WebClientTests.swift

final class WebClientTests: XCTestCase {
    override func tearDown() {
        TestURLProtocol.loadingHandler = nil
    }
    
    struct TestPayload: Codable, Equatable {
        let country: String
    }
    
    func testFetchingDataSuccessfully() {
        let expected = TestPayload(country: "Estonia")
        let request = URLRequest(url: URL(string: "https://www.example.com")!)
        let responseJSONData = try! JSONEncoder().encode(expected)
        TestURLProtocol.loadingHandler = { request in
            let response = HTTPURLResponse(url: request.url!, statusCode: 200, httpVersion: nil, headerFields: nil)!
            return (response, responseJSONData, nil)
        }
    
        let expectation = XCTestExpectation(description: "Loading")
        let configuration = URLSessionConfiguration.ephemeral
        configuration.protocolClasses = [TestURLProtocol.self]
        let client = WebClient(urlSession: URLSession(configuration: configuration))
        client.fetch(request, requestDataType: TestPayload.self) { (result) in
            switch result {
            case .failure(let error):
                XCTFail("Request was not successful: \(error.localizedDescription)")
            case .success(let payload):
                XCTAssertEqual(payload, expected)
            }
            expectation.fulfill()
        }
        wait(for: [expectation], timeout: 1)
    }
}