Singleton macro with blocks using GCD

SingletonClass.m

#define SingletonWithBlock(block)   static dispatch_once_t pred = 0; \
                                      __strong static id _sharedObject = nil; \
                                      dispatch_once(&pred, ^{ \
                                        _sharedObject = block(); \
                                      }); \
                                      return _sharedObject; \

@implementation SingletonClass

+ (id)sharedInstance
{
  SingletonWithBlock(^{
    return [[self alloc] init];
  });
}

@end