如何在协议中声明通用协议属性要求

Question

Struggling for a while with, it will be really helpful if you can shed some light on this:

I have an APIWorkerProtocol which has a property requirement, the required property is a protocol ie DataParserProtocol

protocol APIWorkerProtocol {
    var apiRequestProvider : APIRequestGeneratorProtocol {get}
    var dataParser : DataParserProtocol{get}
    func callAPI(completionHandler: @escaping (APICallResult<Self.ResultType>) -> Void)
}

protocol DataParserProtocol {
    associatedtype ExpectedRawDataType
    associatedtype ResultType
    func parseFetchedData(fetchedData : ExpectedRawDataType) -> APICallResult<ResultType>
}

How can I achieve this?

In this current implementation, this causes an error Protocol 'DataParserProtocol' can only be used as a generic constraint because it has Self or associated type requirements .

Thanks in advance

Ankit

Answer 1

If a protocol make use of Self or associated type requirements (a homogenous protocol), we may note use the protocol as a concrete type.

So instead of using the DataParserProtocol as the concrete type of the dataParser property (blueprinted in the APIWorkerProtocol ), you could add an associatedtype typeholder, say DataParser , to the APIWorkerProtocol , which is constrained to types that conform to the DataParserProtocol .

Also, I'm not sure what the intent with using Self.ResultType as specialization in the completion handler of callAPI(...) is (since Self would refer to the type implementing the APIWorkerProtocol ; a protocol which blueprints no associatedtype ResultType ): did you mean to use the ResultType of the DataParserProtocol type?

Eg

protocol APIWorkerProtocol {
    associatedtype DataParser: DataParserProtocol
    var dataParser : DataParser { get }
    func callAPI(completionHandler: @escaping (APICallResult<DataParser.ResultType>) -> Void)
}

protocol DataParserProtocol {
    associatedtype ExpectedRawDataType
    associatedtype ResultType
    func parseFetchedData(fetchedData: ExpectedRawDataType) -> APICallResult<ResultType>
}