Question

我有以下协议及其扩展名

public protocol RESEndpointReachable: CustomDebugStringConvertible
{
    associatedtype EndpointType: RESEndpointReachable


    //  MARK: - Properties

    /// The name of the endpoint as defined in the REST URI.
    var name: String { get }

    /// An array of possible next endpoints that this endpoint can reach. E.g account's next endpoints would be authenticate and unauthenticate.
    var nextPossibleEndpoints: [EndpointType] { get }


    //  MARK: - Ability

    /// Used to process the endpoint.
    func processRequest(request: RERequest)

    /// Processes the next endpoint that matches the name `name`. Expects an endpoint with the name `name` to exist in `nextPossibleEndpoints`.
    func processNextEndpointWithName(name: String, request: RERequest)
}

public extension RESEndpointReachable
{
    //  MARK: - CustomDebugStringConvertible

    public var debugDescription: String {
        return name
    }


    //  MARK: - RESEndpointReachable

    var nextPossibleEndpoints: [EndpointType] {
        return []
    }

    public func processRequest(request: RERequest)
    {
        //  Check all possible endpoints are being processed
        if let nextEndpoint = nextPossibleEndpoints.first
        {
            fatalError("Unhandled endpoint \(nextEndpoint).")
        }
    }

    public func processNextEndpointWithName(name: String, request: RERequest)
    {
        //  Get the next endpoint that matches the specified name
        let nextEndpoints = nextPossibleEndpoints.filter { $0.name == name }

        if nextEndpoints.count > 1
        {
            fatalError("Multiple next endpoints found with the name '\(name)'.")
        }

        guard let nextEndpoint = nextEndpoints.first else
        {
            fatalError("No next endpoint with the name '\(name)'.")
        }


        //  Process the next endpoint
        nextEndpoint.processRequest(request)
    }
}

在构建时，行associatedtype EndpointType: RESEndpointReachable出现以下错误：Type may not reference itself as a requirement。但据我所知，这就是你在Swift中使用相关类型的方式。

正如您可能已经猜到的那样，我总是希望将EndpointType最终设置为继承自RESEndpointReachable的类型。

Answer 1

Swift团队将此功能称为“递归协议约束”，并且该路线图将在未来版本的Swift中添加。有关此计划功能和其他计划功能的详细信息，请查看Swift团队'Completing Generics' manifesto。

Answer 2

是的，swift不行。协议不能将其自身用作类型约束。一种解决方案是声明一个额外的协议 RESEndpointReachable 本身将采用并约束 RESEndpointReachable 到该超级协议。

我采用了Neuburg M.的书“ iOS 10编程基础与Swift ”中的示例（请参阅第194页）

非法的例子：

  1 protocol Flier {
  2         associatedtype Other : Flier
  3         func flockTogetherWith(_ f: Other)
  4 }
  5 
  6 struct Bird : Flier {
  7         func flockTogetherWith(_ f: Bird) {}
  8 }

解决方案：

  1 protocol Superflier {}
  2 protocol Flier: Superflier {
  3         associatedtype Other : Superflier
  4         func flockTogetherWith(_ f: Other)
  5 }       
  6 
  7 struct Bird : Flier {
  8         func flockTogetherWith(_ f: Bird) {}
  9 }

干杯

具有关联类型的Swift协议 - 类型可能不会将自身引用为要求

2 个答案: