总结：swift中的枚举可以看作变量可以作为case匹配参数的类
Enumerations
枚举的作用：状态列举与匹配
枚举值与类型
If a value (known as a “raw” value) is provided for each enumeration case, the value can be a string, a character, or a value of any integer or floating-point type.
You can define a common set of related cases as part of one enumeration, each of which has a different set of values of appropriate types associated with it.
Enumerations in Swift are first-class types in their own right.
computed properties
instance methods
继承、协议
case
简化
match
.语法
