@implementation UIViewController (TourGuide)

+ (void)load {
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
   SEL viewWillAppearSelector = @selector(viewWillAppear:);
   SEL tourGuideWillAppearSelector =  @selector(tourGuideWillAppear:);
   Method originalMethod = class_getInstanceMethod(self, viewWillAppearSelector);
   Method newMethod = class_getInstanceMethod(self, tourGuideWillAppearSelector);
    
   BOOL methodAdded = class_addMethod([self class],
                                      viewWillAppearSelector,
                                      method_getImplementation(newMethod),
                                      method_getTypeEncoding(newMethod));
     
   if (methodAdded) {
   class_replaceMethod([self class], 
                         tourGuideWillAppearSelector, 
                         method_getImplementation(originalMethod),
                         method_getTypeEncoding(originalMethod));
   } else {
     method_exchangeImplementations(originalMethod, newMethod);
   }
  });
}