Tests-version2

LoginTests.swift

// You implememt a Mock which conforms to the same protocol the ViewController is connected to 
class UserServiceMock: UserServiceInterface {
    
    var resultToReturn: Bool = false
    var errorToReturn: Error?
    
    func login(email: String, password: String, completion: (Bool, Error?)) {
        completion(resultToReturn, errorToReturn)
    }
}

class LoginTests: XCTestCase {
    
    let viewControllerToTest = ViewController()

    func testLoginFailed() {
        
        let mockService = UserServiceMock()
        mockService.resultToReturn = false
        mockService.errorToReturn = Error("no internet")
        
        viewControllerToTest.userService = mockService
        
        // Since this is an async call we need to wait for the callback in the test - otherwise we will not run long enought to get the response
        let loginExpectation = expectation(description: "should call login")
        viewController.login("testEmail", "testPassword", completion: { success in
            loginExpectation.fulfill()
            XCTAssertFalse(success, "the login should fail")
        })
        
        wait(for: loginExpectation, timeout: 0.1)
    }
}