Better null handling

In CvCell in my app I was getting some weird null exceptions that while they do not make a lot of sense to get to the GetCell call, I can do a bit better job of mitigating those exceptions to originate from this library.
Redth · Apr 18, 2024 · ca708e9 · ca708e9
1 parent 0fa6337
commit ca708e9
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Showing 4 changed files with 171 additions and 145 deletions.
diff --git a/VirtualListView/Apple/CvCell.ios.maccatalyst.cs b/VirtualListView/Apple/CvCell.ios.maccatalyst.cs
@@ -9,6 +9,8 @@ namespace Microsoft.Maui;
 
 internal class CvCell : UICollectionViewCell
 {
+	internal const string ReuseIdUnknown = "UNKNOWN";
+
 	public VirtualListViewHandler Handler { get; set; }
 
 	public WeakReference<NSIndexPath> IndexPath { get; set; }

diff --git a/VirtualListView/Apple/CvDataSource.ios.maccatalyst.cs b/VirtualListView/Apple/CvDataSource.ios.maccatalyst.cs
@@ -1,4 +1,5 @@
-using Foundation;
+#nullable enable
+using Foundation;
 using UIKit;
 
 namespace Microsoft.Maui;
@@ -20,54 +21,74 @@ public CvDataSource(VirtualListViewHandler handler)
 
 	public override nint NumberOfSections(UICollectionView collectionView)
 		=> 1;
-
+	
 	public override UICollectionViewCell GetCell(UICollectionView collectionView, NSIndexPath indexPath)
 	{
 		var info = Handler?.PositionalViewSelector?.GetInfo(indexPath.Item.ToInt32());
 
-		var data = Handler?.PositionalViewSelector?.Adapter?.DataFor(info.Kind, info.SectionIndex, info.ItemIndex);
-
-		var reuseId = Handler?.PositionalViewSelector?.ViewSelector?.GetReuseId(info, data);
+		object? data = null;
 
-		var nativeReuseId = info.Kind switch
+		var nativeReuseId = CvCell.ReuseIdUnknown;
+
+		if (info is not null)
 		{
-			PositionKind.Item => itemIdManager.GetReuseId(collectionView, reuseId),
-			PositionKind.SectionHeader => sectionHeaderIdManager.GetReuseId(collectionView, reuseId),
-			PositionKind.SectionFooter => sectionFooterIdManager.GetReuseId(collectionView, reuseId),
-			PositionKind.Header => globalIdManager.GetReuseId(collectionView, reuseId),
-			PositionKind.Footer => globalIdManager.GetReuseId(collectionView, reuseId),
-			_ => "UNKNOWN",
-		};
-
-		var cell = (collectionView.DequeueReusableCell(nativeReuseId, indexPath) as CvCell)!;
+			data = Handler?.PositionalViewSelector?.Adapter?.DataFor(info.Kind, info.SectionIndex, info.ItemIndex);
+
+			if (data is not null)
+			{
+				var reuseId = Handler?.PositionalViewSelector?.ViewSelector?.GetReuseId(info, data);
+
+				nativeReuseId = info.Kind switch
+				{
+					PositionKind.Item => itemIdManager.GetReuseId(collectionView, reuseId),
+					PositionKind.SectionHeader => sectionHeaderIdManager.GetReuseId(collectionView, reuseId),
+					PositionKind.SectionFooter => sectionFooterIdManager.GetReuseId(collectionView, reuseId),
+					PositionKind.Header => globalIdManager.GetReuseId(collectionView, reuseId),
+					PositionKind.Footer => globalIdManager.GetReuseId(collectionView, reuseId),
+					_ => CvCell.ReuseIdUnknown,
+				};
+			}
+		}
+
+		var nativeCell = collectionView.DequeueReusableCell(nativeReuseId, indexPath);
+		if (nativeCell is not CvCell cell)
+			return (UICollectionViewCell)nativeCell;
+
 		cell.SetTapHandlerCallback(TapCellHandler);
 		cell.Handler = Handler;
 		cell.IndexPath = new WeakReference<NSIndexPath>(indexPath);
 
 		cell.ReuseCallback = new WeakReference<Action<IView>>((rv) =>
 		{
-			if (cell?.VirtualView?.TryGetTarget(out var cellVirtualView) ?? false)
-				Handler.VirtualView.ViewSelector.ViewDetached(info, cellVirtualView);
+			if (info is not null && (cell.VirtualView?.TryGetTarget(out var cellView) ?? false))
+				Handler?.VirtualView?.ViewSelector?.ViewDetached(info, cellView);
 		});
 
