UserService.swift

protocol UserNetworkService {

  func loadUsers(withURL url: URL, completion: @escaping (Result<[User], Error>) -> Void)
}


class UserService {
    
  let networkService: UserNetworkService

  init(networkService: UserNetworkService) {
    self.networkService = networkService
  }

  func getUser(withId userId: Int, completion: @escaping (User?) -> Void) {
    // ...
  }
}