Add 'useYoga' property to indicate whether UIView uses Yoga for layout or not

Summary: Now developer could check whether UIView uses Yoga for layout.

Reviewed By: emilsjolander

Differential Revision: D5881242

fbshipit-source-id: 4d1cf0b489b759ff0b9c5091cc51534fd12dcf60

YogaKit/Source/UIView+Yoga.m

@@ -26,6 +26,11 @@ static const void *kYGYogaAssociatedKey = &kYGYogaAssociatedKey;
   return yoga;
 }
 
+- (BOOL)isYogaEnabled
+{
+  return objc_getAssociatedObject(self, kYGYogaAssociatedKey) != nil;
+}
+
 - (void)configureLayoutWithBlock:(YGLayoutConfigurationBlock)block
 {
   if (block != nil) {