在用一系列常量来表示错误状态码或可组合的选项时,很适合 枚举 为其命名。
枚举是一种常量命名方式,某个对象所经历的各种状态就可以定义为一个简单的枚举集(enumeration set)。例如:
enum EOCConnectionState {
EOCConnectionStateDisconnected,
EOCConnectionStateConnecting,
EOCConnectionStateConnected,
};
编译器会为每个枚举值分配一个独有的编号,从0开始,依次加1。一个字节最多可表示0~255共256种(2^8)枚举变量。
定义枚举变量:
enum EOCConnectionState state = EOCConnectionStateDisconnected;
为简便起见,可以使用 typedef
关键字重新定义枚举类型:
enum EOCConnectionState {
EOCConnectionStateDisconnected,
EOCConnectionStateConnecting,
EOCConnectionStateConnected,
};
typedef enum EOCConnectionState EOCConnectionState;
现在就可以不写 enum
,直接定义了:
EOCConnectionState state = EOCConnectionStateDisconnected;
- 指定“底层数据类型”(underlying type)(C++11标准)的语法:
enum EOCConnectionState : NSInteger {
/*...*/
}
- 指定分配的序号:
enum EOCConnectionState {
EOCConnectionStateDisconnected = 1,
EOCConnectionStateConnecting, //接下来的值会依次递增1
EOCConnectionStateConnected
};
- 若定义的枚举选项可以彼此组合,则更应如此。例如:
// UIView.h
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
};
其二进制值如图所示:
二进制值示意图
- 此外,枚举用法也可用于
switch
语句中,例如:
typedef NS_ENUM(NSUInteger, EOCConnectionState) {
EOCConnectionStateDisconnected,
EOCConnectionStateConnecting,
EOCConnectionStateConnected
};
switch (_currentState) {
EOCConnectionStateDisconnected:
//...
break;
EOCConnectionStateConnecting:
//...
break;
EOCConnectionStateConnected:
//...
break;
}
注:若用枚举来定义状态机(state machine),最好不要有
default
分支。
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