di-1.swift

protocol BookService {
    typealias Handler = (Books) -> Void
    
    // Global popular books (Anonymous)
    func fetchPopularBooks(then handler: Handler) 
    
    // Special books based on logged in user preferences
    func fetchRelatedBooks(then handler: Handler) 
}

protocol Auth {
    var isAuthenticated: Bool { get }
}

class BooksViewContoller: UIViewController {

    private let bookService: BookService
    private let auth: Auth

    init(bookService: BookService, auth: Auth) {
        self.bookService = bookService
        self.auth = auth
        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder aDecoder: NSCoder) { fatalError("init(coder:) has not been implemented") }

    override func viewDidLoad() {
        super.viewDidLoad()
        fetchBooks()
    }

    func fetchBooks() {
        let hander: (Books) -> Void = { [weak self] books in
            self?.display(books)
        }

        if auth.isAuthenticated {
            bookService.fetchRelatedBooks(then: hander)
        } else {
            bookService.fetchPopularBooks(then: hander)
        }
    }

    private func display(_ books: Books) {
        print(books)
    }
}