Merge pull request AvaloniaUI#3444 from MarchingCube/fix-selecting-items-control-init

Fix SelectingItemsControl init behavior

Author: grokys

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -5,6 +5,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
+using System.Diagnostics;
 using System.Linq;
 using Avalonia.Collections;
 using Avalonia.Controls.Generators;
@@ -240,17 +241,14 @@ protected SelectionMode SelectionMode
         public override void BeginInit()
         {
             base.BeginInit();
-            ++_updateCount;
-            _updateSelectedIndex = int.MinValue;
+
+            InternalBeginInit();
         }
 
         /// <inheritdoc/>
         public override void EndInit()
         {
-            if (--_updateCount == 0)
-            {
-                UpdateFinished();
-            }
+            InternalEndInit();
 
             base.EndInit();
         }
@@ -437,18 +435,16 @@ protected override void OnContainersRecycled(ItemContainerEventArgs e)
         protected override void OnDataContextBeginUpdate()
         {
             base.OnDataContextBeginUpdate();
-            ++_updateCount;
+
+            InternalBeginInit();
         }
 
         /// <inheritdoc/>
         protected override void OnDataContextEndUpdate()
         {
             base.OnDataContextEndUpdate();
 
-            if (--_updateCount == 0)
-            {
-                UpdateFinished();
-            }
+            InternalEndInit();
         }
 
         protected override void OnKeyDown(KeyEventArgs e)
@@ -1118,6 +1114,26 @@ private void UpdateFinished()
             }
         }
 
+        private void InternalBeginInit()
+        {
+            if (_updateCount == 0)
+            {
+                _updateSelectedIndex = int.MinValue;
+            }
+
+            ++_updateCount;
+        }
+
+        private void InternalEndInit()
+        {
+            Debug.Assert(_updateCount > 0);
+
+            if (--_updateCount == 0)
+            {
+                UpdateFinished();
+            }
+        }
+
         private class Selection : IEnumerable<int>
         {
             private readonly List<int> _list = new List<int>();

tests/Avalonia.Controls.UnitTests/Primitives/SelectingItemsControlTests.cs

@@ -129,6 +129,23 @@ public void SelectedIndex_Should_Be_Minus_1_After_Initialize()
             Assert.Equal(-1, target.SelectedIndex);
         }
 
+        [Fact]
+        public void SelectedIndex_Should_Be_Minus_1_Without_Initialize()
+        {
+            var items = new[]
+            {
+                new Item(),
+                new Item(),
+            };
+
+            var target = new ListBox();
+            target.Items = items;
+            target.Template = Template();
+            target.DataContext = new object();
+
+            Assert.Equal(-1, target.SelectedIndex);
+        }
+
         [Fact]
         public void SelectedIndex_Should_Be_0_After_Initialize_With_AlwaysSelected()
         {