swift - 为什么我们不能将连续的 rawValue 设置为选项集？

Question

今天我尝试在 Playground 中使用 OptionSet 进行一些操作，我注意到了这种模式

 struct Activities: OptionSet {
      let rawValue: Int
      static let eating = Activities(rawValue: 1)
      static let programming = Activities(rawValue: 2)
      static let breathing = Activities(rawValue: 3)
      static let saveGotham = Activities(rawValue: 4) 
}

 let act: Activities = [.eating, .programming, .saveGotham]

 act.contains(.breathing). //true /* this is unexpected */
 act.contains(.saveGotham) //true

尽管该数组不包含值“.breathing”，但它仍会返回 true。我用不同的 rawValue 修改了相同的结构

 struct Activities: OptionSet {
      let rawValue: Int
      static let eating = Activities(rawValue: 1)
      static let programming = Activities(rawValue: 8)
      static let breathing = Activities(rawValue: 16)
      static let saveGotham = Activities(rawValue: 32) 
}

 let act: Activities = [.eating, .programming, .saveGotham]

 act.contains(.breathing). //false
 act.contains(.saveGotham) //true

并得到了想要的输出。如果有人能阐明问题并解释“OptionSet”的实际工作原理，那就太棒了。

谢谢。

Answer 1

OptionSet protocol是为了

... represent bitset types, where individual bits represent members of a set.

在你的情况下，

let act: Activities = [.eating, .programming, .saveGotham]
print(act.rawValue) // 7

存储为包含原始值的按位或的整数(1 | 2 | 4 = 7) 和

  act.contains(.breathing). //true /* this is unexpected */

测试 BITWISE AND 7 & 3 是否为非零(是这种情况)。

因此你不应该使用连续的原始值，而应该使用两个，即每个互斥值由一个表示比特位:

struct Activities: OptionSet {
    let rawValue: Int
    static let eating = Activities(rawValue: 1)
    static let programming = Activities(rawValue: 2)
    static let breathing = Activities(rawValue: 4)
    static let saveGotham = Activities(rawValue: 8)
}

或等同于:

struct Activities: OptionSet {
    let rawValue: Int
    static let eating = Activities(rawValue: 1 << 0)
    static let programming = Activities(rawValue: 1 << 1)
    static let breathing = Activities(rawValue: 1 << 2)
    static let saveGotham = Activities(rawValue: 1 << 3)
}

现在一切正常:

let act: Activities = [.eating, .programming, .saveGotham]
print(act.rawValue) // 11
print(act.contains(.breathing)) // false
print(act.contains(.saveGotham)) // true