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; }