[TVMLKit] Work around Apple breaking API for enum values in TVElementUpdateType. (#5849)

* [TVMLKit] Work around Apple breaking API for enum values in TVElementUpdateType.

Reference: https://trello.com/c/Eyox7vlH/118-41123682-tvelementupdatetype-enum-values-changed
Reference: rdar://41123682 (closed as by design)

* Use this workaround even after XAMCORE_4_0.

Apple's breaking changes don't disappear in XAMCORE_4_0.

* Fix compiler warnings and Export attribute location.

src/TVMLKit/TVViewElement.cs

@@ -1,6 +1,7 @@
 using System;
 using Foundation;
 using ObjCRuntime;
+using UIKit;
 
 namespace TVMLKit {
 
@@ -16,6 +17,39 @@ namespace TVMLKit {
 		public bool IsDispatched { get; set; }
 
 		public bool IsCancelled { get; set; }
+#endif
+	}
+
+	public partial class TVViewElement {
+#if !COREBUILD
+		public virtual TVElementUpdateType UpdateType {
+			[Export ("updateType")]
+			get {
+				var value = _UpdateType;
+				switch ((long) value) {
+				case 2:
+					if (UIDevice.CurrentDevice.CheckSystemVersion (12, 0)) {
+						return TVElementUpdateType.Styles;
+					} else {
+						return TVElementUpdateType.Children;
+					}
+				case 3:
+					if (UIDevice.CurrentDevice.CheckSystemVersion (12, 0)) {
+						return TVElementUpdateType.Children;
+					} else {
+						return TVElementUpdateType.Self;
+					}
+				case 4:
+					if (UIDevice.CurrentDevice.CheckSystemVersion (12, 0)) {
+						return TVElementUpdateType.Self;
+					} else {
+						return TVElementUpdateType.Styles;
+					}
+				default:
+					return value;
+				}
+			}
+		}
 #endif
 	}
 }

src/tvmlkit.cs

@@ -86,10 +86,17 @@ namespace TVMLKit {
 	public enum TVElementUpdateType : long {
 		None,
 		Subtree,
+#if XAMCORE_4_0
+		[TV (10,0)]
+		Styles,
+		Children,
+		Self,
+#else
 		Children,
 		Self,
 		[TV (10,0)]
 		Styles,
+#endif
 	}
 
 	[TV (9,0)]
@@ -819,8 +826,9 @@ namespace TVMLKit {
 			set;
 		}
 
+		[Internal][Sealed]
 		[Export ("updateType")]
-		TVElementUpdateType UpdateType { get; }
+		TVElementUpdateType _UpdateType { get; }
 
 		[Export ("resetProperty:")]
 		void Reset (TVElementResettableProperty resettableProperty);