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.h

@@ -20,6 +20,10 @@ typedef void (^YGLayoutConfigurationBlock)(YGLayout *);
  The YGLayout that is attached to this view. It is lazily created.
  */
 @property (nonatomic, readonly, strong) YGLayout *yoga;
+/**
+ Indicates whether or not Yoga is enabled
+ */
+@property (nonatomic, readonly, assign) BOOL isYogaEnabled;
 
 /**
  In ObjC land, every time you access `view.yoga.*` you are adding another `objc_msgSend`

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) {