非ARC和ARC下创建单利模式的宏定义，可以直接套用

本例子是将创建单例模式的.h和.m文件抽出来，用一个宏来定义，放在头文件中：

// 帮助实现单例设计模式

// .h文件的实现
#define SingletonH(methodName) + (instancetype)shared##methodName;

// .m文件的实现
#if __has_feature(objc_arc) // 是ARC
#define SingletonM(methodName) 
static id _instace = nil; 
+ (id)allocWithZone:(struct _NSZone *)zone 
{ 
if (_instace == nil) { 
static dispatch_once_t onceToken; 
dispatch_once(&onceToken, ^{ 
_instace = [super allocWithZone:zone]; 
}); 
} 
return _instace; 
} 

- (id)init 
{ 
static dispatch_once_t onceToken; 
dispatch_once(&onceToken, ^{ 
_instace = [super init]; 
}); 
return _instace; 
} 

+ (instancetype)shared##methodName 
{ 
return [[self alloc] init]; 
} 
+ (id)copyWithZone:(struct _NSZone *)zone 
{ 
return _instace; 
} 

+ (id)mutableCopyWithZone:(struct _NSZone *)zone 
{ 
return _instace; 
}

#else // 不是ARC

#define SingletonM(methodName) 
static id _instace = nil; 
+ (id)allocWithZone:(struct _NSZone *)zone 
{ 
if (_instace == nil) { 
static dispatch_once_t onceToken; 
dispatch_once(&onceToken, ^{ 
_instace = [super allocWithZone:zone]; 
}); 
} 
return _instace; 
} 

- (id)init 
{ 
static dispatch_once_t onceToken; 
dispatch_once(&onceToken, ^{ 
_instace = [super init]; 
}); 
return _instace; 
} 

+ (instancetype)shared##methodName 
{ 
return [[self alloc] init]; 
} 

- (oneway void)release 
{ 

} 

- (id)retain 
{ 
return self; 
} 

- (NSUInteger)retainCount 
{ 
return 1; 
} 
+ (id)copyWithZone:(struct _NSZone *)zone 
{ 
    return _instace; 
} 
 
+ (id)mutableCopyWithZone:(struct _NSZone *)zone 
{ 
    return _instace; 
}

#endif