一般情況下我們在開發中可以采用C風格的enum 關鍵字定義枚舉類型

enum{   
    UIViewAnimationTransitionNone,  
    UIViewAnimationTransitionFlipFromLeft,  
    UIViewAnimationTransitionFlipFromRight,  
    UIViewAnimationTransitionCurlUp,  
    UIViewAnimationTransitionCurlDown,  
}

位移動操作枚舉定義

enum {  
    UIViewAutoresizingNone                 = 0,  
    UIViewAutoresizingFlexibleLeftMargin   = 1 << 0,  
    UIViewAutoresizingFlexibleWidth        = 1 << 1,  
    UIViewAutoresizingFlexibleRightMargin  = 1 << 2,  
    UIViewAutoresizingFlexibleTopMargin    = 1 << 3,  
    UIViewAutoresizingFlexibleHeight       = 1 << 4,  
    UIViewAutoresizingFlexibleBottomMargin = 1 << 5  
};  
typedef NSUInteger UIViewAutoresizing;
//使用NSUInteger的地方可以使用UIViewAutoresizing,//UIViewAutoresizing相當于NSUInteger的一個別名使用
//因此一個UIViewAutoresizing的變量可以直接賦值給NSUInteger  

枚舉值一般是4個字節的int值，在64位系統上是8個字節

typedef NS_ENUM(NSInteger, UIBarButtonItemStyle) {
    UIBarButtonItemStylePlain,// 默認從0開始
    UIBarButtonItemStyleBordered,
    UIBarButtonItemStyleDone,
};

typedef NS_OPTIONS(NSUInteger, UIViewAutoresizing) {  
    UIViewAutoresizingNone                 = 0,  
    UIViewAutoresizingFlexibleLeftMargin   = 1 << 0,  
    UIViewAutoresizingFlexibleWidth        = 1 << 1,  
    UIViewAutoresizingFlexibleRightMargin  = 1 << 2,  
    UIViewAutoresizingFlexibleTopMargin    = 1 << 3,  
    UIViewAutoresizingFlexibleHeight       = 1 << 4,  
    UIViewAutoresizingFlexibleBottomMargin = 1 << 5  
}; 

這兩個宏的定義在framework的NSObjCRuntime中

包

定義

從枚舉定義來看，NS_ENUM和NS_OPTIONS本質是一樣的。NS_ENUM是通用情況，NS_OPTIONS一般用來定義具有位移操作或特點的情況