From 21574f5607d687c24d1c831bd808733d71650439 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Fri, 3 Mar 2023 13:59:00 +0100
Subject: [PATCH] Expose pre/post collection changed events.

Instead of implementing `ICollectionChangedListener` on `SelectionNodeBase`. We may want to expose this publicly at some point.
---
 src/Avalonia.Controls/ItemsSourceView.cs      | 39 +++++++++++--------
 .../Selection/SelectionNodeBase.cs            | 38 +++++++++---------
 2 files changed, 41 insertions(+), 36 deletions(-)

diff --git a/src/Avalonia.Controls/ItemsSourceView.cs b/src/Avalonia.Controls/ItemsSourceView.cs
index 1fa8f6a5cf..416b909219 100644
--- a/src/Avalonia.Controls/ItemsSourceView.cs
+++ b/src/Avalonia.Controls/ItemsSourceView.cs
@@ -27,6 +27,7 @@ namespace Avalonia.Controls
 
         private readonly IList _inner;
         private NotifyCollectionChangedEventHandler? _collectionChanged;
+        private NotifyCollectionChangedEventHandler? _preCollectionChanged;
         private NotifyCollectionChangedEventHandler? _postCollectionChanged;
         private bool _listening;
 
@@ -70,7 +71,7 @@ namespace Avalonia.Controls
         /// Gets a value that indicates whether the items source can provide a unique key for each item.
         /// </summary>
         /// <remarks>
-        /// TODO: Not yet implemented in Avalonia.
+        /// Not implemented in Avalonia, preserved here for ItemsRepeater's usage.
         /// </remarks>
         internal bool HasKeyIndexMapping => false;
 
@@ -92,6 +93,25 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <summary>
+        /// Occurs when a collection has finished changing and all <see cref="CollectionChanged"/>
+        /// event handlers have been notified.
+        /// </summary>
+        internal event NotifyCollectionChangedEventHandler? PreCollectionChanged
+        {
+            add
+            {
+                AddListenerIfNecessary();
+                _preCollectionChanged += value;
+            }
+
+            remove
+            {
+                _preCollectionChanged -= value;
+                RemoveListenerIfNecessary();
+            }
+        }
+
         /// <summary>
         /// Occurs when a collection has finished changing and all <see cref="CollectionChanged"/>
         /// event handlers have been notified.
@@ -229,6 +249,7 @@ namespace Avalonia.Controls
 
         void ICollectionChangedListener.PreChanged(INotifyCollectionChanged sender, NotifyCollectionChangedEventArgs e)
         {
+            _preCollectionChanged?.Invoke(this, e);
         }
 
         void ICollectionChangedListener.Changed(INotifyCollectionChanged sender, NotifyCollectionChangedEventArgs e)
@@ -241,22 +262,6 @@ namespace Avalonia.Controls
             _postCollectionChanged?.Invoke(this, e);
         }
 
-        internal void AddListener(ICollectionChangedListener listener)
-        {
-            if (Inner is INotifyCollectionChanged incc)
-            {
-                CollectionChangedEventManager.Instance.AddListener(incc, listener);
-            }
-        }
-
-        internal void RemoveListener(ICollectionChangedListener listener)
-        {
-            if (Inner is INotifyCollectionChanged incc)
-            {
-                CollectionChangedEventManager.Instance.RemoveListener(incc, listener);
-            }
-        }
-
         /// <summary>
         /// Retrieves the index of the item that has the specified unique identifier (key).
         /// </summary>
diff --git a/src/Avalonia.Controls/Selection/SelectionNodeBase.cs b/src/Avalonia.Controls/Selection/SelectionNodeBase.cs
index 22db0cbb6c..caeff61f07 100644
--- a/src/Avalonia.Controls/Selection/SelectionNodeBase.cs
+++ b/src/Avalonia.Controls/Selection/SelectionNodeBase.cs
@@ -2,7 +2,6 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
-using Avalonia.Controls.Utils;
 
 namespace Avalonia.Controls.Selection
 {
@@ -10,7 +9,7 @@ namespace Avalonia.Controls.Selection
     /// Base class for selection models.
     /// </summary>
     /// <typeparam name="T">The type of the element being selected.</typeparam>
-    public abstract class SelectionNodeBase<T> : ICollectionChangedListener
+    public abstract class SelectionNodeBase<T>
     {
         private IEnumerable? _source;
         private bool _rangesEnabled;
@@ -24,12 +23,28 @@ namespace Avalonia.Controls.Selection
             get => _source;
             set
             {
+                void OnPreChanged(object? sender, NotifyCollectionChangedEventArgs e) => OnSourceCollectionChangeStarted();
+                void OnChanged(object? sender, NotifyCollectionChangedEventArgs e) => OnSourceCollectionChanged(e);
+                void OnPostChanged(object? sender, NotifyCollectionChangedEventArgs e) => OnSourceCollectionChangeFinished();
+
                 if (_source != value)
                 {
-                    ItemsView?.RemoveListener(this);
+                    if (ItemsView is not null)
+                    {
+                        ItemsView.PreCollectionChanged -= OnPreChanged;
+                        ItemsView.CollectionChanged -= OnChanged;
+                        ItemsView.PostCollectionChanged -= OnPostChanged;
+                    }
+
                     _source = value;
                     ItemsView = value is not null ? ItemsSourceView.GetOrCreate<T>(value) : null;
-                    ItemsView?.AddListener(this);
+
+                    if (ItemsView is not null)
+                    {
+                        ItemsView.PreCollectionChanged += OnPreChanged;
+                        ItemsView.CollectionChanged += OnChanged;
+                        ItemsView.PostCollectionChanged += OnPostChanged;
+                    }
                 }
             }
         }
@@ -73,21 +88,6 @@ namespace Avalonia.Controls.Selection
             }
         }
 
-        void ICollectionChangedListener.PreChanged(INotifyCollectionChanged sender, NotifyCollectionChangedEventArgs e)
-        {
-            OnSourceCollectionChangeStarted();
-        }
-
-        void ICollectionChangedListener.Changed(INotifyCollectionChanged sender, NotifyCollectionChangedEventArgs e)
-        {
-            OnSourceCollectionChanged(e);
-        }
-
-        void ICollectionChangedListener.PostChanged(INotifyCollectionChanged sender, NotifyCollectionChangedEventArgs e)
-        {
-            OnSourceCollectionChangeFinished();
-        }
-
         /// <summary>
         /// Called when the source collection starts changing.
         /// </summary>