使用协议定义具有不同值的KeyPaths集合

Question

假设您想要跟踪一个或多个唯一标识符。在本例中，A有两个被认为是A独有的属性，而B只有一个属性是B独有的。

protocol HavingUID {
   // Some way to use KeyPath?
}

struct A : HavingUID {
   var unique1 : String  
   var unique2 : Int
}

struct B : HavingUID {
   var unique1 : Double
}

let a1 = A(unique1:"val", unique2: 1)
let a2 = A(unique1:"val", unique2: 2)
let b1 = B(unique1:0.5)
let b2 = B(unique1:0.0)
let b3 = B(unique1:0.2)

let arrA : [HavingUID] = [a1,a2]
let arrB : [HavingUID] = [b1,b2,b3]

// How to check arrA and arrB for duplicate UID properties?

如果只有一个唯一的键，我们可以这样做：

protocol HavingUID {
   typealias UID
   static var uidKey : KeyPath<Self, UID> {get}
}
struct A : HavingUID {
   static var uidKey = A.\unique1
   var unique1 : String
}
struct B : HavingUID {
   static var uidKey = B.\uniqueB
   var uniqueB : Int
}

...but，这将我限制为一个密钥。

Answer 1

无论何时需要使用唯一标识符，都应该使用全局结构或枚举来跟踪它们。这里有两种简单的方法可以做到这一点：

结构：

struct UniqueIdentifiers {
    static let id1 = "identifier_one"
    static let id2 = "identifier_two"
    static let id3 = "identifier_three"
}

let currentID = UniqueIdentifiers.id1

枚举：

enum UniqueIdentifiers: String {
    case id1 = "identifier_one"
    case id2 = "identifier_two"
    case id3 = "identifier_three"
}

let currentID = UniqueIdentifiers.id1.rawValue