单例模式的作用
可以保证在程序运行过程，一个类只有一个实例，而且该实例易于供外界访问
从而方便地控制了实例个数，并节约系统资源
单例模式的使用场合
在整个应用程序中，共享一份资源（这份资源只需要创建初始化1次）
#define interfaceSingle(name)+ (instancetype)share##name
#if __has_feature(objc_arc)
//如果是ARC
#define implementationSingle(name)+ (instancetype)share##name \
{ \
return [[self alloc] init]; \
} \
static id _instance; \
+ (instancetype)allocWithZone:(struct _NSZone *)zone \
{ \
static dispatch_once_t onceToken; \
dispatch_once(&onceToken, ^{ \
_instance = [super allocWithZone:zone]; \
}); \
return _instance; \
} \
- (id)copyWithZone:(NSZone *)zone \
{ \
return _instance; \
} \
- (id)mutableCopyWithZone:(NSZone *)zone \
{ \
return _instance; \
}
#else
//如果不是ARC
#define implementationSingle(name)+ (instancetype)share##name \
{ \
return [[self alloc] init]; \
} \
static id _instance; \
+ (instancetype)allocWithZone:(struct _NSZone *)zone \
{ \
static dispatch_once_t onceToken; \
dispatch_once(&onceToken, ^{ \
_instance = [super allocWithZone:zone]; \
}); \
return _instance; \
} \
- (id)copyWithZone:(NSZone *)zone \
{ \
return _instance; \
} \
- (id)mutableCopyWithZone:(NSZone *)zone \
{ \
return _instance; \
}\
- (oneway void)release \
{} \
- (instancetype)retain \
{ \
return _instance; \
} \
- (NSUInteger)retainCount \
{ \
return MAXFLOAT; \
}
