype = GetPrototype;
+ _getPrototype ??= GetPrototype;
+ ItemsViewLayout.GetPrototype = _getPrototype;
 
  Delegator = CreateDelegator();
  CollectionView.Delegate = Delegator;

src/Controls/src/Core/Handlers/Items/iOS/ItemsViewDelegator.cs

@@ -12,15 +12,17 @@ public class ItemsViewDelegator<TItemsView, TViewController> : UICollectionViewD
  where TItemsView : ItemsView
  where TViewController : ItemsViewController<TItemsView>
  {
+ readonly WeakReference<TViewController> _viewController;
+
  public ItemsViewLayout ItemsViewLayout { get; }
- public TViewController ViewController { get; }
+ public TViewController ViewController => _viewController.TryGetTarget(out var vc) ? vc : null;
 
  protected float PreviousHorizontalOffset, PreviousVerticalOffset;
 
  public ItemsViewDelegator(ItemsViewLayout itemsViewLayout, TViewController itemsViewController)
  {
  ItemsViewLayout = itemsViewLayout;
- ViewController = itemsViewController;
+ _viewController = new(itemsViewController);
  }
 
  public override void Scrolled(UIScrollView scrollView)
@@ -45,8 +47,12 @@ public override void Scrolled(UIScrollView scrollView)
  LastVisibleItemIndex = lastVisibleItemIndex
  };
 
- var itemsView = ViewController.ItemsView;
- var source = ViewController.ItemsSource;
+ var viewController = ViewController;
+ if (viewController is null)
+ return;
+
+ var itemsView = viewController.ItemsView;
+ var source = viewController.ItemsSource;
  itemsView.SendScrolled(itemsViewScrolledEventArgs);
 
  PreviousHorizontalOffset = (float)contentOffsetX;
@@ -119,15 +125,19 @@ public override void CellDisplayingEnded(UICollectionView collectionView, UIColl
 
  protected virtual (bool VisibleItems, NSIndexPath First, NSIndexPath Center, NSIndexPath Last) GetVisibleItemsIndexPath()
  {
- var indexPathsForVisibleItems = ViewController.CollectionView.IndexPathsForVisibleItems.OrderBy(x => x.Row).ToList();
+ var collectionView = ViewController?.CollectionView;
+ if (collectionView is null)
+ return default;
+
+ var indexPathsForVisibleItems = collectionView.IndexPathsForVisibleItems.OrderBy(x => x.Row).ToList();
 
  var visibleItems = indexPathsForVisibleItems.Count > 0;
  NSIndexPath firstVisibleItemIndex = null, centerItemIndex = null, lastVisibleItemIndex = null;
 
  if (visibleItems)
  {
  firstVisibleItemIndex = indexPathsForVisibleItems.First();
- centerItemIndex = GetCenteredIndexPath(ViewController.CollectionView);
+ centerItemIndex = GetCenteredIndexPath(collectionView);
  lastVisibleItemIndex = indexPathsForVisibleItems.Last();
  }
 
@@ -166,7 +176,7 @@ static NSIndexPath GetCenteredIndexPath(UICollectionView collectionView)
 
  public override CGSize GetSizeForItem(UICollectionView collectionView, UICollectionViewLayout layout, NSIndexPath indexPath)
  {
- return ViewController.GetSizeForItem(indexPath);
+ return ViewController?.GetSizeForItem(indexPath) ?? CGSize.Empty;
  }
  }
 }

src/Controls/src/Core/Handlers/Items/iOS/ItemsViewLayout.cs

@@ -19,6 +19,7 @@ public abstract class ItemsViewLayout : UICollectionViewFlowLayout
  CGSize _adjustmentSize0;
  CGSize _adjustmentSize1;
  CGSize _currentSize;
+ WeakReference<Func<UICollectionViewCell>> _getPrototype;
 
  const double ConstraintSizeTolerance = 0.00001;
 
@@ -28,7 +29,11 @@ public abstract class ItemsViewLayout : UICollectionViewFlowLayout
 
  public nfloat ConstrainedDimension { get; set; }
 
- public Func<UICollectionViewCell> GetPrototype { get; set; }
+ public Func<UICollectionViewCell> GetPrototype
+ {
+ get => _getPrototype is not null && _getPrototype.TryGetTarget(out var func) ? func : null;
+ set => _getPrototype = new(value);
+ }
 
  internal ItemSizingStrategy ItemSizingStrategy { get; private set; }
 

src/Controls/src/Core/Handlers/Items/iOS/LoopObservableItemsSource.cs

@@ -1,4 +1,5 @@
 #nullable disable
+using System;
 using System.Collections;
 using Foundation;
 using ObjCRuntime;
@@ -26,8 +27,12 @@ protected override NSIndexPath[] CreateIndexesFrom(int startIndex, int count)
  return base.CreateIndexesFrom(startIndex, count);
  }
 
+ var collectionView = CollectionView;
+ if (collectionView is null)
+ return Array.Empty<NSIndexPath>();
+
  return IndexPathHelpers.GenerateLoopedIndexPathRange(Section,
- (int)CollectionView.NumberOfItemsInSection(Section), LoopBy, startIndex, count);
+ (int)collectionView.NumberOfItemsInSection(Section), LoopBy, startIndex, count);
  }
  }
 }

src/Controls/src/Core/Handlers/Items/iOS/ObservableItemsSource.cs

@@ -10,17 +10,15 @@ namespace Microsoft.Maui.Controls.Handlers.Items
 {
  internal class ObservableItemsSource : IObservableItemsViewSource
  {
- readonly UICollectionViewController _collectionViewController;
- protected readonly UICollectionView CollectionView;
+ readonly WeakReference<UICollectionViewController> _collectionViewController;
  readonly bool _grouped;
  readonly int _section;
  readonly IEnumerable _itemsSource;
  bool _disposed;
 
  public ObservableItemsSource(IEnumerable itemSource, UICollectionViewController collectionViewController, int group = -1)
  {
- _collectionViewController = collectionViewController;
- CollectionView = _collectionViewController.CollectionView;
+ _collectionViewController = new(collectionViewController);
 
