public protocol DataSource: class {
    associatedtype AbstractType
    func source() -> AbstractType
}

class ViewController<T: DataSource>: UIViewController {
    
    weak var dataSouce: T?
    typealias AbstractType = T.AbstractType
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let source: AbstractType? = dataSouce?.source()
        print("Source: ", source)
    }
}

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate, DataSource {

    var vc: ViewController<AppDelegate>?
    var window: UIWindow?
    
    typealias AbstractType = Date

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        // Override point for customization after application launch.
        
        vc = ViewController<AppDelegate>()
        vc?.dataSouce = self
        window = UIWindow(frame: UIScreen.main.bounds)
        window?.rootViewController = vc
        window?.makeKeyAndVisible()
        return true
    }

    // MARK: - DataSource
    func source() -> AbstractType {
        return Date()
    }
}