Protocol

ServiceImplementations.swift

struct UserData {
    
}

protocol UserServiceProtocol {
    var user: UserData? { get set }
    func load()
}

class UserGraphQLService: UserServiceProtocol {
    var user: UserData? = nil
    func load() {
        // async load from disk.
        self.user = UserData()
    }
}

class UserDiskService: UserServiceProtocol {
    var user: UserData? = nil
    func load() {
        // async load from graphql endpoint.
        self.user = UserData()
    }
}

class SomeViewController: UIViewController {
    var userService: UserServiceProtocol
    init(userService: UserServiceProtocol) {
        self.userService = userService
        super.init(nibName: nil, bundle: nil)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        userService.load()
    }
}