arc 模式下 使用宏定义单例模式（gcd 实现单例模式）

// .h文件
#define HMSingletonH(name) + (instancetype)shared##name;

// .m文件
#define HMSingletonM(name) \
static id _instance; \
 \
+ (id)allocWithZone:(struct _NSZone *)zone \
{ \
    static dispatch_once_t onceToken; \
    dispatch_once(&onceToken, ^{ \
        _instance = [super allocWithZone:zone]; \
    }); \
    return _instance; \
} \
 \
+ (instancetype)shared##name \
{ \
    static dispatch_once_t onceToken; \
    dispatch_once(&onceToken, ^{ \
        _instance = [[self alloc] init]; \
    }); \
    return _instance; \
} \
 \
- (id)copyWithZone:(NSZone *)zone \
{ \
    return _instance; \
}