Merge pull request #4785 from AvaloniaUI/fixes/4781-treeview-setting-bound-selected-item-null

Fixes/4781 treeview setting bound selected item null

src/Avalonia.Controls/TreeView.cs

@@ -117,10 +117,8 @@ namespace Avalonia.Controls
                 if (value != null)
                 {
                     if (selectedItems.Count != 1 || selectedItems[0] != value)
-                    {
-                        _syncingSelectedItems = true;
-                        SelectSingleItem(value);
-                        _syncingSelectedItems = false;
+                    {                        
+                        SelectSingleItem(value);                        
                     }
                 }
                 else if (SelectedItems.Count > 0)
@@ -219,8 +217,12 @@ namespace Avalonia.Controls
 
         private void SelectSingleItem(object item)
         {
-            SelectedItems.Clear();
+            _syncingSelectedItems = true;
+            SelectedItems.Clear();            
             SelectedItems.Add(item);
+            _syncingSelectedItems = false;
+
+            SetAndRaise(SelectedItemProperty, ref _selectedItem, item);            
         }
 
         /// <summary>

tests/Avalonia.Controls.UnitTests/TreeViewTests.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Linq;
 using Avalonia.Collections;
 using Avalonia.Controls.Presenters;
@@ -13,6 +14,7 @@ using Avalonia.Interactivity;
 using Avalonia.LogicalTree;
 using Avalonia.Styling;
 using Avalonia.UnitTests;
+using ReactiveUI;
 using Xunit;
 
 namespace Avalonia.Controls.UnitTests
@@ -454,6 +456,41 @@ namespace Avalonia.Controls.UnitTests
             }
         }
 
+        [Fact]
+        public void Bound_SelectedItem_Should_Not_Be_Cleared_when_Changing_Selection()
+        {
+            using (Application())
+            {
+                var dataContext = new TestDataContext();
+
+                var target = new TreeView
+                {
+                    Template = CreateTreeViewTemplate(),
+                    DataContext = dataContext
+                };
+
+                target.Bind(TreeView.ItemsProperty, new Binding("Items"));
+                target.Bind(TreeView.SelectedItemProperty, new Binding("SelectedItem"));
+
+                var visualRoot = new TestRoot();
+                visualRoot.Child = target;
+
+                CreateNodeDataTemplate(target);
+                ApplyTemplates(target);
+
+                var selectedValues = new List<object>();
+
+                dataContext.WhenAnyValue(x => x.SelectedItem)
+                    .Subscribe(x => selectedValues.Add(x));
+
+                _mouse.Click((Interactive)target.Presenter.Panel.Children[0], MouseButton.Left);
+                _mouse.Click((Interactive)target.Presenter.Panel.Children[2], MouseButton.Left);
+
+                Assert.Equal(3, selectedValues.Count);
+                Assert.Equal(new[] { null, "Item 0", "Item 2" }, selectedValues.ToArray());
+            }
+        }
+
         [Fact]
         public void LogicalChildren_Should_Be_Set()
         {
@@ -1288,5 +1325,26 @@ namespace Avalonia.Controls.UnitTests
         private class DerivedTreeView : TreeView
         {
         }
+
+        private class TestDataContext : ReactiveObject
+        {
+            private string _selectedItem;
+
+            public TestDataContext()
+            {
+                Items = new ObservableCollection<string>(Enumerable.Range(0, 5).Select(i => $"Item {i}"));
+            }
+
+            public ObservableCollection<string> Items { get; }
+
+            public string SelectedItem
+            {
+                get { return _selectedItem; }
+                set
+                {
+                    this.RaiseAndSetIfChanged(ref _selectedItem, value);
+                }
+            }
+        }
     }
 }