MyViewModelTests.swift

class MyViewModelTests: XCTestCase {
    func testFetchData() {
        let dataFetcher = MockDataFetcher()
        let viewModel = MyViewModel(dataFetcher: dataFetcher)

        let expectation = XCTestExpectation(description: "Fetch data")

        dataFetcher.fetchDataResult = "test data"
        viewModel.fetchData()
        
        let cancellable = viewModel.dataPublisher
            .sink(receiveCompletion: { completion in
                switch completion {
                case .finished:
                    break
                case .failure(let error):
                    XCTFail("Publisher failed with error: \(error)")
                }
            }, receiveValue: { value in
                XCTAssertEqual(value, "test data")
            })

        expectation.fulfill()
        wait(for: [expectation], timeout: 1.0)
    }
}

class MockDataFetcher: DataFetcher {
    var fetchDataResult: String?
    func fetchData() -> AnyPublisher<String, Error> {
        if let result = fetchDataResult {
            return Just(result)
                .setFailureType(to: Error.self)
                .eraseToAnyPublisher()
        } else {
            return Fail(error: MockError.unknown)
                .eraseToAnyPublisher()
        }
    }
}

enum MockError: Error {
    case unknown
}