给出以下枚举：
enum Audience {
case Public
case Friends
case Private
}
如何从观众常数下面得到字符串“公共”？
let audience = Audience.Public
不知道在哪个Swift版本中添加了这个功能，但现在(Swift 2.1)，你只需要这个代码：
enum Audience: String {
case Public
case Friends
case Private
}
let audience = Audience.Public.rawValue // "Public"
When strings are used for raw values,the implicit value for each case
is the text of that case’s name.[…]
06001
In the example above,CompassPoint.South has an implicit raw value of
“South”,and so on.You access the raw value of an enumeration case with its rawValue
property:06002
07000
