用开关转换

switch 语句也可用于尝试转换为不同的类型：

func checkType(_ value: Any) -> String {
    switch value {

    // The `is` operator can be used to check a type
    case is Double:
        return "value is a Double"

    // The `as` operator will cast. You do not need to use `as?` in a `switch`.
    case let string as String:
        return "value is the string: \(string)"

    default:
        return "value is something else"
    }

}

checkType("Cadena")  // "value is the string: Cadena"
checkType(6.28)      // "value is a Double"
checkType(UILabel()) // "value is something else"