[UIKit] Enable nullability and clean up UIDragDropSessionExtensions.

src/UIKit/UIDragDropSessionExtensions.cs

@@ -9,44 +9,49 @@
 
 #if !TVOS
 
-// Disable until we get around to enable + fix any issues.
-#nullable disable
+#nullable enable
 
 namespace UIKit {
 
-	/// <summary>Contains methods for working with drag-and-drop sessions, including a default implementation of <see cref="UIKit.IUIDropSession.LoadObjects(ObjCRuntime.Class,System.Action{Foundation.INSItemProviderReading[]})" />.</summary>
-	///     <remarks>To be added.</remarks>
+	/// <summary>Contains extension methods for working with drag-and-drop sessions.</summary>
 	public static class UIDragDropSessionExtensions {
 
-		/// <typeparam name="T">To be added.</typeparam>
-		///         <param name="session">To be added.</param>
-		///         <param name="completion">To be added.</param>
-		///         <summary>To be added.</summary>
-		///         <returns>To be added.</returns>
-		///         <remarks>To be added.</remarks>
-		public static NSProgress LoadObjects<T> (this IUIDropSession session, Action<T []> completion) where T : NSObject, INSItemProviderReading
+		/// <summary>Loads objects of the specified type from the drop session and invokes the completion handler when done.</summary>
+		/// <typeparam name="T">The type of objects to load from the drop session.</typeparam>
+		/// <param name="session">The drop session to load objects from.</param>
+		/// <param name="completion">The completion handler to invoke with the loaded objects.</param>
+		/// <returns>An <see cref="NSProgress" /> object that can be used to track the loading progress.</returns>
+		public static NSProgress LoadObjects<T> (this IUIDropSession session, Action<T []?> completion) where T : NSObject, INSItemProviderReading
 		{
+			// allowing null for 'completion' doesn't make much sense, but we have tests verifying that it's allowed,
+			// so make the implementation work with a null 'completion' handler.
+			ArgumentNullException.ThrowIfNull (session);
+
 			return session.LoadObjects (new Class (typeof (T)), (v) => {
 				var arr = v as T [];
 				if (arr is null && v is not null) {
 					arr = new T [v.Length];
 					for (int i = 0; i < arr.Length; i++) {
 						if (v [i] is not null)
-							arr [i] = Runtime.ConstructNSObject<T> (v [i].Handle);
+							arr [i] = Runtime.ConstructNSObject<T> (v [i].Handle)!;
 					}
 				}
+				GC.KeepAlive (v);
 
-				completion (arr);
+				if (completion is not null)
+					completion (arr);
 			});
 		}
 
+		/// <summary>Returns <see langword="true" /> if the specified <paramref name="session" /> can instantiate items of the specified <paramref name="type" />.</summary>
 		/// <param name="session">The session to query.</param>
-		///         <param name="type">The type of object to query about.</param>
-		///         <summary>Returns <see langword="true" /> if the specified <paramref name="session" /> can instantiate items of the specified <paramref name="type" />.</summary>
-		///         <returns>To be added.</returns>
-		///         <remarks>To be added.</remarks>
+		/// <param name="type">The type of object to query about.</param>
+		/// <returns><see langword="true" /> if the session can load objects of the specified type; otherwise, <see langword="false" />.</returns>
 		public static bool CanLoadObjects (this IUIDragDropSession session, Type type)
 		{
+			ArgumentNullException.ThrowIfNull (session);
+			ArgumentNullException.ThrowIfNull (type);
+
 			return session.CanLoadObjects (new Class (type));
 		}
 	}

tests/cecil-tests/HandleSafety.KnownFailures.cs

@@ -218,7 +218,6 @@ public partial class HandleSafetyTest {
 			"UIKit.UIAlertView..ctor (System.String, System.String, UIKit.IUIAlertViewDelegate, System.String, System.String[])",
 			"UIKit.UIAppearance.Equals (System.Object)",
 			"UIKit.UIContentSizeCategoryExtensions.Compare (UIKit.UIContentSizeCategory, UIKit.UIContentSizeCategory)",
-			"UIKit.UIDragDropSessionExtensions/<>c__DisplayClass0_0`1.<LoadObjects>b__0 (Foundation.INSItemProviderReading[])",
 			"UIKit.UIFontFeature.ObjCRuntime.INativeObject.get_Handle ()",
 			"UIKit.UIPasteboard.set_Images (UIKit.UIImage[])",
 			"UIKit.UIToolbar.SetItems (UIKit.UIBarButtonItem[], System.Boolean)",

tests/monotouch-test/UIKit/UIDragDropSessionExtensionsTest.cs

@@ -25,12 +25,30 @@ public void LoadObjectsTest ()
 				Assert.Ignore ("Ignoring tests: Requires iOS11+");
 
 			var test = new DropSession ();
+			Assert.That (test.CalledCanLoadObjects, Is.False, "CalledCanLoadObjects #1");
 			test.CanLoadObjects (typeof (UIImage));
+			Assert.That (test.CalledCanLoadObjects, Is.True, "CalledCanLoadObjects #2");
+
+			Assert.That (test.CalledLoadObjects, Is.False, "CalledLoadObjects #1");
 			test.LoadObjects<UIImage> (null);
+			Assert.That (test.CalledLoadObjects, Is.True, "CalledLoadObjects #2");
+
+			test.CalledLoadObjects = false;
+			Assert.That (test.CalledLoadObjects, Is.False, "CalledLoadObjects #1");
+			var calledCallback = false;
+			test.LoadObjects<UIImage> ((arr) => {
+				calledCallback = true;
+			});
+			Assert.That (calledCallback, Is.True, "calledCallback");
+			Assert.That (test.CalledLoadObjects, Is.True, "CalledLoadObjects #2");
+
 		}
 	}
 
 	class DropSession : NSObject, IUIDropSession {
+		public bool CalledLoadObjects;
+		public bool CalledCanLoadObjects;
+
 		public IUIDragSession LocalDragSession => throw new NotImplementedException ();
 
 		public UIDropSessionProgressIndicatorStyle ProgressIndicatorStyle { get => throw new NotImplementedException (); set => throw new NotImplementedException (); }
@@ -43,6 +61,7 @@ class DropSession : NSObject, IUIDropSession {
 
 		public bool CanLoadObjects (Class itemProviderReadingClass)
 		{
+			CalledCanLoadObjects = true;
 			Assert.That (itemProviderReadingClass.Handle, Is.EqualTo (new Class (typeof (UIImage)).Handle), "UIDragDropSessionExtensionsTest did not convert the type properly for 'CanLoadObjects'.");
 			return true;
 		}
@@ -54,7 +73,9 @@ public bool HasConformingItems (string [] typeIdentifiers)
 
 		public NSProgress LoadObjects (Class itemProviderReadingClass, Action<INSItemProviderReading []> completion)
 		{
+			CalledLoadObjects = true;
 			Assert.That (itemProviderReadingClass.Handle, Is.EqualTo (new Class (typeof (UIImage)).Handle), "UIDragDropSessionExtensionsTest did not convert the type properly for 'LoadObjects'.");
+			completion (Array.Empty<INSItemProviderReading> ());
 			return new NSProgress ();
 		}