Testing using mocks

testing_mocks.swift

import XCTest
@testable import WeatherApp

extension XCTestCase { // JSON support
    func jsonData(payload: Any) throws -> Data {
        let options: JSONSerialization.WritingOptions = [.prettyPrinted, .sortedKeys]
        let data = try JSONSerialization.data(withJSONObject: payload, options: options)
        return data
    }
}

class WeatherAppTests: XCTestCase {

    override func setUp() {
        super.setUp()
        // Put setup code here. This method is called before the invocation of each test method in the class.
    }

    override func tearDown() {
        // Put teardown code here. This method is called after the invocation of each test method in the class.
        super.tearDown()
    }

    func testOpenWeatherMap() {
        let exp = expectation(description: "Get weather data")

        let session = MockURLSession()

        self.prepareRetreiveWeatherSessionDataSucess(session)

        let controller = OpenWeatherMapNetworkController(session: session)
        let input = Input(location: "Campana", unit: TemperatureUnit.metric)
        controller.fetchCurrentWeatherData(input: input) { (result: Either<NetworkControllerError, WeatherData>) in
            switch result {
            case .left:
                XCTFail("no data returned by fetchWeatherData()")
            case .right(let data):
                let city = input.location
                let condition = data.condition
                let temperature = data.temperature

                XCTAssertEqual(city, "Campana")
                XCTAssertEqual(condition, "Clear")
                XCTAssertEqual(temperature, 42.8)

                exp.fulfill()
            }
        }
        waitForExpectations(timeout: 10, handler: nil)
    }

    func testOpenWeatherShouldFailWithError() {
        let exp = expectation(description: "Get weather data")

        let session = MockURLSession()
        let error = NSError(domain: "asd", code: 123, userInfo: nil)
        session.error = error as Error

        self.prepareRetreiveWeatherSessionDataSucess(session)

        let controller = OpenWeatherMapNetworkController(session: session)
        let input = Input(location: "Campana", unit: TemperatureUnit.metric)
        controller.fetchCurrentWeatherData(input: input) { (result: Either<NetworkControllerError, WeatherData>) in
            switch result {
            case .left(let error):
                XCTAssertNotNil(error)
                exp.fulfill()
            case .right:
                XCTFail("It should fail")
            }
        }
        waitForExpectations(timeout: 10, handler: nil)
    }
}

extension WeatherAppTests { // MARK: Session mock production
    fileprivate func prepareRetreiveWeatherSessionDataSucess(_ session: MockURLSession) {
        let dataPayload: [String: Any] = [
            "weather": [
                [
                    "id": 800,
                    "main": "Clear",
                    "description": "clear sky",
                    "icon": "01n"
                ]
            ],
            "main":
                [
                    "temp": 42.8,
                    "pressure": 1019,
                    "humidity": 100,
                    "temp_min": 42.8,
                    "temp_max": 42.8
            ]
        ]

        do {
            let data = try self.jsonData(payload: dataPayload)
            session.data = data
        } catch {
            print(error)
        }
    }
}

class MockURLSessionDataTask: URLSessionDataTask {
    // I know, against Apple naming convention,
    // but I rather have the `Mock` part right on front
    // to prevent autocompletion to introduce errors.
    private let closure: () -> Void

    init(closure: @escaping () -> Void) {
        self.closure = closure
    }

    override func resume() {
        self.closure()
    }
}

class MockURLSession: URLSession {
    typealias CompletionHandler = (Data?, URLResponse?, Error?) -> Void

    var data: Data?
    var error: Error?
    var response: URLResponse?

    override func dataTask(with request: URLRequest,
                           completionHandler: @escaping CompletionHandler) -> URLSessionDataTask {
        let data = self.data
        let response = self.response
        let error = self.error

        let dataTask = MockURLSessionDataTask(closure: {
            completionHandler(data, response, error)
        })

        return dataTask
    }
}