NS_ENUM & NS_OPTIONS

1. NS_ENUM and NS_OPTIONS are perferred.

Introduced in Foundation with iOS 6 / OS X Mountain Lion, the NS_ENUM
and NS_OPTIONS macros are the new, preferred way to declare enum
types.

在OC中，推荐使用NS_ENUM 和 NS_OPTIONS来定义枚举.
可以看到Apple公司自己就是使用这两个宏来定义枚举的.

2. 通用模式 - NS_ENUM

NS_ENUM supports the use of one or two arguments. The first argument is always the integer type used for the values of the enum. The second argument is an optional type name for the macro. When specifying a type name, you must precede the macro with 'typedef' like so:
typedef NS_ENUM(NSInteger, NSComparisonResult) {
...
};
If you do not specify a type name, do not use 'typedef'. For example:
NS_ENUM(NSInteger) {
...
};
Refer from NSObjcRuntime.h

typedef NS_ENUM(NSInteger, EZLConnectionState)
{
    EZLConnectionStateDisconnected = -1,
    EZLConnectionStateConnected    = 0,
    EZLConnectionStateConnecting   = 1
};

等价于:

enum EZLConnectionState : NSInteger
{
    EZLConnectionStateDisconnected = -1,
    EZLConnectionStateConnected    = 0,
    EZLConnectionStateConnecting   = 1
};

typedef enum EZLConnectionState : NSInteger EZLConnectionState;

可以看出NS_ENUM实质是将C风格的enum定义和typedef合二为一.

3. 位操作模式 - NS_OPTIONS

typedef NS_OPTIONS(NSUInteger, UIControlState) {
    UIControlStateNormal       = 0,
    UIControlStateHighlighted  = 1 << 0,                  // used when UIControl isHighlighted is set
    UIControlStateDisabled     = 1 << 1,
    UIControlStateSelected     = 1 << 2,                  // flag usable by app (see below)
    UIControlStateFocused NS_ENUM_AVAILABLE_IOS(9_0) = 1 << 3, // Applicable only when the screen supports focus
    UIControlStateApplication  = 0x00FF0000,              // additional flags available for application use
    UIControlStateReserved     = 0xFF000000               // flags reserved for internal framework use
};

References:
http://nshipster.com/ns_enum-ns_options/
http://www.cnblogs.com/langtianya/p/3888924.html