ios用枚举表示状态，选项，状态码

枚举只是一种常量命名方式，入下列枚举表示链接状态

enum EOCConnectionState:NSInteger{    EOCConnectionStateDisconnected = 1,    EOCConnectionStateConnecting,    EOCConnectionStateConnected,};
编译器会为每一个枚举分配一个独有的编号，从0开始，然而定义没巨变量的方式却不太简洁，如下
enum EOCConnectionState state = EOCConnectionStateConnected;
另一种方式
enum EOCConnectionState{    EOCConnectionStateDisconnected,    EOCConnectionStateConnecting,    EOCConnectionStateConnected,};typedef enum EOCConnectionState EOCConnectionState;这样使用会简洁一些
EOCConnectionState state = EOCConnectionStateConnected;c++11标准修订类枚举的某些特性，其中一项就是可以指明用何种“底层数据类型”来保存枚举类型的变量。这样的好处是，可以向前申明枚举变量。如下
enum EOCConnectionState:NSInteger{    EOCConnectionStateDisconnected = 1,    EOCConnectionStateConnecting,    EOCConnectionStateConnected,};
上述代码把EOCConnectionStateDisconnected的值设为1而不是默认分配的0.
还有一种情况应该使用枚举类型，那就是定义选项的时候，若这些选项可以彼此组合，则更应该如此，和选项之久可以通过“按位或操作符”来组合，例如
enum UIViewAutoresizing{    UIViewAutoresizingNone                 = 0,    UIViewAutoresizingFlexibleLeftMargin   = 1 << 0,    UIViewAutoresizingFlexibleWidth        = 1 << 1,    UIViewAutoresizingFlexibleRightMargin  = 1 << 2,    UIViewAutoresizingFlexibleTopMargin    = 1 << 3,    UIViewAutoresizingFlexibleHeight       = 1 << 4,    UIViewAutoresizingFlexibleBottomMargin = 1 << 5};
Foundation框架中定义了一写辅助的宏，用这些宏来定义枚举类型时，也可以指定用于保存枚举值的底层数据类型，这些宏具有向后兼容能力
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};
typedef NS_ENUM(NSUInteger, UIViewAutoresizing) {    EOCConnectionStateDisconnected ,    EOCConnectionStateConnecting,    EOCConnectionStateConnected,};
从枚举定义来看，NS_ENUM和NS_OPTIONS本质是一样的，仅仅从字面上来区分其用途。NS_ENUM是通用情况，NS_OPTIONS一般用来定义具有位移操作或特点的情况。
注：我们总习惯在switch语句中加上default分支，然而，若是枚举来定义状态机，最好不要有default分支，这样的话，如果稍后又加了一种状态，那么编译器就会发出浸膏信息，提示新加入的状态并未在switch分支中处理