[uikit] UIAccessibility.RequestGuidedAccessSession completion handler is an ObjC block. Fix #59196 (#2917)

Reference:
https://bugzilla.xamarin.com/show_bug.cgi?id=59196

src/UIKit/UIAccessibility.cs

@@ -197,16 +197,21 @@ namespace XamCore.UIKit {
 		// UIAccessibility.h
 		[iOS (7,0)]
 		[DllImport (Constants.UIKitLibrary)]
-		extern static void UIAccessibilityRequestGuidedAccessSession (/* BOOL */ bool enable, /* void(^completionHandler)(BOOL didSucceed) */ Action<bool> completionHandler);
+		extern unsafe static void UIAccessibilityRequestGuidedAccessSession (/* BOOL */ bool enable, /* void(^completionHandler)(BOOL didSucceed) */ void * completionHandler);
 
 		[iOS (7,0)]
 		public static void RequestGuidedAccessSession (bool enable, Action<bool> completionHandler)
 		{
-			callback = completionHandler;
-			if (callback == null)
-				UIAccessibilityRequestGuidedAccessSession (enable, null);
-			else
-				UIAccessibilityRequestGuidedAccessSession (enable, RequestGuidedAccessSessionBridge);
+			unsafe {
+				BlockLiteral *block_ptr_handler;
+				BlockLiteral block_handler;
+				block_handler = new BlockLiteral ();
+				block_ptr_handler = &block_handler;
+				block_handler.SetupBlock (callback, completionHandler);
+
+				UIAccessibilityRequestGuidedAccessSession (enable, (void*) block_ptr_handler);
+				block_ptr_handler->CleanupBlock ();
+			}
 		}
 
 		[iOS (7,0)]
@@ -219,18 +224,18 @@ namespace XamCore.UIKit {
 			return tcs.Task;
 		}
 		
-		static Action<bool> callback;
+		internal delegate void InnerRequestGuidedAccessSession (IntPtr block, bool enable);
+		static readonly InnerRequestGuidedAccessSession callback = TrampolineRequestGuidedAccessSession;
 
-		[MonoPInvokeCallback (typeof (Action<bool>))]
-		static void RequestGuidedAccessSessionBridge (bool didSucceed)
+		[MonoPInvokeCallback (typeof (InnerRequestGuidedAccessSession))]
+		static unsafe void TrampolineRequestGuidedAccessSession (IntPtr block, bool enable)
 		{
-			if (callback != null) {
-				callback (didSucceed);
-				callback = null;
-			}
+			var descriptor = (BlockLiteral *) block;
+			var del = (Action<bool>) (descriptor->Target);
+			if (del != null)
+				del (enable);
 		}
 
-
 		[iOS (8,0)]
 		[DllImport (Constants.UIKitLibrary)]
 		static extern bool UIAccessibilityDarkerSystemColorsEnabled ();

tests/monotouch-test/UIKit/AccessibilityTest.cs

@@ -34,9 +34,9 @@ namespace MonoTouchFixtures.UIKit {
 				Assert.Inconclusive ("Requires iOS7 or later");
 
 			// should not affect execution since it needs to be a "supervised" device (and allowed in MDM)
-			//			UIAccessibility.RequestGuidedAccessSession (true, delegate (bool didSuccess) { 
-			//				Assert.Fail ("should not be reached");
-			//});
+			UIAccessibility.RequestGuidedAccessSession (true, delegate (bool didSuccess) {
+				Assert.False (didSuccess, "devices are not supervised by default");
+			});
 			UIAccessibility.RequestGuidedAccessSession (false, null);
 		}
 	}