非线程安全,避免生成多个实例

SingletonMarco.h

#define SINGLETON(__class)                                      \
+ (instancetype)sharedInstance;

#define DEF_SINGLETON(__class)                                  \
static __class * __singleton__ = nil;                           \
+ (instancetype)sharedInstance {                                \
    static dispatch_once_t once;                                \
    dispatch_once(&once, ^{                                     \
        __singleton__ = [[super allocWithZone:NULL] init];      \
    });                                                         \
    return __singleton__;                                       \
}                                                               \
+ (id)allocWithZone:(struct _NSZone *)zone {                    \
    return [__class sharedInstance];                            \
}                                                               \
- (id)copyWithZone:(struct _NSZone *)zone {                     \
    return [__class sharedInstance];                            \
}