Fix repeater holding onto elements (#1723)

* If a consumer changes the ItemsSource with a non recycling layout then the items from the old source would be perpetually held by repeater, a potentially substantial leak.

* Add a test

Author: StephenLPeters

dev/Repeater/APITests/RepeaterTests.cs

@@ -28,6 +28,7 @@
 using System.Collections.ObjectModel;
 using System.Threading;
 using System.Collections.Generic;
+using Windows.UI.Xaml.Tests.MUXControls.ApiTests.RepeaterTests.Common.Mocks;
 
 namespace Windows.UI.Xaml.Tests.MUXControls.ApiTests.RepeaterTests
 {
@@ -145,6 +146,35 @@ public void ValidateGetSetItemsSource()
             });
         }
 
+        [TestMethod]
+        public void VerifyClearingItemsSourceClearsElements()
+        {
+            var data = new ObservableCollection<string>(Enumerable.Range(0, 4).Select(i => "Item #" + i));
+            var mapping = (List<ContentControl>)null;
+
+            RunOnUIThread.Execute(() =>
+            {
+                mapping = Enumerable.Range(0, data.Count).Select(i => new ContentControl { Width = 40, Height = 40 }).ToList();
+
+                var dataSource = MockItemsSource.CreateDataSource(data, supportsUniqueIds: false);
+                var elementFactory = MockElementFactory.CreateElementFactory(mapping);
+                ItemsRepeater repeater = new ItemsRepeater();
+                repeater.ItemsSource = dataSource;
+                repeater.ItemTemplate = elementFactory;
+                // This was an issue only for NonVirtualizing layouts
+                repeater.Layout = new MyCustomNonVirtualizingStackLayout();
+                Content = repeater;
+                Content.UpdateLayout();
+
+                repeater.ItemsSource = null;
+            });
+
+            foreach(var item in mapping)
+            {
+                Verify.IsNull(item.Parent);
+            }
+        }
+
         [TestMethod]
         public void ValidateGetSetBackground()
         {

dev/Repeater/ItemsRepeater.cpp

@@ -546,6 +546,8 @@ void ItemsRepeater::OnDataSourcePropertyChanged(const winrt::ItemsSourceView& ol
                     ClearElementImpl(element);
                 }
             }
+
+            Children().Clear();
         }
 
         InvalidateMeasure();