Handle null SelectionModel.Source.

src/Avalonia.Controls/SelectionModel.cs

@@ -36,13 +36,29 @@ namespace Avalonia.Controls
             get => _rootNode?.Source;
             set
             {
-                using (var operation = new Operation(this))
+                var wasNull = _rootNode.Source == null;
+
+                if (_rootNode.Source != null)
                 {
-                    ClearSelection(resetAnchor: true);
+                    using (var operation = new Operation(this))
+                    {
+                        ClearSelection(resetAnchor: true);
+                    }
                 }
 
                 _rootNode.Source = value;
+
                 RaisePropertyChanged("Source");
+
+                if (wasNull)
+                {
+                    var e = new SelectionModelSelectionChangedEventArgs(
+                        null,
+                        SelectedIndices,
+                        null,
+                        SelectedItems);
+                    OnSelectionChanged(e);
+                }
             }
         }
 

src/Avalonia.Controls/SelectionModelChangeSet.cs

@@ -16,30 +16,38 @@ namespace Avalonia.Controls
 
         public SelectionModelSelectionChangedEventArgs CreateEventArgs()
         {
-            var deselectedCount = 0;
-            var selectedCount = 0;
+            var deselectedIndexCount = 0;
+            var selectedIndexCount = 0;
+            var deselectedItemCount = 0;
+            var selectedItemCount = 0;
 
             foreach (var change in _changes)
             {
-                deselectedCount += change.DeselectedCount;
-                selectedCount += change.SelectedCount;
+                deselectedIndexCount += change.DeselectedCount;
+                selectedIndexCount += change.SelectedCount;
+
+                if (change.Items != null)
+                {
+                    deselectedItemCount += change.DeselectedCount;
+                    selectedItemCount += change.SelectedCount;
+                }
             }
 
             var deselectedIndices = new SelectedItems<IndexPath, SelectionNodeOperation>(
                 _changes,
-                deselectedCount,
+                deselectedIndexCount,
                 GetDeselectedIndexAt);
             var selectedIndices = new SelectedItems<IndexPath, SelectionNodeOperation>(
                 _changes,
-                selectedCount,
+                selectedIndexCount,
                 GetSelectedIndexAt);
             var deselectedItems = new SelectedItems<object?, SelectionNodeOperation>(
                 _changes,
-                deselectedCount,
+                deselectedItemCount,
                 GetDeselectedItemAt);
             var selectedItems = new SelectedItems<object?, SelectionNodeOperation>(
                 _changes,
-                selectedCount,
+                selectedItemCount,
                 GetSelectedItemAt);
 
             return new SelectionModelSelectionChangedEventArgs(
@@ -71,7 +79,7 @@ namespace Avalonia.Controls
             List<SelectionNodeOperation> infos,
             int index)
         {
-            static int GetCount(SelectionNodeOperation info) => info.DeselectedCount;
+            static int GetCount(SelectionNodeOperation info) => info.Items != null ? info.DeselectedCount : 0;
             static List<IndexRange>? GetRanges(SelectionNodeOperation info) => info.DeselectedRanges;
             return GetItemAt(infos, index, GetCount, GetRanges);
         }
@@ -80,7 +88,7 @@ namespace Avalonia.Controls
             List<SelectionNodeOperation> infos,
             int index)
         {
-            static int GetCount(SelectionNodeOperation info) => info.SelectedCount;
+            static int GetCount(SelectionNodeOperation info) => info.Items != null ? info.SelectedCount : 0;
             static List<IndexRange>? GetRanges(SelectionNodeOperation info) => info.SelectedRanges;
             return GetItemAt(infos, index, GetCount, GetRanges);
         }

src/Avalonia.Controls/SelectionNode.cs

@@ -48,8 +48,11 @@ namespace Avalonia.Controls
             {
                 if (_source != value)
                 {
-                    ClearSelection();
-                    UnhookCollectionChangedHandler();
+                    if (_source != null)
+                    {
+                        ClearSelection();
+                        UnhookCollectionChangedHandler();
+                    }
 
                     _source = value;
 

tests/Avalonia.Controls.UnitTests/SelectionModelTests.cs

@@ -1424,6 +1424,48 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(2, raised);
         }
 
+        [Fact]
+        public void Raises_SelectionChanged_With_No_Source()
+        {
+            var target = new SelectionModel();
+            var raised = 0;
+
+            target.SelectionChanged += (s, e) =>
+            {
+                Assert.Empty(e.DeselectedIndices);
+                Assert.Empty(e.DeselectedItems);
+                Assert.Equal(new[] { new IndexPath(1) }, e.SelectedIndices);
+                Assert.Empty(e.SelectedItems);
+                ++raised;
+            };
+
+            target.Select(1);
+
+            Assert.Equal(new[] { new IndexPath(1) }, target.SelectedIndices);
+            Assert.Empty(target.SelectedItems);
+        }
+
+        [Fact]
+        public void Raises_SelectionChanged_With_Items_After_Source_Is_Set()
+        {
+            var target = new SelectionModel();
+            var raised = 0;
+
+            target.Select(1);
+            target.SelectionChanged += (s, e) =>
+            {
+                Assert.Empty(e.DeselectedIndices);
+                Assert.Empty(e.DeselectedItems);
+                Assert.Equal(new[] { new IndexPath(1) }, e.SelectedIndices);
+                Assert.Equal(new[] { "bar" }, e.SelectedItems);
+                ++raised;
+            };
+
+            target.Source = new[] { "foo", "bar", "baz" };
+
+            Assert.Equal(1, raised);
+        }
+
         private int GetSubscriberCount(AvaloniaList<object> list)
         {
             return ((INotifyCollectionChangedDebug)list).GetCollectionChangedSubscribers()?.Length ?? 0;