学步园

在iOS6和Mac OS 10.8以后Apple引入了两个宏来重新定义这两个枚举类型，实际上是将enum定义和typedef合二为一，并且采用不同的宏来从代码角度来区分。

NS_OPTIONS一般用来定义位移相关操作的枚举值，我们可以参考UIKit.Framework的头文件，可以看到大量的枚举定义。

    typedef NS_ENUM(NSInteger, UIViewAnimationTransition) {  
        UIViewAnimationTransitionNone,//默认从0开始  
        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  

    }; 

http://blog.csdn.net/annkie/article/details/9877643