Swift 关联类型继承

Question

我想编写两种协议，一种是通用的，另一种是专门用于处理一些网络请求的。

对于更通用的DORequest 协议，一切正常，但我无法让DOPagedRequest 工作。问题是由associatedtype Response 引起的，我想在DOPagedRequest 中更具体。

这是我的协议：

public protocol DORequest {
    associatedtype Response: DOResponse
    var method: String { get }
    var path: String { get }
}

public protocol DOResponse: Codable { }

public protocol DOPagedRequest: DORequest where Response: DOPagedResponse {
    var page: Int? { get }
    var perPage: Int? { get }
}

public protocol DOPagedResponse: Codable {
    var links: ResponseLinks { get }
}

public struct ResponseLinks: Codable {

    public struct Pages: Codable {
        var next: String?
    }

    public var pages: Pages?
}

以及它们的具体实现示例：

// Implementation of DORequest with no errors
public struct Get: DORequest {

    public struct Response: DOResponse {
        public let account: String
    }

    public let method = "GET"
    public let path = "account"

    public init() { }
}

// Implementation of DOPagedRequest with errors:
//  Type 'List' does not conform to protocol 'DORequest'
//  Type 'List' does not conform to protocol 'DOPagedRequest'
public struct List: DOPagedRequest {

    public var tag: String?
    public var page: Int?
    public var perPage: Int?

    public struct Response: DOPagedResponse {
        public var links: ResponseLinks

        public let droplets: [String]
    }

    public let method = "GET"
    public let path = "droplets"

    public init(tag: String? = nil, page: Int = 0, perPage: Int = 200) {
        self.tag = tag
        self.page = page
        self.perPage = perPage
    }
}

我可能遗漏了一些关于 Swift 的关联类型的信息。

Answer 1

你忘了继承DOResponse。

public protocol DOPagedResponse: DOResponse {

Answer 2

您的代码中有几处对我来说没有多大意义。

我不明白的重点是响应的性质：

DORequest 中的回复是：associatedtype Response: DOResponse

DOPagedRequest 中的响应是 where Response: DOPagedResponse。

因此，当您声明 public struct List: DOPagedRequest 时，编译器将无法确定 Response 符合哪个类型。

它是Response 类型的DOResponse 还是DOPagedResponse 类型？

我建议你做一些事情来统一这两个协议，即：DOResponse 和 DOPagedResponse 在同一个保护伞下，如下所示：

public protocol GeneralResponse {}

public protocol DOResponse: Codable, GeneralResponse { }
public protocol DOPagedResponse: Codable, GeneralResponse {
   var links: ResponseLinks { get }
}

public protocol DORequest {
  associatedtype Response: GeneralResponse
  var method: String { get }
   var path: String { get }
}

public protocol DOPagedRequest {
   var page: Int? { get }
   var perPage: Int? { get }
}