MVC Structure in swift - ios

Question

Do we need to create the Web services singleton class file out of the MVC folder, while following MVC Structure or it stays in Model folder together with the Mapper classes

Answer 1

It is a good practice to avoid using singletons. Although they are fast to create, they are not very scalable, difficult to test, etc, Try to use dependency injections instead.
Singletons are easier to create, easier to use, but it fix you to using specific object that is difficult to replace, if needed. For example you have Store class that supplying name of current user to your VC and it had NetworkClient that fetching User from the network. You can declare both of them as Singletons. It means that any VC had to use same Store.sharedInstance, and store had to use NetworkClient .sharedInstance. Imagine that you want to create UnitTests. As your Store depends on sharedInstance of Client you can't test them separately. It is difficult to test without network, or with timeouts etc. And who's responsible if test failed. Store itself? Network client? Mapper?
In Dependecy Injection pattern instead of using singular instance of store and client, you creating variable in AppDelegate for example, an provide them to all instances you need. Store instead of using shared instance of network client, using some instance that conforms to protocol. Developer can choose what client dependency Store had to use, inject appropriate depends on situation. Usually injection happening during initialisation. In case of tests you can supply you classes with small predictable mocks.
protocol UserStoring: class {
var userName: String { get }
}
final class Store: UserStoring {
private var client: NetworkClient
init (client: NetworkClient) {
self.client = client
}
private var cachedUser: User? = nil
var userName: String {
guard let user = cachedUser else {
client.fetchUser(userId: "current") { userFromClient in
self.cachedUser = userFromClient
}
return " ... "
}
return user.name
}
}
protocol DataFetching {
func fetchUser(userId: String, handler: #escaping (User)->() )
}
final class NetworkClient: DataFetching {
private var session: URLSession
private var baseURL: URL
private var mapper: Mapper
init(session: URLSession, baseURL: URLSession, mapper: Mapper) {
self.session = session
self.baseURL = baseURL
self.mapper = mapper
}
func fetchUser(userId: String, handler: #escaping (User)->() ) {
session.dataTask(with: baseURL.appendingPathComponent("user/\(userId)")) { (data, response, error) in
let user = self.mapper.map(data)
handler(user)
}
}
}
You can even to create a singleton that will keep all shared services in one place, but as you have UIApplication singleton, so you can utilise it: UIApplication.sharedApplication.dependencyService and initialise it in didFinishLaunch.
protocol DependencyServiceSharing {
var client: DataFetching { get }
var mapper: DataMapping { get }
var store: UserStoring { get }
}
class DependencyService: DependencyServiceSharing {
let client: DataFetching
lazy var mapper: DataMapping = Mapper()
lazy var store: UserStoring = Store(client: client)
init(basePath: String) {
client = NetworkClient(session: URLSession.shared, baseURL: URL(string: basePath) , mapper: self.mapper)
}
}
protocol DependencyServiceKeeping: class {
var dependencyService: DependencyServiceSharing? { get }
}
extension UIApplication: DependencyServiceKeeping {
var dependencyService: DependencyServiceSharing? {
return (delegate as? DependencyServiceKeeping)?.dependencyService
}
}
class AppDelegate: UIResponder, UIApplicationDelegate, DependencyServiceKeeping {
var dependencyService: DependencyServiceSharing?
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
dependencyService = DependencyService(basePath: "https://google.com")
...