单例宏

#define DEFINE_SINGLETON_HEADER(className) \  
\  
+ (className *)shared##className;  
  
#define DEFINE_SINGLETON_IMPLEMENTATION(className) \  
\  
static className *shared##className = nil; \  
static dispatch_once_t pred; \  
\  
+ (className *)shared##className { \  
    dispatch_once(&pred, ^{ \  
        shared##className = [[super allocWithZone:NULL] init]; \  
    }); \  
    return shared##className; \  
} \  
\  
+ (id)allocWithZone:(NSZone *)zone { \  
    return [self shared##className];\  
} \  
\  
- (id)copyWithZone:(NSZone *)zone { \  
return self; \  
}  

使用如下

@interface MySingleton : NSObject  
DEFINE_SINGLETON_HEADER(MySingleton);  
/**other method declare**/  
@end  

比如

@implementation MySingleton  
DEFINE_SINGLETON_IMPLEMENTATION(MySingleton)  
  
- (instancetype)init {  
    if (sharedMySingleton) {  
        return sharedMySingleton;  
    }  
    if (self = [super init]) {  
        //todo init other  
    }  
    return self;  
}  
@end