OptionSet 协议

OptionSetType 是一种用于表示位掩码类型的协议，其中各个位表示集合的成员。一组逻辑和/或函数强制执行正确的语法：

struct Features : OptionSet {
  let rawValue : Int
  static let none = Features(rawValue: 0)
  static let feature0 = Features(rawValue: 1 << 0)
  static let feature1 = Features(rawValue: 1 << 1)
  static let feature2 = Features(rawValue: 1 << 2)
  static let feature3 = Features(rawValue: 1 << 3)
  static let feature4 = Features(rawValue: 1 << 4)
  static let feature5 = Features(rawValue: 1 << 5)
  static let all: Features = [feature0, feature1, feature2, feature3, feature4, feature5]
}

Features.feature1.rawValue //2
Features.all.rawValue //63

var options: Features = [.feature1, .feature2, .feature3]

options.contains(.feature1) //true
options.contains(.feature4) //false

options.insert(.feature4)
options.contains(.feature4) //true

var otherOptions : Features = [.feature1, .feature5]

options.contains(.feature5) //false

options.formUnion(otherOptions)
options.contains(.feature5) //true

options.remove(.feature5)
options.contains(.feature5) //false