  _section = group < 0 ? 0 : group;
  _grouped = group >= 0;
@@ -35,6 +33,8 @@ public ObservableItemsSource(IEnumerable itemSource, UICollectionViewController
  internal event NotifyCollectionChangedEventHandler CollectionViewUpdating;
  internal event NotifyCollectionChangedEventHandler CollectionViewUpdated;
 
+ internal UICollectionView CollectionView => _collectionViewController.TryGetTarget(out var controller) ? controller.CollectionView : null;
+
  public int Count { get; private set; }
 
  public int Section => _section;
@@ -125,9 +125,13 @@ void CollectionChanged(object sender, NotifyCollectionChangedEventArgs args)
 
  void CollectionChanged(NotifyCollectionChangedEventArgs args)
  {
+ if (!_collectionViewController.TryGetTarget(out var controller))
+ return;
+
  // Force UICollectionView to get the internal accounting straight
- if (!CollectionView.Hidden)
- CollectionView.NumberOfItemsInSection(_section);
+ var collectionView = controller.CollectionView;
+ if (!collectionView.Hidden)
+ collectionView.NumberOfItemsInSection(_section);
 
  switch (args.Action)
  {
@@ -153,14 +157,18 @@ void CollectionChanged(NotifyCollectionChangedEventArgs args)
 
  void Reload()
  {
+ if (!_collectionViewController.TryGetTarget(out var controller))
+ return;
+
  var args = new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset);
 
  Count = ItemsCount();
 
  OnCollectionViewUpdating(args);
 
- CollectionView.ReloadData();
- CollectionView.CollectionViewLayout.InvalidateLayout();
+ var collectionView = controller.CollectionView;
+ collectionView.ReloadData();
+ collectionView.CollectionViewLayout.InvalidateLayout();
 
  OnCollectionViewUpdated(args);
  }
@@ -177,7 +185,7 @@ void Add(NotifyCollectionChangedEventArgs args)
  var startIndex = args.NewStartingIndex > -1 ? args.NewStartingIndex : IndexOf(args.NewItems[0]);
 
  // Queue up the updates to the UICollectionView
- Update(() => CollectionView.InsertItems(CreateIndexesFrom(startIndex, count)), args);
+ Update(c => c.InsertItems(CreateIndexesFrom(startIndex, count)), args);
  }
 
  void Remove(NotifyCollectionChangedEventArgs args)
@@ -196,7 +204,7 @@ void Remove(NotifyCollectionChangedEventArgs args)
  var count = args.OldItems.Count;
  Count -= count;
 
- Update(() => CollectionView.DeleteItems(CreateIndexesFrom(startIndex, count)), args);
+ Update(c => c.DeleteItems(CreateIndexesFrom(startIndex, count)), args);
  }
 
  void Replace(NotifyCollectionChangedEventArgs args)
@@ -209,7 +217,7 @@ void Replace(NotifyCollectionChangedEventArgs args)
 
  // We are replacing one set of items with a set of equal size; we can do a simple item range update
 
- Update(() => CollectionView.ReloadItems(CreateIndexesFrom(startIndex, newCount)), args);
+ Update(c => c.ReloadItems(CreateIndexesFrom(startIndex, newCount)), args);
  return;
  }
 
@@ -228,14 +236,14 @@ void Move(NotifyCollectionChangedEventArgs args)
  var oldPath = NSIndexPath.Create(_section, args.OldStartingIndex);
  var newPath = NSIndexPath.Create(_section, args.NewStartingIndex);
 
- Update(() => CollectionView.MoveItem(oldPath, newPath), args);
+ Update(c => c.MoveItem(oldPath, newPath), args);
  return;
  }
 
  var start = Math.Min(args.OldStartingIndex, args.NewStartingIndex);
  var end = Math.Max(args.OldStartingIndex, args.NewStartingIndex) + count;
 
- Update(() => CollectionView.ReloadItems(CreateIndexesFrom(start, end)), args);
+ Update(c => c.ReloadItems(CreateIndexesFrom(start, end)), args);
  }
 
  internal int ItemsCount()
@@ -281,15 +289,19 @@ internal int IndexOf(object item)
  return -1;
  }
 
- void Update(Action update, NotifyCollectionChangedEventArgs args)
+ void Update(Action<UICollectionView> update, NotifyCollectionChangedEventArgs args)
  {
- if (CollectionView.Hidden)
+ if (!_collectionViewController.TryGetTarget(out var controller))
+ return;
+
+ var collectionView = controller.CollectionView;
+ if (collectionView.Hidden)
  {
  return;
  }
 
  OnCollectionViewUpdating(args);
- update();
+ update(collectionView);
  OnCollectionViewUpdated(args);
  }
 

src/Controls/src/Core/Handlers/Items/iOS/ReorderableItemsViewDelegator.cs

@@ -19,7 +19,7 @@ public override NSIndexPath GetTargetIndexPathForMove(UICollectionView collectio
  {
  NSIndexPath targetIndexPath;
 
- var itemsView = ViewController.ItemsView;
+ var itemsView = ViewController?.ItemsView;
  if (itemsView?.IsGrouped == true)
  {
  if (originalIndexPath.Section == proposedIndexPath.Section || itemsView.CanMixGroups)