Fix TreeView missing children issue

dev/TreeView/InteractionTests/TreeViewTests.cs

@@ -2815,6 +2815,34 @@ public void ItemsSourceResyncTest()
             }
         }
 
+        [TestMethod]
+        [TestProperty("TestSuite", "B")]
+        public void ItemsSourceSwitchForthAndBackTest()
+        {
+            // TreeView databinding only works on RS5+
+            if (IsLowerThanRS5())
+            {
+                return;
+            }
+
+            using (var setup = new TestSetupHelper("TreeView Tests"))
+            {
+                SetContentMode(true);
+
+                ClickButton("SwapItemsSource");
+                Wait.ForIdle();
+                ClickButton("GetItemCount");
+                Verify.AreEqual("2", ReadResult());
+
+                ClickButton("SwapItemsSource");
+                Wait.ForIdle();
+                ClickButton("ExpandRootNode");
+                Wait.ForIdle();
+                ClickButton("GetItemCount");
+                Verify.AreEqual("4", ReadResult());
+            }
+        }
+
         private void ClickButton(string buttonName)
         {
             var button = new Button(FindElement.ByName(buttonName));

dev/TreeView/TestUI/TreeViewPage.xaml

@@ -74,6 +74,7 @@
                     <Button x:Name="ResetItemsSource" AutomationProperties.Name="ResetItemsSource" Content="ResetItemsSource" HorizontalAlignment="Stretch" Margin="1" Click="ResetItemsSource_Click"/>
                     <Button x:Name="ResetItemsSourceAsync" AutomationProperties.Name="ResetItemsSourceAsync" Content="ResetItemsSourceAsync" HorizontalAlignment="Stretch" Margin="1" Click="ResetItemsSourceAsync_Click"/>
                     <Button x:Name="ExpandRootNode" AutomationProperties.Name="ExpandRootNode" Content="ExpandRootNode" HorizontalAlignment="Stretch" Margin="1" Click="ExpandRootNode_Click"/>
+                    <Button x:Name="SwapItemsSource" AutomationProperties.Name="SwapItemsSource" Content="SwapItemsSource" HorizontalAlignment="Stretch" Margin="1" Click="SwapItemsSource_Click"/>
                 </StackPanel>
             </ScrollViewer>
             <ScrollViewer Grid.Column="1">

dev/TreeView/TestUI/TreeViewPage.xaml.cs

@@ -998,5 +998,17 @@ private void ExpandRootNode_Click(object sender, RoutedEventArgs e)
         {
             TestTreeViewItemsSource[0].IsExpanded = !TestTreeViewItemsSource[0].IsExpanded;
         }
+
+        private void SwapItemsSource_Click(object sender, RoutedEventArgs e)
+        {
+            if (ContentModeTestTreeView.ItemsSource == TestTreeView2ItemsSource)
+            {
+                this.ContentModeTestTreeView.ItemsSource = TestTreeViewItemsSource;
+            }
+            else
+            {
+                this.ContentModeTestTreeView.ItemsSource = TestTreeView2ItemsSource;
+            }
+        }
     }
 }

dev/TreeView/TreeViewItem.cpp

@@ -322,16 +322,7 @@ void TreeViewItem::OnPropertyChanged(const winrt::DependencyPropertyChangedEvent
         }
         else if (property == s_ItemsSourceProperty)
         {
-            winrt::IInspectable value = args.NewValue();
-
-            auto treeViewNode = winrt::get_self<TreeViewNode>(node);
-            treeViewNode->ItemsSource(value);
-            if (IsInContentMode())
-            {
-                // The children have changed, validate and update GlyphOpacity
-                bool hasChildren = HasUnrealizedChildren() || treeViewNode->HasChildren();
-                GlyphOpacity(hasChildren ? 1.0 : 0.0);
-            }
+            SetItemsSource(node, args.NewValue());
         }
         else if (property == s_HasUnrealizedChildrenProperty)
         {
@@ -341,6 +332,18 @@ void TreeViewItem::OnPropertyChanged(const winrt::DependencyPropertyChangedEvent
     }
 }
 
+void TreeViewItem::SetItemsSource(winrt::TreeViewNode const& node, winrt::IInspectable const& value)
+{
+    auto treeViewNode = winrt::get_self<TreeViewNode>(node);
+    treeViewNode->ItemsSource(value);
+    if (IsInContentMode())
+    {
+        // The children have changed, validate and update GlyphOpacity
+        bool hasChildren = HasUnrealizedChildren() || treeViewNode->HasChildren();
+        GlyphOpacity(hasChildren ? 1.0 : 0.0);
+    }
+}
+
 void TreeViewItem::OnExpandContentTimerTick(const winrt::IInspectable& /*sender*/, const winrt::IInspectable& /*e*/)
 {
     if (m_expandContentTimer)

dev/TreeView/TreeViewItem.h

@@ -39,6 +39,8 @@ class TreeViewItem :
 
     void OnPropertyChanged(const winrt::DependencyPropertyChangedEventArgs& args);
 
+    void SetItemsSource(winrt::TreeViewNode const& node, winrt::IInspectable const& value);
+
 public:
     // IFrameworkElementOverrides
     void OnApplyTemplate();

dev/TreeView/TreeViewList.cpp

@@ -100,7 +100,18 @@ void TreeViewList::OnContainerContentChanging(const winrt::IInspectable& /*sende
     {
         auto targetItem = args.ItemContainer().as<winrt::TreeViewItem>();
         auto targetNode = NodeFromContainer(targetItem);
+
         auto treeViewItem = winrt::get_self<TreeViewItem>(targetItem);
+        auto treeViewNode = winrt::get_self<TreeViewNode>(targetNode);
+
+        if (auto itemsSource = targetItem.ItemsSource())
+        {
+            if (treeViewNode->ItemsSource() == nullptr)
+            {
+                treeViewItem->SetItemsSource(targetNode, itemsSource);
+            }
+        }   
+
         treeViewItem->UpdateIndentation(targetNode.Depth());
         treeViewItem->UpdateSelectionVisual(winrt::get_self<TreeViewNode>(targetNode)->SelectionState());
     }