专门用于通用协议的Swift协议 [英] Swift protocol specializing generic protocol

问题描述

是否可以有专门用于通用协议的协议?我想要这样的东西:

Is it possible to have a protocol that specializes a generic protocol? I want something like this:

protocol Protocol: RawRepresentable {
  typealias RawValue = Int
  ...
}

这确实可以编译，但是当我尝试从Protocol实例访问init或rawValue时，其类型为RawValue而不是Int.

This does compile, but when I try to access the init or rawValue from a Protocol instance, its type is RawValue instead of Int.

推荐答案

在Swift 4中，您可以向协议添加约束:

In Swift 4, you can add constraints to your protocol:

protocol MyProtocol: RawRepresentable where RawValue == Int {
}

现在，在MyProtocol上定义的所有方法都将具有Int rawValue.例如:

And now all methods defined on MyProtocol will have an Int rawValue. For example:

extension MyProtocol {
    var asInt: Int {
        return rawValue
    }
}

enum Number: Int, MyProtocol {
    case zero
    case one
    case two
}

print(Number.one.asInt)
// prints 1

采用RawRepresentable但RawValue不是Int的类型不能采用您的约束协议:

Types that adopt RawRepresentable but whose RawValue is not Int can not adopt your constrained protocol:

enum Names: String {
    case arthur
    case barbara
    case craig
}

// Compiler error
extension Names : MyProtocol { }

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