AuthorViewModelTests.swift

class FakeAuthorService: AuthorServiceProtocol {
    func fetchAllAuthors() -> [Author] {
        [
            Author(firstName: "Frank", lastName: "Herbert", booksWritten: 27),
            Author(firstName: "Roald", lastName: "Dahl", booksWritten: 19),
            Author(firstName: "Haruki", lastName: "Marukami", booksWritten: 14)
        ]
    }
}

class AuthorViewModelTests: XCTestCase {
    func testFetchAllAuthors() throws {
        let viewModel = AuthorViewModel(service: FakeAuthorService())
        let authors = viewModel.allAuthors()
        
        XCTAssertEqual(authors.count, 3)
        XCTAssertEqual(authors.first!.firstName, "Frank")
    }
}