From 377d24fd05e7fb597c4f9237c1596ed4fbf86f19 Mon Sep 17 00:00:00 2001
From: Shane Neuville <shane94@hotmail.com>
Date: Fri, 24 Mar 2017 12:58:28 -0600
Subject: [PATCH] Setup a ConditionalWeakTable in ListProxy to hold weak
 references so that a ListView will work when connected to a Weak Source
 (#802)

---
 Xamarin.Forms.Core.UnitTests/ListProxyTests.cs | 92 ++++++++++++++++++++++++++
 Xamarin.Forms.Core/ListProxy.cs                | 12 +++-
 2 files changed, 102 insertions(+), 2 deletions(-)

diff --git a/Xamarin.Forms.Core.UnitTests/ListProxyTests.cs b/Xamarin.Forms.Core.UnitTests/ListProxyTests.cs
index 6fa85d2..bf7f6ee 100644
--- a/Xamarin.Forms.Core.UnitTests/ListProxyTests.cs
+++ b/Xamarin.Forms.Core.UnitTests/ListProxyTests.cs
@@ -422,5 +422,97 @@ namespace Xamarin.Forms.Core.UnitTests
 				return Items.GetEnumerator ();
 			}
 		}
+
+		[Test]
+		public void WeakToWeak()
+		{
+			WeakCollectionChangedList list = new WeakCollectionChangedList();
+			var proxy = new ListProxy(list);
+
+			Assert.True(list.AddObject());
+
+			GC.Collect();
+			GC.WaitForPendingFinalizers();
+			GC.Collect();
+
+			Assert.IsTrue(list.AddObject());
+
+			proxy = null;
+
+			GC.Collect();
+			GC.WaitForPendingFinalizers();
+			GC.Collect();
+
+			Assert.IsFalse(list.AddObject());
+		}
+
+		public class WeakCollectionChangedList : List<object>, INotifyCollectionChanged
+		{
+			List<WeakHandler> handlers = new List<WeakHandler>();
+
+			public WeakCollectionChangedList()
+			{
+
+			}
+			public event NotifyCollectionChangedEventHandler CollectionChanged
+			{
+				add { handlers.Add(new WeakHandler(this, value)); }
+				remove { throw new NotImplementedException(); }
+			}
+
+
+			public bool AddObject()
+			{
+				bool invoked = false;
+				var me = new object();
+
+				foreach (var handler in handlers.ToList())
+				{
+					if (handler.IsActive)
+					{
+						invoked = true;
+						handler.Handler.DynamicInvoke(this, new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, me));
+					}
+					else
+					{
+						handlers.Remove(handler);
+					}
+				}
+
+				return invoked;
+			}
+
+			class WeakHandler
+			{
+				WeakReference source;
+				WeakReference originalHandler;
+
+				public bool IsActive
+				{
+					get { return this.source != null && this.source.IsAlive && this.originalHandler != null && this.originalHandler.IsAlive; }
+				}
+
+				public NotifyCollectionChangedEventHandler Handler
+				{
+					get
+					{
+						if (this.originalHandler == null)
+						{
+							return default(NotifyCollectionChangedEventHandler);
+						}
+						else
+						{
+							return (NotifyCollectionChangedEventHandler)this.originalHandler.Target;
+						}
+					}
+				}
+
+				public WeakHandler(object source, NotifyCollectionChangedEventHandler originalHandler)
+				{
+					this.source = new WeakReference(source);
+					this.originalHandler = new WeakReference(originalHandler);
+				}
+			}
+		}
 	}
 }
diff --git a/Xamarin.Forms.Core/ListProxy.cs b/Xamarin.Forms.Core/ListProxy.cs
index c2f5c9d..b25da16 100644
--- a/Xamarin.Forms.Core/ListProxy.cs
+++ b/Xamarin.Forms.Core/ListProxy.cs
@@ -3,6 +3,7 @@ using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
 using System.Linq;
+using System.Runtime.CompilerServices;
 using Xamarin.Forms.Internals;
 
 namespace Xamarin.Forms
@@ -12,6 +13,7 @@ namespace Xamarin.Forms
 		readonly ICollection _collection;
 		readonly IList _list;
 		readonly int _windowSize;
+		readonly ConditionalWeakTable<ListProxy, WeakNotifyProxy> _sourceToWeakHandlers;
 
 		IEnumerator _enumerator;
 		int _enumeratorIndex;
@@ -30,6 +32,7 @@ namespace Xamarin.Forms
 
 			ProxiedEnumerable = enumerable;
 			_collection = enumerable as ICollection;
+			_sourceToWeakHandlers = new ConditionalWeakTable<ListProxy, WeakNotifyProxy>();
 
 			if (_collection == null && enumerable is IReadOnlyCollection<object>)
 				_collection = new ReadOnlyListAdapter((IReadOnlyCollection<object>)enumerable);
@@ -40,7 +43,7 @@ namespace Xamarin.Forms
 
 			var changed = enumerable as INotifyCollectionChanged;
 			if (changed != null)
-				new WeakNotifyProxy(this, changed);
+				_sourceToWeakHandlers.Add(this, new WeakNotifyProxy(this, changed));
 		}
 
 		public IEnumerable ProxiedEnumerable { get; }
@@ -362,10 +365,15 @@ namespace Xamarin.Forms
 		{
 			readonly WeakReference<INotifyCollectionChanged> _weakCollection;
 			readonly WeakReference<ListProxy> _weakProxy;
+			readonly ConditionalWeakTable<ListProxy, NotifyCollectionChangedEventHandler> _sourceToWeakHandlers;
 
 			public WeakNotifyProxy(ListProxy proxy, INotifyCollectionChanged incc)
 			{
-				incc.CollectionChanged += OnCollectionChanged;
+				_sourceToWeakHandlers = new ConditionalWeakTable<ListProxy, NotifyCollectionChangedEventHandler>();
+				NotifyCollectionChangedEventHandler handler = new NotifyCollectionChangedEventHandler(OnCollectionChanged);
+
+				_sourceToWeakHandlers.Add(proxy, handler);
+				incc.CollectionChanged += handler;
 
 				_weakProxy = new WeakReference<ListProxy>(proxy);
 				_weakCollection = new WeakReference<INotifyCollectionChanged>(incc);
-- 
2.7.4