protocol AuthorServiceProtocol {
    func fetchAllAuthors() -> [Author]
}

class AuthorWebService: AuthorServiceProtocol {
    func fetchAllAuthors() -> [Author] {
        // fetch and return authors
    }
}

class AuthorViewModel {
    private let service : AuthorServiceProtocol
    
    init(service: AuthorServiceProtocol = AuthorWebService()) {
        self.service = service
    }
    
    func allAuthors() -> [Author] {
        service.fetchAllAuthors()
    }
}