fix(dragdrop): Avoid double subscription (since ClearItem not always …

…invoked) + make sure to subscribe event on CanDragItems change

src/Uno.UI/UI/Xaml/Controls/ListViewBase/ListViewBase.DragDrop.cs

@@ -6,6 +6,7 @@
 using System.Collections.ObjectModel;
 using System.Linq;
 using System.Reflection;
+using Windows.ApplicationModel.DataTransfer;
 using Windows.Foundation;
 using Windows.Foundation.Collections;
 using Uno.Extensions;
@@ -17,9 +18,15 @@ namespace Windows.UI.Xaml.Controls
 {
 	public partial class ListViewBase
 	{
+		private const string ReorderOwnerFormatId = DataPackage.UnoPrivateDataPrefix + "__list__view__base__source__";
+		private const string ReorderItemFormatId = DataPackage.UnoPrivateDataPrefix + "__list__view__base__source__item__";
+		private const string ReorderContainerFormatId = DataPackage.UnoPrivateDataPrefix + "__list__view__base__source__container__";
+		private const string DragItemsFormatId = DataPackage.UnoPrivateDataPrefix + "__list__view__base__items__";
+
 		public event DragItemsStartingEventHandler DragItemsStarting;
 		public event TypedEventHandler<ListViewBase, DragItemsCompletedEventArgs> DragItemsCompleted;
 
+		#region CanReorderItems (DP)
 		public static DependencyProperty CanReorderItemsProperty { get; } = DependencyProperty.Register(
 			nameof(CanReorderItems),
 			typeof(bool),
@@ -31,19 +38,69 @@ public bool CanReorderItems
 			get => (bool)GetValue(CanReorderItemsProperty);
 			set => SetValue(CanReorderItemsProperty, value);
 		}
+		#endregion
 
+		#region CanDragItems (DP)
 		public static DependencyProperty CanDragItemsProperty { get; } = DependencyProperty.Register(
 			nameof(CanDragItems),
 			typeof(bool),
 			typeof(ListViewBase),
-			new FrameworkPropertyMetadata(default(bool)));
+			new FrameworkPropertyMetadata(default(bool), OnCanDragItemsChanged));
 
 		public bool CanDragItems
 		{
 			get => (bool)GetValue(CanDragItemsProperty);
 			set => SetValue(CanDragItemsProperty, value);
 		}
 
+		private static void OnCanDragItemsChanged(DependencyObject snd, DependencyPropertyChangedEventArgs args)
+		{
+			if (snd is ListViewBase that && args.NewValue is bool canDragItems)
+			{
+				var items = that.MaterializedContainers.OfType<UIElement>();
+				if (canDragItems)
+				{
+					items.ForEach(PrepareContainerForDragDropCore);
+				}
+				else
+				{
+					items.ForEach(ClearContainerForDragDrop);
+				}
+			}
+		} 
+		#endregion
+
+		private void PrepareContainerForDragDrop(UIElement itemContainer)
+		{
+			if (CanDragItems)
+			{
+				PrepareContainerForDragDropCore(itemContainer);
+			}
+		}
+
+		private static void PrepareContainerForDragDropCore(UIElement itemContainer)
+		{
+			// Known issue: the ContainerClearedForItem might not be invoked properly for all items on some platforms.
+			// This patch is acceptable as event handlers are static (so they won't leak).
+			itemContainer.DragStarting -= OnItemContainerDragStarting;
+			itemContainer.DropCompleted -= OnItemContainerDragCompleted;
+
+			itemContainer.CanDrag = true;
+			itemContainer.DragStarting += OnItemContainerDragStarting;
+			itemContainer.DropCompleted += OnItemContainerDragCompleted;
+		}
+
+		private static void ClearContainerForDragDrop(UIElement itemContainer)
+		{
+			itemContainer.DragStarting -= OnItemContainerDragStarting;
+			itemContainer.DropCompleted -= OnItemContainerDragCompleted;
+
+			itemContainer.DragEnter -= OnReorderUpdated;
+			itemContainer.DragOver -= OnReorderUpdated;
+			itemContainer.DragLeave -= OnReorderCompleted;
+			itemContainer.Drop -= OnReorderCompleted;
+		}
+
 		private static void OnItemContainerDragStarting(UIElement sender, DragStartingEventArgs innerArgs)
 		{
 			if (ItemsControlFromItemContainer(sender) is ListViewBase that && that.CanDragItems)
@@ -70,6 +127,12 @@ private static void OnItemContainerDragStarting(UIElement sender, DragStartingEv
 					args.Data.SetData(ReorderItemFormatId, draggedItem);
 					args.Data.SetData(ReorderContainerFormatId, sender);
 
+					// For safety only, avoids double subscription
+					that.DragEnter -= OnReorderUpdated;
+					that.DragOver -= OnReorderUpdated;
+					that.DragLeave -= OnReorderCompleted;
+					that.Drop -= OnReorderCompleted;
+
 					that.DragEnter += OnReorderUpdated;
 					that.DragOver += OnReorderUpdated;
 					that.DragLeave += OnReorderCompleted;

src/Uno.UI/UI/Xaml/Controls/ListViewBase/ListViewBase.cs

@@ -26,7 +26,6 @@
 using Uno.Client;
 using System.Threading.Tasks;
 using System.Threading;
-using Windows.ApplicationModel.DataTransfer;
 using Windows.Foundation;
 using Uno.UI;
 
@@ -657,29 +656,18 @@ protected override void PrepareContainerForItemOverride(DependencyObject element
 			}
 		}
 
-		private const string ReorderOwnerFormatId = DataPackage.UnoPrivateDataPrefix + "__list__view__base__source__";
-		private const string ReorderItemFormatId = DataPackage.UnoPrivateDataPrefix + "__list__view__base__source__item__";
-		private const string ReorderContainerFormatId = DataPackage.UnoPrivateDataPrefix + "__list__view__base__source__container__";
-		private const string DragItemsFormatId = DataPackage.UnoPrivateDataPrefix + "__list__view__base__items__";
-
 		internal override void ContainerPreparedForItem(object item, SelectorItem itemContainer, int itemIndex)
 		{
 			base.ContainerPreparedForItem(item, itemContainer, itemIndex);
 
-			if (CanDragItems)
-			{
-				itemContainer.CanDrag = true;
-				itemContainer.DragStarting += OnItemContainerDragStarting;
-				itemContainer.DropCompleted += OnItemContainerDragCompleted;
-			}
+			PrepareContainerForDragDrop(itemContainer);
 
 			ContainerContentChanging?.Invoke(this, new ContainerContentChangingEventArgs(item, itemContainer, itemIndex));
 		}
 
 		internal override void ContainerClearedForItem(object item, SelectorItem itemContainer)
 		{
-			itemContainer.DragStarting -= OnItemContainerDragStarting;
-			itemContainer.DropCompleted -= OnItemContainerDragCompleted;
+			ClearContainerForDragDrop(itemContainer);
 
 			base.ContainerClearedForItem(item, itemContainer);
 		}