From b4d615b95574f7a4969f7ebc6c5d17f533136985 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Tue, 25 Aug 2020 12:39:53 +0200
Subject: [PATCH] Use CollectionChangedEventManager in ItemsControl.

---
 src/Avalonia.Controls/ItemsControl.cs | 30 +++++++++++++++++++--------
 1 file changed, 21 insertions(+), 9 deletions(-)

diff --git a/src/Avalonia.Controls/ItemsControl.cs b/src/Avalonia.Controls/ItemsControl.cs
index 1aa7945901..a3dfe33641 100644
--- a/src/Avalonia.Controls/ItemsControl.cs
+++ b/src/Avalonia.Controls/ItemsControl.cs
@@ -18,7 +18,7 @@ namespace Avalonia.Controls
     /// <summary>
     /// Displays a collection of items.
     /// </summary>
-    public class ItemsControl : TemplatedControl, IItemsPresenterHost
+    public class ItemsControl : TemplatedControl, IItemsPresenterHost, ICollectionChangedListener
     {
         /// <summary>
         /// The default value for the <see cref="ItemsPanel"/> property.
@@ -53,7 +53,6 @@ namespace Avalonia.Controls
         private IEnumerable _items = new AvaloniaList<object>();
         private int _itemCount;
         private IItemContainerGenerator _itemContainerGenerator;
-        private IDisposable _itemsCollectionChangedSubscription;
 
         /// <summary>
         /// Initializes static members of the <see cref="ItemsControl"/> class.
@@ -150,6 +149,19 @@ namespace Avalonia.Controls
             ItemContainerGenerator.Clear();
         }
 
+        void ICollectionChangedListener.PreChanged(INotifyCollectionChanged sender, NotifyCollectionChangedEventArgs e)
+        {
+        }
+
+        void ICollectionChangedListener.Changed(INotifyCollectionChanged sender, NotifyCollectionChangedEventArgs e)
+        {
+        }
+
+        void ICollectionChangedListener.PostChanged(INotifyCollectionChanged sender, NotifyCollectionChangedEventArgs e)
+        {
+            ItemsCollectionChanged(sender, e);
+        }
+
         /// <summary>
         /// Gets the item at the specified index in a collection.
         /// </summary>
@@ -315,12 +327,14 @@ namespace Avalonia.Controls
         /// <param name="e">The event args.</param>
         protected virtual void ItemsChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            _itemsCollectionChangedSubscription?.Dispose();
-            _itemsCollectionChangedSubscription = null;
-
             var oldValue = e.OldValue as IEnumerable;
             var newValue = e.NewValue as IEnumerable;
 
+            if (oldValue is INotifyCollectionChanged incc)
+            {
+                CollectionChangedEventManager.Instance.RemoveListener(incc, this);
+            }
+
             UpdateItemCount();
             RemoveControlItemsFromLogicalChildren(oldValue);
             AddControlItemsToLogicalChildren(newValue);
@@ -418,11 +432,9 @@ namespace Avalonia.Controls
             PseudoClasses.Set(":empty", items == null || items.Count() == 0);
             PseudoClasses.Set(":singleitem", items != null && items.Count() == 1);
 
-            var incc = items as INotifyCollectionChanged;
-
-            if (incc != null)
+            if (items is INotifyCollectionChanged incc)
             {
-                _itemsCollectionChangedSubscription = incc.WeakSubscribe(ItemsCollectionChanged);
+                CollectionChangedEventManager.Instance.AddListener(incc, this);
             }
         }