iOS官方推荐的单例写法

#define HMSingleton_h(name)  + (instancetype)shared##name;

#if __has_feature(objc_arc) // 是arc环境

#define HMSingleton_m(name)  + (instancetype)shared##name {\

return [[self alloc] init];\

}\

\

+ (instancetype)allocWithZone:(struct _NSZone *)zone {\

static dispatch_once_t onceToken;\

static id instance = nil;\

dispatch_once(&onceToken, ^{ \

instance = [super allocWithZone:zone];\

});\

return instance;\

}\

\

- (id)copyWithZone:(nullable NSZone *)zone {\

return self;\

}

#else // MRC环境

#define HMSingleton_m(name)  + (instancetype)shared##name {\

return [[self alloc] init];\

}\

\

+ (instancetype)allocWithZone:(struct _NSZone *)zone {\

static dispatch_once_t onceToken;\

static id instance = nil;\

dispatch_once(&onceToken, ^{ \

instance = [super allocWithZone:zone];\

});\

return instance;\

}\

\

- (id)copyWithZone:(nullable NSZone *)zone {\

return self;\

}\

- (oneway void)release {\

\

}\

\

- (instancetype)autorelease {\

return self;\

}\

\

- (instancetype)retain {\

return self;\

}\

\

- (NSUInteger)retainCount {\

return 1;\

}

#endif