Updated ItemsControlRegionAdapter to fix previous-binding and allow i…

…tems to be added

src/Avalonia/Prism.Avalonia/Navigation/Regions/ItemsControlRegionAdapter.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using Avalonia.Controls;
 using Prism.Properties;
 
@@ -10,18 +11,14 @@ namespace Prism.Navigation.Regions
     /// </summary>
     public class ItemsControlRegionAdapter : RegionAdapterBase<ItemsControl>
     {
-        /// <summary>
-        /// Initializes a new instance of <see cref="ItemsControlRegionAdapter"/>.
-        /// </summary>
+        /// <summary>Initializes a new instance of <see cref="ItemsControlRegionAdapter"/>.</summary>
         /// <param name="regionBehaviorFactory">The factory used to create the region behaviors to attach to the created regions.</param>
         public ItemsControlRegionAdapter(IRegionBehaviorFactory regionBehaviorFactory)
             : base(regionBehaviorFactory)
         {
         }
 
-        /// <summary>
-        /// Adapts an <see cref="ItemsControl"/> to an <see cref="IRegion"/>.
-        /// </summary>
+        /// <summary>Adapts an <see cref="ItemsControl"/> to an <see cref="IRegion"/>.</summary>
         /// <param name="region">The new region being used.</param>
         /// <param name="regionTarget">The object to adapt.</param>
         protected override void Adapt(IRegion region, ItemsControl regionTarget)
@@ -33,30 +30,46 @@ protected override void Adapt(IRegion region, ItemsControl regionTarget)
                 throw new ArgumentNullException(nameof(regionTarget));
 
             // NOTE: In Avalonia, Items will never be null
+            // Removed: Avalonia v11.1.1
+            // Prism.Wpf throws, but we keep it rollin' baby!
+            /*
             bool itemsSourceIsSet = regionTarget.ItemCount > 0;
             itemsSourceIsSet = itemsSourceIsSet || regionTarget.HasBinding(ItemsControl.ItemsSourceProperty);
 
             if (itemsSourceIsSet)
             {
                 throw new InvalidOperationException(Resources.ItemsControlHasItemsSourceException);
             }
+            */
 
             // If control has child items, move them to the region and then bind control to region. Can't set ItemsSource if child items exist.
             if (regionTarget.ItemCount > 0)
             {
                 foreach (object childItem in regionTarget.Items)
-                {
                     region.Add(childItem);
-                }
 
                 // Control must be empty before setting ItemsSource
                 regionTarget.Items.Clear();
             }
 
+            // Detect when an item has been added/removed to the ItemsControl's backing region. Copy
+            // all items to a new collection and bind to the region's ItemsSource
+            region.Views.CollectionChanged += (s, e) =>
+            {
+                var enumerator = region.Views.GetEnumerator();
+                List<object> items = new();
+                while (enumerator.MoveNext())
+                {
+                    items.Add(enumerator.Current);
+                }
+
+                regionTarget.ItemsSource = items;
+            };
+
             // Avalonia v11-Preview5 needs IRegion implement IList. Enforcing it to return AvaloniaList<object> fixes this.
             // Avalonia v11-Preview8 ItemsControl.Items is readonly (#10827).
-            ////regionTarget.Items = region.Views as Avalonia.Collections.AvaloniaList<object>;
-            regionTarget.ItemsSource = region.Views as Avalonia.Collections.AvaloniaList<object>;
+            // Removed: Avalonia v11.1.1
+            ////regionTarget.ItemsSource = region.Views as Avalonia.Collections.AvaloniaList<object>;
         }
 
         /// <summary>