在Swift中，如何使用关联类型转换为协议？ [英] In Swift, how to cast to protocol with associated type?

问题描述

在下面的代码中，我想测试 x 是不是 SpecialController 。如果是这样，我想要 currentValue 作为 SpecialValue 。你怎么做到这一点？如果不是使用强制转换，那么使用其他技术。

最后一行不能编译。有错误的是： ProtocolSpecialController只能用作通用约束，因为它具有Self或相关类型的要求。

  protocol SpecialController {
 associatedtype SpecialValueType：SpecialValue 
 var currentValue：SpecialValueType？ {get} 
} 
 ... 
 var x：AnyObject = ... 
如果让sc = x as？ SpecialController {//不会编译
  

解决方案

当前不支持使用与关联类型相关的协议作为实际类型。然而，这对编译器来说在技术上是可行的;而且它可能会在未来的版本中实现语言。

在你的情况下，一个简单的解决方案是定义一个'影子协议'， SpecialController 派生自并允许你通过一个协议要求来访问 currentValue ，类型会将其擦除：

  / /这假设SpecialValue没有关联类型 - 如果是的话，可以通过添加TypeErasedSpecialValue重复
 //相同的逻辑，然后使用它。 
协议SpecialValue {
 // ... 
} 
 
协议TypeErasedSpecialController {
 var typeErasedCurrentValue：SpecialValue？ {get} 
} 
 
协议SpecialController：TypeErasedSpecialController {
 associatedtype SpecialValueType：SpecialValue 
 var currentValue：SpecialValueType？ {get} 
} 
 
扩展SpecialController {
 var typeErasedCurrentValue：SpecialValue？ {return currentValue} 
} 
 
扩展字符串：SpecialValue {} 
 $ b $ struct S：SpecialController {
 var currentValue：String？ 
} 
 
 var x：Any = S（currentValue：Hello World！）
如果让sc = x as？ TypeErasedSpecialController {
 print（sc.typeErasedCurrentValue as Any）//可选（Hello World！）
} 
  

In the following code, I want to test if x is a SpecialController. If it is, I want to get the currentValue as a SpecialValue. How do you do this? If not with a cast, then some other technique.

The last line there won't compile. There error is: 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

解决方案

Unfortunately, Swift doesn't currently support the use of protocols with associated types as actual types. This however is technically possible for the compiler to do; and it may well be implemented in a future version of the language.

A simple solution in your case is to define a 'shadow protocol' that SpecialController derives from, and allows you to access currentValue through a protocol requirement that type erases it:

// 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!")
}

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