[leak-fix] Fix Picker.ItemsSource memory leak (Fixes #36272)

src/Controls/src/Core/Picker/Picker.cs

@@ -79,12 +79,17 @@ public partial class Picker : View, IFontElement, ITextElement, ITextAlignmentEl
 
 		readonly Lazy<PlatformConfigurationRegistry<Picker>> _platformConfigurationRegistry;
 
+		ItemsSourceCollectionChangedSubscription _itemsSourceCollectionChangedSubscription;
+		// WeakNotifyCollectionChangedProxy weakly references its handler, so the Picker must keep this delegate alive.
+		NotifyCollectionChangedEventHandler _collectionChangedEventHandler;
+
 		/// <summary>Initializes a new instance of the Picker class.</summary>
 		public Picker()
 		{
 			((INotifyCollectionChanged)Items).CollectionChanged += OnItemsCollectionChanged;
 			_platformConfigurationRegistry = new Lazy<PlatformConfigurationRegistry<Picker>>(() => new PlatformConfigurationRegistry<Picker>(this));
 		}
+
 		/// <summary>Gets a value that indicates whether the font for the searchbar text is bold, italic, or neither. This is a bindable property.</summary>
 		public FontAttributes FontAttributes
 		{
@@ -338,7 +343,7 @@ void OnItemsSourceChanged(IList oldValue, IList newValue)
 				UnsubscribeFromItemsSourceCollection();
 			}
 
-			if (Handler is not null)
+			if (!_isItemsSourceSubscriptionPaused)
 			{
 				SubscribeToItemsSourceCollection(newValue as INotifyCollectionChanged);
 			}
@@ -356,8 +361,40 @@ void OnItemsSourceChanged(IList oldValue, IList newValue)
 			}
 		}
 
+		sealed class ItemsSourceCollectionChangedSubscription
+		{
+			readonly WeakNotifyCollectionChangedProxy _proxy = new();
+			bool _isFinalizationSuppressed;
+
+			~ItemsSourceCollectionChangedSubscription() => _proxy.Unsubscribe();
+
+			public void Subscribe(INotifyCollectionChanged source, NotifyCollectionChangedEventHandler handler)
+			{
+				if (_isFinalizationSuppressed)
+				{
+					GC.ReRegisterForFinalize(this);
+					_isFinalizationSuppressed = false;
+				}
+
+				_proxy.Subscribe(source, handler);
+			}
+
+			public void Unsubscribe()
+			{
+				_proxy.Unsubscribe();
+
+				if (_isFinalizationSuppressed)
+					return;
+
+				GC.SuppressFinalize(this);
+				_isFinalizationSuppressed = true;
+			}
+		}
+
 		readonly Queue<Action> _pendingIsOpenActions = new Queue<Action>();
 		INotifyCollectionChanged _subscribedItemsSourceCollection;
+		// Preserve source updates before the first handler; pause only after explicit detachment.
+		bool _isItemsSourceSubscriptionPaused;
 
 		void OnIsOpenPropertyChanged(bool oldValue, bool newValue)
 		{
@@ -373,21 +410,27 @@ void OnIsOpenPropertyChanged(bool oldValue, bool newValue)
 
 		protected override void OnHandlerChanged()
 		{
+			var wasItemsSourceSubscriptionPaused = _isItemsSourceSubscriptionPaused;
+
 			if (Handler is null)
 			{
+				_isItemsSourceSubscriptionPaused = true;
 				UnsubscribeFromItemsSourceCollection();
 			}
 
 			base.OnHandlerChanged();
 
 			if (Handler is not null)
 			{
+				_isItemsSourceSubscriptionPaused = false;
 				SubscribeToItemsSourceCollection(ItemsSource as INotifyCollectionChanged);
 
-				// Keep display items in sync if this Picker is detached and later reattached.
-				if (ItemsSource is not null)
+				// Observable sources stay synchronized before the first handler; rebuild only after a
+				// detached interval or for non-observable sources that may have changed in place.
+				if (ItemsSource is not null &&
+					(wasItemsSourceSubscriptionPaused || ItemsSource is not INotifyCollectionChanged))
 				{
-					ResetItems();
+					ResyncItemsAndReconcileSelection();
 				}
 			}
 
@@ -418,18 +461,15 @@ void SubscribeToItemsSourceCollection(INotifyCollectionChanged collection)
 			}
 
 			UnsubscribeFromItemsSourceCollection();
+			var subscription = _itemsSourceCollectionChangedSubscription ??= new ItemsSourceCollectionChangedSubscription();
+			_collectionChangedEventHandler ??= CollectionChanged;
+			subscription.Subscribe(collection, _collectionChangedEventHandler);
 			_subscribedItemsSourceCollection = collection;
-			_subscribedItemsSourceCollection.CollectionChanged += CollectionChanged;
 		}
 
 		void UnsubscribeFromItemsSourceCollection()
 		{
-			if (_subscribedItemsSourceCollection is null)
-			{
-				return;
-			}
-
-			_subscribedItemsSourceCollection.CollectionChanged -= CollectionChanged;
+			_itemsSourceCollectionChangedSubscription?.Unsubscribe();
 			_subscribedItemsSourceCollection = null;
 		}
 
@@ -516,6 +556,9 @@ void ResyncItemsAndReconcileSelection()
 
 			Handler?.UpdateValue(nameof(IPicker.Items));
 
+			if (SelectedItem is null && TryApplyPendingSelectedIndex(forceClamp: true))
+				return;
+
 			if (SelectedItem is not null)
 			{
 				ClampSelectedIndex(ItemsSource.IndexOf(SelectedItem));