Made accesses to ApplicationIconBadgeNumber always use the main queue.

In Xamarin.iOS, any accessess made to a UIKit property that's labeled as `nonatomic` on a thread other than the main thread results in an exception being thrown. This was causing installation saves to fail quietly (very bad).

We solve this problem by forcing usage of these properties onto the main queue, and although this provides an avenue for deadlock, it's better than simply having silent failures in most cases.

Author: richardjrossiii

Parse/ParseInstallation.iOS.cs

@@ -10,6 +10,7 @@
 using Foundation;
 using UIKit;
 using System.Threading.Tasks;
+using CoreFoundation;
 
 namespace Parse {
   public partial class ParseInstallation : ParseObject {
@@ -54,16 +55,39 @@ public void SetDeviceTokenFromData(NSData deviceToken) {
     public int Badge {
       get {
         if (CurrentInstallationController.IsCurrent(this)) {
-          SetProperty<int>((int)UIApplication.SharedApplication.ApplicationIconBadgeNumber);
+          RunOnUIQueue(() =>
+            SetProperty<int>((int)UIApplication.SharedApplication.ApplicationIconBadgeNumber)
+          );
         }
         return GetProperty<int>();
       }
       set {
         int badge = value;
         // Set the UI, too
-        UIApplication.SharedApplication.ApplicationIconBadgeNumber = (nint)badge;
+        RunOnUIQueue(() => {
+          UIApplication.SharedApplication.ApplicationIconBadgeNumber = (nint)badge;
+        });
         SetProperty<int>(badge);
       }
     }
+
+    /// <summary>
+    /// Runs (synchronously) the on user interface queue.
+    ///
+    /// Note that this could cause deadlocks, if the main thread is waiting on
+    /// the background thread we're calling this from.
+    ///
+    /// However, Xamarin's native bindings always threw exceptions when calling
+    /// into UIKit code from a background thread, so this at least helps us in
+    /// some deadlock scenarios.
+    /// </summary>
+    /// <param name="action">Action.</param>
+    private static void RunOnUIQueue(Action action) {
+      if (NSThread.IsMain) {
+        action();
+      } else {
+        DispatchQueue.MainQueue.DispatchSync(action);
+      }
+    }
   }
-}
+}