一、枚舉類(lèi)型、enum
枚舉類(lèi)型

ypedef NS_ENUM(NSInteger, UIViewAnimationTransition) {  
    UIViewAnimationTransitionNone,//默認(rèn)從0開(kāi)始  
    UIViewAnimationTransitionFlipFromLeft,  
    UIViewAnimationTransitionFlipFromRight,  
    UIViewAnimationTransitionCurlUp,  
    UIViewAnimationTransitionCurlDown,  
};  
  
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  
};  

從枚舉定義來(lái)看，NS_ENUM和NS_OPTIONS本質(zhì)是一樣的，僅僅從字面上來(lái)區(qū)分其用途。NS_ENUM是通用情況，NS_OPTIONS一般用來(lái)定義具有位移操作或特點(diǎn)的情況（bitmask)，也適用于可以同時(shí)選擇多個(gè)選項(xiàng)的情況。