[Core] Ensure new object reuses view models

Xamarin.PropertyEditing/Extensions.cs

@@ -107,6 +107,21 @@ namespace Xamarin.PropertyEditing
 			self.Add (with);
 		}
 
+		public static void Reset<T> (this ICollection<T> self, IEnumerable<T> newContents)
+		{
+			if (self == null)
+				throw new ArgumentNullException (nameof(self));
+			if (newContents == null)
+				throw new ArgumentNullException (nameof(newContents));
+
+			if (self is ObservableCollectionEx<T> oce) {
+				oce.Reset (newContents);
+			} else {
+				self.Clear();
+				self.AddItems (newContents);
+			}
+		}
+
 		public static void Move (this IList self, int index, int moveTo)
 		{
 			if (self == null)

Xamarin.PropertyEditing/ViewModels/ObjectPropertyViewModel.cs

@@ -90,13 +90,16 @@ namespace Xamarin.PropertyEditing.ViewModels
 				return;
 
 			using (await AsyncWork.RequestAsyncWork (this)) {
-				ValueModel.SelectedObjects.Clear();
+				
 
 				await base.UpdateCurrentValueAsync ();
 				ValueType = CurrentValue?.ValueDescriptor as ITypeInfo;
 
-				if (CurrentValue?.Value != null)
-					ValueModel.SelectedObjects.Add (CurrentValue.Value);
+				if (CurrentValue?.Value != null) {
+					ValueModel.SelectedObjects.Reset (new[] { CurrentValue.Value });
+				} else {
+					ValueModel.SelectedObjects.Clear ();
+				}
 
 				SetCanDelve (ValueModel.SelectedObjects.Count > 0);
 			}