Merge pull request #4876 from donandren/issues/4875

fix notification for SelectedItem changed

src/Avalonia.Controls/Selection/SelectionModel.cs

@@ -442,7 +442,8 @@ namespace Avalonia.Controls.Selection
                 RaisePropertyChanged(nameof(SelectedIndex));
             }
 
-            if (e.Action == NotifyCollectionChangedAction.Remove && e.OldStartingIndex <= oldSelectedIndex)
+            if ((e.Action == NotifyCollectionChangedAction.Remove && e.OldStartingIndex <= oldSelectedIndex) ||
+                e.Action == NotifyCollectionChangedAction.Reset)
             {
                 RaisePropertyChanged(nameof(SelectedItem));
             }

tests/Avalonia.Controls.UnitTests/Selection/InternalSelectionModelTests.cs

@@ -205,6 +205,49 @@ namespace Avalonia.Controls.UnitTests.Selection
             Assert.Equal(2, target.SelectedIndex);
         }
 
+        [Fact]
+        public void Raises_Selection_Changed_On_Items_Reset()
+        {
+            var items = new ResettingCollection(new[] { "foo", "bar", "baz" });
+            var target = CreateTarget(source: items);
+
+            target.SelectedIndex = 1;
+
+            var changed = new List<string>();
+
+            target.PropertyChanged += (s, e) => changed.Add(e.PropertyName);
+
+            var oldSelectedIndex = target.SelectedIndex;
+            var oldSelectedItem = target.SelectedItem;
+
+            items.Reset(new string[0]);
+
+            Assert.NotEqual(oldSelectedIndex, target.SelectedIndex);
+            Assert.NotEqual(oldSelectedItem, target.SelectedItem);
+
+            Assert.Equal(-1, target.SelectedIndex);
+            Assert.Equal(null, target.SelectedItem);
+
+            Assert.Contains(nameof(target.SelectedIndex), changed);
+            Assert.Contains(nameof(target.SelectedItem), changed);
+        }
+
+        [Fact]
+        public void Preserves_SelectedItem_On_Items_Reset()
+        {
+            var items = new ResettingCollection(new[] { "foo", "bar", "baz" });
+            var target = CreateTarget(source: items);
+
+            target.SelectedItem = "foo";
+
+            Assert.Equal(0, target.SelectedIndex);
+
+            items.Reset(new string[] { "baz", "foo", "bar" });
+
+            Assert.Equal("foo", target.SelectedItem);
+            Assert.Equal(1, target.SelectedIndex);
+        }
+
         [Fact]
         public void Preserves_Selection_On_Source_Changed()
         {
@@ -222,7 +265,7 @@ namespace Avalonia.Controls.UnitTests.Selection
             bool nullSource = false)
         {
             source ??= !nullSource ? new[] { "foo", "bar", "baz" } : null;
-            
+
             var result = new InternalSelectionModel
             {
                 SingleSelect = singleSelect,