-		if (info.SectionIndex < 0 || info.ItemIndex < 0)
-			info.IsSelected = false;
-		else
-			info.IsSelected = Handler?.IsItemSelected(info.SectionIndex, info.ItemIndex) ?? false;
+		if (info is not null)
+		{
+			if (info.SectionIndex < 0 || info.ItemIndex < 0)
+				info.IsSelected = false;
+			else
+				info.IsSelected = Handler?.IsItemSelected(info.SectionIndex, info.ItemIndex) ?? false;
+		}
 
-		if (cell.NeedsView)
+		if (cell.NeedsView && info is not null && data is not null)
 		{
 			var view = Handler?.PositionalViewSelector?.ViewSelector?.CreateView(info, data);
-			cell.SetupView(view);
+			if (view is not null)
+				cell.SetupView(view);
 		}
 
-		cell.UpdatePosition(info);
-
-		if (cell.VirtualView.TryGetTarget(out var cellVirtualView))
+		if (info is not null)
 		{
-			Handler?.PositionalViewSelector?.ViewSelector?.RecycleView(info, data, cellVirtualView);
+			cell.UpdatePosition(info);
+
+			if (data is not null && (cell.VirtualView?.TryGetTarget(out var cellVirtualView) ?? false))
+			{
+				Handler?.PositionalViewSelector?.ViewSelector?.RecycleView(info, data, cellVirtualView);
 
-			Handler.VirtualView.ViewSelector.ViewAttached(info, cellVirtualView);
+				Handler?.VirtualView?.ViewSelector?.ViewAttached(info, cellVirtualView);
+			}
 		}
 
 		return cell;

diff --git a/VirtualListView/Apple/CvDelegate.ios.maccatalyst.cs b/VirtualListView/Apple/CvDelegate.ios.maccatalyst.cs
@@ -11,6 +11,7 @@ public CvDelegate(VirtualListViewHandler handler, UICollectionView collectionVie
 	{
 		Handler = handler;
 		NativeCollectionView = new WeakReference<UICollectionView>(collectionView);
+		collectionView.RegisterClassForCell(typeof(CvCell), CvCell.ReuseIdUnknown);
 	}
 
 	internal readonly WeakReference<UICollectionView> NativeCollectionView;
@@ -27,20 +28,22 @@ public override void ItemDeselected(UICollectionView collectionView, NSIndexPath
 	void HandleSelection(UICollectionView collectionView, NSIndexPath indexPath, bool selected)
 	{
 		//UIView.AnimationsEnabled = false;
-		var selectedCell = collectionView.CellForItem(indexPath) as CvCell;
-
-		if ((selectedCell?.PositionInfo?.Kind ?? PositionKind.Header) == PositionKind.Item)
+		if (collectionView.CellForItem(indexPath) is CvCell selectedCell
+		    && (selectedCell.PositionInfo?.Kind ?? PositionKind.Header) == PositionKind.Item)
 		{
 			selectedCell.UpdateSelected(selected);
 
-			var itemPos = new ItemPosition(
-				selectedCell.PositionInfo.SectionIndex,
-				selectedCell.PositionInfo.ItemIndex);
+			if (selectedCell.PositionInfo is not null)
+			{
+				var itemPos = new ItemPosition(
+					selectedCell.PositionInfo.SectionIndex,
+					selectedCell.PositionInfo.ItemIndex);
 
-			if (selected)
-				Handler?.VirtualView?.SelectItem(itemPos);
-			else
-				Handler?.VirtualView?.DeselectItem(itemPos);
+				if (selected)
+					Handler?.VirtualView?.SelectItem(itemPos);
+				else
+					Handler?.VirtualView?.DeselectItem(itemPos);
+			}
 		}
 	}