Make sure ItemsPresenter is updated before selection change.

Having a separate subscription to `Items.CollectionChanged` in both `ItemsControl` and `ItemsPresenter` meant that the `ItemsPresenter` sometimes doesn't update itself in time for a selection change, resulting in #3148. Make `ItemsControl` trigger `ItemsPresenter.ItemsChanged` rather than both controls listening to the event separately.

src/Avalonia.Controls/ItemsControl.cs

@@ -356,6 +356,7 @@ namespace Avalonia.Controls
             var oldValue = e.OldValue as IEnumerable;
             var newValue = e.NewValue as IEnumerable;
 
+            Presenter?.ItemsChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
             UpdateItemCount();
             RemoveControlItemsFromLogicalChildren(oldValue);
             AddControlItemsToLogicalChildren(newValue);
@@ -370,6 +371,9 @@ namespace Avalonia.Controls
         /// <param name="e">The event args.</param>
         protected virtual void ItemsCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
         {
+            UpdateItemCount();
+            Presenter?.ItemsChanged(e);
+
             switch (e.Action)
             {
                 case NotifyCollectionChangedAction.Add:
@@ -381,8 +385,6 @@ namespace Avalonia.Controls
                     break;
             }
 
-            UpdateItemCount();
-
             var collection = sender as ICollection;
             PseudoClasses.Set(":empty", collection == null || collection.Count == 0);
             PseudoClasses.Set(":singleitem", collection != null && collection.Count == 1);

src/Avalonia.Controls/Presenters/IItemsPresenter.cs

@@ -1,12 +1,16 @@
 // Copyright (c) The Avalonia Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
+using System.Collections.Specialized;
+
 namespace Avalonia.Controls.Presenters
 {
     public interface IItemsPresenter : IPresenter
     {
         IPanel Panel { get; }
 
+        void ItemsChanged(NotifyCollectionChangedEventArgs e);
+
         void ScrollIntoView(object item);
     }
 }

src/Avalonia.Controls/Presenters/ItemsPresenterBase.cs

@@ -63,7 +63,7 @@ namespace Avalonia.Controls.Presenters
                 _itemsSubscription?.Dispose();
                 _itemsSubscription = null;
 
-                if (_createdPanel && value is INotifyCollectionChanged incc)
+                if (!IsHosted && _createdPanel && value is INotifyCollectionChanged incc)
                 {
                     _itemsSubscription = incc.WeakSubscribe(ItemsCollectionChanged);
                 }
@@ -130,6 +130,8 @@ namespace Avalonia.Controls.Presenters
             private set;
         }
 
+        protected bool IsHosted => TemplatedParent is IItemsPresenterHost;
+
         /// <inheritdoc/>
         public override sealed void ApplyTemplate()
         {
@@ -144,6 +146,15 @@ namespace Avalonia.Controls.Presenters
         {
         }
 
+        /// <inheritdoc/>
+        void IItemsPresenter.ItemsChanged(NotifyCollectionChangedEventArgs e)
+        {
+            if (Panel != null)
+            {
+                ItemsChanged(e);
+            }
+        }
+
         /// <summary>
         /// Creates the <see cref="ItemContainerGenerator"/> for the control.
         /// </summary>
@@ -215,7 +226,7 @@ namespace Avalonia.Controls.Presenters
 
             _createdPanel = true;
 
-            if (_itemsSubscription == null && Items is INotifyCollectionChanged incc)
+            if (!IsHosted && _itemsSubscription == null && Items is INotifyCollectionChanged incc)
             {
                 _itemsSubscription = incc.WeakSubscribe(ItemsCollectionChanged);
             }

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -302,13 +302,24 @@ namespace Avalonia.Controls.Primitives
         /// <inheritdoc/>
         protected override void ItemsCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
         {
-            base.ItemsCollectionChanged(sender, e);
-
             if (_updateCount > 0)
             {
+                base.ItemsCollectionChanged(sender, e);
                 return;
             }
 
+            switch (e.Action)
+            {
+                case NotifyCollectionChangedAction.Add:
+                    _selection.ItemsInserted(e.NewStartingIndex, e.NewItems.Count);
+                    break;
+                case NotifyCollectionChangedAction.Remove:
+                    _selection.ItemsRemoved(e.OldStartingIndex, e.OldItems.Count);
+                    break;
+            }
+
+            base.ItemsCollectionChanged(sender, e);
+
             switch (e.Action)
             {
                 case NotifyCollectionChangedAction.Add:
@@ -318,14 +329,12 @@ namespace Avalonia.Controls.Primitives
                     }
                     else
                     {
-                        _selection.ItemsInserted(e.NewStartingIndex, e.NewItems.Count);
                         UpdateSelectedItem(_selection.First(), false);
                     }
 
                     break;
 
                 case NotifyCollectionChangedAction.Remove:
-                    _selection.ItemsRemoved(e.OldStartingIndex, e.OldItems.Count);
                     UpdateSelectedItem(_selection.First(), false);
                     ResetSelectedItems();
                     break;