MockSessionExample.swift

func testRetrieveProductsValidResponse() {
    // we have a locally stored product list in JSON format to test against.
    let testBundle = Bundle(forClass: type(of: self))
    let filepath = testBundle.pathForResource("products", ofType: "txt")
    let data = Data(contentsOfFile: filepath!)
    let urlResponse = HTTPURLResponse(url: URL(string: "https://anyurl.doesntmatter.com")!, statusCode: 200, httpVersion: nil, headerFields: nil)
    // setup our mock response with the above data and fake response.
    MockSession.mockResponse = (data, urlResponse: urlResponse, error: nil)
    let requestsClass = RequestManager()
    // All our network calls are in here.
    requestsClass.Session = MockSession.self
    // Replace NSURLSession with our own MockSession.
    // We still need to test as if it's asynchronous. Because well, it is.
    let expectation = XCTestExpectation(description: "ready")
    // For this test, no need to pass in anything useful since it doesn't affect our mocked response.
    // This particular function fetches JSON, converts it to custom objects, and returns them.
    requestsClass.retrieveProducts("N/A", products: { (products) -> () in
        XCTAssertTrue(products.count == 7)
        expectation.fulfill()
    }) { (error) -> () in
        XCTAssertFalse(error == Errors.NetworkError, "Its a network error")
        XCTAssertFalse(error == Errors.ParseError, "Its a parsing error")
        XCTFail("Error not covered by previous asserts.")
        expectation.fulfill()
    }
    waitForExpectations(timeout: 3.0, handler: nil)
}