swift Enumerations
enum、case、switch
CaseIterable allCases
要区别枚举变量和关联值
枚举变量参与枚举运算；
关联值和rawvalue不参与。
Associated Values
case qrCode(String)
case let .qrCode(productCode):
Raw Values
As an alternative to associated values, enumeration cases can come prepopulated with default values (called raw values), which are all of the same type.
同一类型的继承
enum ASCIIControlCharacter: Character