MyViewTests.swift

class MyViewTests: XCTestCase {
    func testMyView() throws {
        let dataFetcher = MockDataFetcher()
        let viewModel = MyViewModel(dataFetcher: dataFetcher)
        let view = MyView(viewModel: viewModel)

        let window = UIWindow()
        window.rootViewController = UIHostingController(rootView: view)
        window.makeKeyAndVisible()

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

        dataFetcher.fetchDataResult = "test data"
        viewModel.fetchData()
        let label = try XCTUnwrap(window.rootViewController?.view.subviews.first as? UILabel)
        XCTAssertEqual(label.text, "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 Result.success(result).publisher
        } else {
            return Result.failure(MockError.unknown).publisher
        }
    }
}

enum MockError: Error {
    case unknown
}