mocking_in_swift_test.swift

func testExecuteRequest_failure_noData() throws {

        /// create mock to return nil
        class NetworkMock : NetworkClientProtocol{
            func executeRequest(request: URLRequest, completion: @escaping (Data?, Error?) -> Void) {
                completion(nil, nil)
            }
        }

        /// inject mock and execute request
        var networkResultOptional : Result<Data, Error>?
        let client = NetworkClient(p: NetworkMock())
        client.executeRequest(request: URLRequest.station(station: Station(id: "BHM"))){ result in
            networkResultOptional = result
        }

        /// assert
        guard let networkResult = networkResultOptional else {
            return XCTFail("NetworkClient Result is nil")
        }
        switch networkResult{
        case .success(let data):
            return XCTFail("NetworkClient was meant to fail with a noData error but returned success with data \(data)")
        case .failure(let error):
            XCTAssertEqual(error.localizedDescription, NetworkError.noData.localizedDescription)
        }

}