Occasionally clean up WeakReferences in Styles whose targets have been collected (#8124)

Author: hartez

Xamarin.Forms.Core/Interactivity/AttachedCollection.cs

@@ -8,6 +8,9 @@ internal class AttachedCollection<T> : ObservableCollection<T>, ICollection<T>,
 	{
 		readonly List<WeakReference> _associatedObjects = new List<WeakReference>();
 
+		const int CleanupTrigger = 128;
+		int _cleanupThreshold = CleanupTrigger;
+
 		public AttachedCollection()
 		{
 		}
@@ -64,6 +67,7 @@ protected virtual void OnAttachedTo(BindableObject bindable)
 			lock (_associatedObjects)
 			{
 				_associatedObjects.Add(new WeakReference(bindable));
+				CleanUpWeakReferences();
 			}
 			foreach (T item in this)
 				item.AttachTo(bindable);
@@ -123,5 +127,16 @@ protected override void SetItem(int index, T item)
 				item.AttachTo(bindable);
 			}
 		}
+
+		void CleanUpWeakReferences()
+		{
+			if (_associatedObjects.Count < _cleanupThreshold)
+			{
+				return;
+			}
+
+			_associatedObjects.RemoveAll(t => !t.IsAlive);
+			_cleanupThreshold = _associatedObjects.Count + CleanupTrigger;
+		}
 	}
 }

Xamarin.Forms.Core/Style.cs

@@ -10,6 +10,9 @@ public sealed class Style : IStyle
 	{
 		internal const string StyleClassPrefix = "Xamarin.Forms.StyleClass.";
 
+		const int CleanupTrigger = 128;
+		int _cleanupThreshold = CleanupTrigger;
+
 		readonly BindableProperty _basedOnResourceProperty = BindableProperty.CreateAttached("BasedOnResource", typeof(Style), typeof(Style), default(Style),
 			propertyChanged: OnBasedOnResourceChanged);
 
@@ -93,6 +96,8 @@ void IStyle.Apply(BindableObject bindable)
 			if (BaseResourceKey != null)
 				bindable.SetDynamicResource(_basedOnResourceProperty, BaseResourceKey);
 			ApplyCore(bindable, BasedOn ?? GetBasedOnResource(bindable));
+
+			CleanUpWeakReferences();
 		}
 
 		public Type TargetType { get; }
@@ -178,5 +183,16 @@ bool ValidateBasedOn(Style value)
 				return true;
 			return value.TargetType.IsAssignableFrom(TargetType);
 		}
+
+		void CleanUpWeakReferences()
+		{
+			if (_targets.Count < _cleanupThreshold)
+			{
+				return;
+			}
+
+			_targets.RemoveAll(t => !t.TryGetTarget(out _));
+			_cleanupThreshold = _targets.Count + CleanupTrigger;
+		}
 	}
 }