Use ItemCount to determine PseudoClass in ItemsControl for better consistency

src/Avalonia.Controls/ItemsControl.cs

@@ -70,7 +70,7 @@ static ItemsControl()
         /// </summary>
         public ItemsControl()
         {
-            PseudoClasses.Add(":empty");
+            UpdatePseudoClasses(0);
             SubscribeToItems(_items);
         }
 
@@ -323,6 +323,16 @@ protected override void OnKeyDown(KeyEventArgs e)
             base.OnKeyDown(e);
         }
 
+        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        {
+            base.OnPropertyChanged(change);
+
+            if (change.Property == ItemCountProperty)
+            {
+                UpdatePseudoClasses(change.NewValue.GetValueOrDefault<int>());
+            }
+        }
+
         /// <summary>
         /// Called when the <see cref="Items"/> property changes.
         /// </summary>
@@ -371,10 +381,6 @@ protected virtual void ItemsCollectionChanged(object sender, NotifyCollectionCha
             }
 
             Presenter?.ItemsChanged(e);
-
-            var collection = sender as ICollection;
-            PseudoClasses.Set(":empty", collection == null || collection.Count == 0);
-            PseudoClasses.Set(":singleitem", collection != null && collection.Count == 1);
         }
 
         /// <summary>
@@ -431,9 +437,6 @@ private void RemoveControlItemsFromLogicalChildren(IEnumerable items)
         /// <param name="items">The items collection.</param>
         private void SubscribeToItems(IEnumerable items)
         {
-            PseudoClasses.Set(":empty", items == null || items.Count() == 0);
-            PseudoClasses.Set(":singleitem", items != null && items.Count() == 1);
-
             if (items is INotifyCollectionChanged incc)
             {
                 CollectionChangedEventManager.Instance.AddListener(incc, this);
@@ -469,6 +472,12 @@ private void UpdateItemCount()
             }
         }
 
+        private void UpdatePseudoClasses(int itemCount)
+        {
+            PseudoClasses.Set(":empty", itemCount == 0);
+            PseudoClasses.Set(":singleitem", itemCount == 1);
+        }
+
         protected static IInputElement GetNextControl(
             INavigableContainer container,
             NavigationDirection direction,

tests/Avalonia.Controls.UnitTests/ItemsControlTests.cs

@@ -379,6 +379,17 @@ public void Empty_Class_Should_Be_Cleared_When_Items_Added()
             Assert.DoesNotContain(":empty", target.Classes);
         }
 
+        [Fact]
+        public void Empty_Class_Should_Be_Set_When_Items_Not_Set()
+        {
+            var target = new ItemsControl()
+            {
+                Template = GetTemplate(),
+            };
+
+            Assert.Contains(":empty", target.Classes);
+        }
+
         [Fact]
         public void Empty_Class_Should_Be_Set_When_Empty_Collection_Set()
         {
@@ -393,6 +404,118 @@ public void Empty_Class_Should_Be_Set_When_Empty_Collection_Set()
             Assert.Contains(":empty", target.Classes);
         }
 
+        [Fact]
+        public void Item_Count_Should_Be_Set_When_Items_Added()
+        {
+            var target = new ItemsControl()
+            {
+                Template = GetTemplate(),
+                Items = new[] { 1, 2, 3 },
+            };
+
+            Assert.Equal(3, target.ItemCount);
+        }
+
+        [Fact]
+        public void Item_Count_Should_Be_Set_When_Items_Changed()
+        {
+            var items = new ObservableCollection<int>() { 1, 2, 3 };
+
+            var target = new ItemsControl()
+            {
+                Template = GetTemplate(),
+                Items = items,
+            };
+
+            items.Add(4);
+
+            Assert.Equal(4, target.ItemCount);
+
+            items.Clear();
+
+            Assert.Equal(0, target.ItemCount);
+        }
+
+        [Fact]
+        public void Empty_Class_Should_Be_Set_When_Items_Collection_Cleared()
+        {
+            var items = new ObservableCollection<int>() { 1, 2, 3 };
+
+            var target = new ItemsControl()
+            {
+                Template = GetTemplate(),
+                Items = items,
+            };
+
+            items.Clear();
+
+            Assert.Contains(":empty", target.Classes);
+        }
+
+        [Fact]
+        public void Empty_Class_Should_Not_Be_Set_When_Items_Collection_Count_Increases()
+        {
+            var items = new ObservableCollection<int>() { };
+
+            var target = new ItemsControl()
+            {
+                Template = GetTemplate(),
+                Items = items,
+            };
+
+            items.Add(1);
+
+            Assert.DoesNotContain(":empty", target.Classes);
+        }
+
+        [Fact]
+        public void Single_Item_Class_Should_Be_Set_When_Items_Collection_Count_Increases_To_One()
+        {
+            var items = new ObservableCollection<int>() { };
+
+            var target = new ItemsControl()
+            {
+                Template = GetTemplate(),
+                Items = items,
+            };
+
+            items.Add(1);
+
+            Assert.Contains(":singleitem", target.Classes);
+        }
+
+        [Fact]
+        public void Empty_Class_Should_Not_Be_Set_When_Items_Collection_Cleared()
+        {
+            var items = new ObservableCollection<int>() { 1, 2, 3 };
+
+            var target = new ItemsControl()
+            {
+                Template = GetTemplate(),
+                Items = items,
+            };
+
+            items.Clear();
+
+            Assert.DoesNotContain(":singleitem", target.Classes);
+        }
+
+        [Fact]
+        public void Single_Item_Class_Should_Not_Be_Set_When_Items_Collection_Count_Increases_Beyond_One()
+        {
+            var items = new ObservableCollection<int>() { 1 };
+
+            var target = new ItemsControl()
+            {
+                Template = GetTemplate(),
+                Items = items,
+            };
+
+            items.Add(2);
+
+            Assert.DoesNotContain(":singleitem", target.Classes);
+        }
+
         [Fact]
         public void Setting_Presenter_Explicitly_Should_Set_Item_Parent()
         {