Swift: Pass type from property to generic function

Question

For my networking module, I have this protocol that I adopt for accessing different parts of the API:

protocol Router: URLRequestConvertible {
    var baseUrl: URL { get }        
    var route: Route { get }        
    var method: HTTPMethod { get }        
    var headers: [String: String]? { get }        
    var encoding: ParameterEncoding? { get }        
    var responseResultType: Decodable.Type? { get }
}

I'm adopting this with enums that look like this:

enum TestRouter: Router {
    case getTestData(byId: Int)
    case updateTestData(byId: Int)

    var route: Route {
        switch self {
        case .getTestData(let id): return Route(path: "/testData/\(id)")
        case .updateTestData(let id): return Route(path: "/testDataOtherPath/\(id)")
        }
    }

    var method: HTTPMethod {
        switch self {
        case .getTestData: return .get
        case .updateTestData: return .put
        }
    }

    var headers: [String : String]? {
        return [:]
    }

    var encoding: ParameterEncoding? {
        return URLEncoding.default
    }

    var responseResultType: Decodable.Type? {
        switch self {
        case .getTestData: return TestData.self
        case .updateTestData: return ValidationResponse.self
        }
    }
}

I want to use Codable for decoding nested Api responses. Every response consists of a token and a result which content is depending on the request route.

For making the request I want to use the type specified in the responseResultType property in the enum above.

struct ApiResponse<Result: Decodable>: Decodable {
    let token: String
    let result: Result
}

extension Router {
    func asURLRequest() throws -> URLRequest {
        // Construct URL
        var completeUrl = baseUrl.appendingPathComponent(route.path, isDirectory: false)
        completeUrl = URL(string: completeUrl.absoluteString.removingPercentEncoding ?? "")!
        // Create URL Request...
        var urlRequest = URLRequest(url: completeUrl)
        // ... with Method
        urlRequest.httpMethod = method.rawValue
        // Add headers
        headers?.forEach { urlRequest.addValue($0.value, forHTTPHeaderField: $0.key) }
        // Encode URL Request with the parameters
        if encoding != nil {
            return try encoding!.encode(urlRequest, with: route.parameters)
        } else {
            return urlRequest
        }
    }

    func requestAndDecode(completion: @escaping (Result?) -> Void) {
        NetworkAdapter.sessionManager.request(urlRequest).validate().responseData { response in
            let responseObject = try? JSONDecoder().decode(ApiResponse<self.responseResultType!>, from: response.data!)
            completion(responseObject.result)
        }
    }
}

But in my requestAndDecode method It throws an compiler error (Cannot invoke 'decode' with an argument list of type '(Any.Type, from: Data)'). I can't use ApiResponse<self.responseResultType!> like that.

I could make this function generic and call it like this:

TestRouter.getTestData(byId: 123).requestAndDecode(TestData.self, completion:)

but then I'd have to pass the response type everytime I want to use this endpoint.

What I want to achieve is that the extension function requestAndDecode takes it response type information from itself, the responseResultType property.

Is this possible?

score 2 · Accepted Answer · answered Jul 05 '18 at 20:53

Ignoring the actual error report you have a fundamental problem with requestAndDecode: it is a generic function whose type parameters are determined at the call site which is declared to return a value of type Result yet it attempts to return a value of type self.responseResultType whose value is an unknown type.

If Swift's type system supported this it would require runtime type checking, potential failure, and your code would have to handle that. E.g. you could pass TestData to requestAndDecode while responseResultType might be ValidationResponse...

Change the JSON call to:

JSONDecoder().decode(ApiResponse<Result>.self ...

and the types statically match (even though the actual type that Result is is unknown).

You need to rethink your design. HTH

You're right, I mixed the two versions of the function (first with generics and the other one without). I updated the code. — heyfrank, Jul 06 '18 at 07:46
Thanks for your answer! I understood why it's not possible. This question is similar and has also good explanations in it (https://stackoverflow.com/questions/30905060/using-a-type-variable-in-a-generic) — heyfrank, Jul 06 '18 at 07:50

score 0 · Answer 2 · answered Mar 20 '21 at 09:16

Create a Generic function with Combine and AlomFire. You can use it for all method(get, post, put, delete)

 func fatchData<T: Codable>(requestType: String, url: String, params: [String : Any]?, myType: T.Type, completion: @escaping (Result<T, Error>) -> Void) {

    var method = HTTPMethod.get
    switch requestType {
    case "Get":
        method = HTTPMethod.get
    case "Post":
        method = HTTPMethod.post
        print("requestType  \(requestType) \(method) ")
    case "Put":
        method = HTTPMethod.put
    default:
        method = HTTPMethod.delete
    }

    print("url  \(url) \(method)  \(AppConstant.headers)  ")
    task =  AF.request(url, method: method, parameters: params, encoding: JSONEncoding.default, headers: AppConstant.headers)
        .publishDecodable(type: myType.self)
        .sink(receiveCompletion: { (completion) in
            switch completion{
            case .finished:
                ()
            case .failure(let error):
               // completion(.failure(error))
                print("error  \(error)")
            }
        }, receiveValue: {
            [weak self ](response) in
            
            print("response \(response)")

            switch response.result{
            
            case .success(let model):
                completion(.success(model))
                print("error success")

            case .failure(let error):
                completion(.failure(error))

                print("error failure \(error.localizedDescription)")

            }
        }
    )
}

Swift: Pass type from property to generic function

2 Answers2

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