Local and Network Data Manager

Question

I'm facing a strange issue i'm not able to track down.

I have a DataManager that is responsable to return data to my TableViewController.
This is the code

typealias NewsListCompletionBlock = (Result<NewsList>) -> Void
class DataManager {
    var subManager: DataManagerProtocol
    init() {
        if (Config.isUITests()) {
            subManager = LocalDataManager()
        } else {
            subManager = NetworkDataManager()
        }
    }
}
// MARK: News data manager
extension DataManager {
    func getNewsList(completion:@escaping NewsListCompletionBlock) {
        subManager.performRequest(route: nil, decoder: JSONDecoder(), 
    completion: completion)
    }
}

This initializes a subManager based on if we are running UITests or we are running the real application.
Real application works as expected, but the LocalDataManager returns an error i can't track down.

class LocalDataManager: DataManagerProtocol {
@discardableResult
func performRequest<T>(route: APIRouter?, decoder: JSONDecoder = JSONDecoder(), completion: @escaping (Result<T>) -> Void) -> DataRequest where T : Decodable {
    let newsListString = """
    {
    "news" : [
        {
            "id": 262,
            "category": "e",
            "title": "Scott Johnson & Paul Fowler Workshop",
            "subtitle": "Wedding Photography Workshop | 20th-24th May 2019, Italy",
            "link": "",
            "date": "2018-08-31"
        }
    ]
    }
    """
    if let jsonData = newsListString.data(using: .utf8)
    {
        let result :Result<NewsList>
        do {
            let newsListObject = try JSONDecoder().decode(NewsList.self, from: jsonData)
            result = Result.success(newsListObject)
        } catch {
            result = Result.failure(NSError(domain: "", code: 123, userInfo: nil))
        }
        completion(result)
    }
}
}

This is the code of the LocalDataManager. As you can see i simulate the call of the completion block providing dummy data, but when i compile i get

DataManager/LocalDataManager.swift:38:24: Cannot convert value of type 'Result<NewsList>' to expected argument type 'Result<_>'

Answer 1

This is because you are using the NewsList explicitly in the place of generic parameter.

Changing the code to follows will work.

if let jsonData = newsListString.data(using: .utf8)
    {
        let result :Result<T>
        do {
            let newsListObject = try JSONDecoder().decode(T.self, from: jsonData)
            result = Result.success(newsListObject)
        } catch {
            result = Result.failure(NSError(domain: "", code: 123, userInfo: nil))
        }
        completion(result)
    }