单例创建 dispatch_once @synchronized

1、@synchronized

static InstanceClass *instance;


+ (InstanceClass *)defaultInstance{


   @synchronized (self){


     if (instance == nil) {

        instance = [[InstanceClass alloc] init];


     }
    

    }


   return instance;


}

2、GCD

static InstanceClass *instance;


+ (InstanceClass *)defaultInstance{


   static dispatch_once_t onceToken;
    

   dispatch_once(&onceToken, ^{
        

     instance = [[InstanceClass alloc] init];


   });
    
    

   return instance;


}



两种方式都能起到线程安全的作用，尤其是在多cpu的环境下