swift - 在 Swift 中，如何转换为具有关联类型的协议(protocol)？

Question

在下面的代码中，我想测试x 是否是一个SpecialController。如果是，我想获取 currentValue 作为 SpecialValue。你怎么做到这一点？如果不使用类型转换，则使用其他一些技术。

那里的最后一行不会编译。错误是:Protocol "SpecialController"can only be used as a generic constraint because it has Self or associated type requirements.

protocol SpecialController {
    associatedtype SpecialValueType : SpecialValue
    var currentValue: SpecialValueType? { get }
}
...
var x: AnyObject = ...
if let sc = x as? SpecialController {  // does not compile

Answer 1

不幸的是，Swift 目前不支持将具有关联类型的协议(protocol)用作实际类型。然而这is technically possible为编译器做；它may well be implemented in a future version of the language .

在您的情况下，一个简单的解决方案是定义 SpecialController 派生的“影子协议(protocol)”，并允许您通过类型删除它的协议(protocol)要求访问 currentValue :

// This assumes SpecialValue doesn't have associated types – if it does, you can
// repeat the same logic by adding TypeErasedSpecialValue, and then using that.
protocol SpecialValue {
  // ...
}

protocol TypeErasedSpecialController {
  var typeErasedCurrentValue: SpecialValue? { get }
}

protocol SpecialController : TypeErasedSpecialController {
  associatedtype SpecialValueType : SpecialValue
  var currentValue: SpecialValueType? { get }
}

extension SpecialController {
  var typeErasedCurrentValue: SpecialValue? { return currentValue }
}

extension String : SpecialValue {}

struct S : SpecialController {
  var currentValue: String?
}

var x: Any = S(currentValue: "Hello World!")
if let sc = x as? TypeErasedSpecialController {
  print(sc.typeErasedCurrentValue as Any) // Optional("Hello World!")
}