OptionSet
OptionSet作用类似于OC中按位枚举-NS_OPTIONS
- 要存放枚举值，所以
OptionSet遵循了RawRepresentable，提供了rawValue属性 - 为了方便对rawValue进行位运算等操作，
OptionSet要求rawValue必须遵循FixedWidthInteger(FixedWidthInteger的父protocol-BinaryInteger支持位运算) OptionSet还遵循了SetAlgebra- 能够方便的进行集合操作
SetAlgebra继承的ExpressibleByArrayLiteral能够用arrayliteral的形式来给OptionSet赋值
struct ShippingOptions: OptionSet {
let rawValue: Int
static let nextDay = ShippingOptions(rawValue: 1 << 0)
static let secondDay = ShippingOptions(rawValue: 1 << 1)
static let priority = ShippingOptions(rawValue: 1 << 2)
static let standard = ShippingOptions(rawValue: 1 << 3)
static let express: ShippingOptions = [.nextDay, .secondDay]
static let all: ShippingOptions = [.express, .priority, .standard]
}
