Unit Testing and Why it Matters

Vodafone Proprietary classified as C2 - Internal
Unit Testing
and Why it Matters
Yahya Saddiq
1 February 2020

2
Hello!
I am Yahya Saddiq, hands-on iOS Lead, _VOIS
at Vodafone.
A father of two, fitness enthusiast and coding
is my passion.
You can find me on LinkedIn.

The Beginning
2 February 20203
• Unit Testing is really important to anyone who is getting started in software
development, whether you’ve just discovered it, self teaching or going to school.
• Even if you’ve got yourself into new job, may be you haven’t been exposed to it yet
cause maybe your organization doesn’t do that.

The Beginning
2 February 20204
• It’s really important to learn it from the beginning cause you never know at what
point at your career or job interview ask you about unit testing and currently
that’s inevitable.
• It’s really important to have the conceptual ideas down your head and being able
to explain it to someone.
Unit Testing is Awesome
😎

What is Unit Testing?
2 February 20205

2 February 20206

2 February 20207
• Unit testing helps you to find failures and embarrassing mistakes from being
introduced into your production app
• Helps you have a software that actually works reliably.
• Unit Testing is a White Box testing

2 February 20208
• The purpose is to validate that each unit of the software code performs as
expected.
• Unit Testing is done during the development of an application by you (developers).
• Unit Tests isolate a section of code and verify its correctness.
• A unit may be an individual
– Function
– Method
– Procedure
– Module
– or Object.

Why We Should Do it?
2 February 20209
• We write unit tests, because we want our code to work and to keep it working.
• Unit tests are a great way to write better code.
• Tests help you find most of the bugs early on in the process.
• More importantly, writing code in a test-based development mindset helps you write
modular code that’s easy to maintain.
Easy to test code,
is easy to maintain and debug code.

2 February 202010
When it comes to testing, there’s good news, and bad news.
• The bad news is that there can be disadvantages to unit tests.
– More code
– More to maintain
– No silver bullet
– Takes longer

2 February 202011
Although there is no silver bullet, there is a silver lining.
• Testing has the following advantages
– Protection against regression
– Less coupled code
– Confidence
– Quick feedback
– Focus
– Refactoring
– Executable documentation
Maximizes laziness
😴

How The Process Works in Real Software
World?
2 February 202012

The 5 characteristics of Unit Testing
2 February 202013
•Fast.
•Independent/Isolated.
•Repeatable.
•Self-validating.
•Timely
Following the FIRST principles,
keeps your tests clear and helpful.

A simple gallery app
2 February 202014
We are going to test a simple app, in which:
– The app fetches popular photos from 500px API and lists photos in
a UITableView.
– Each cell in the table view shows a title, a description and the
created date of a photo.
– Users are not allowed to click photos which are not labeled
for_sale.

A simple gallery app
2 February 202015
• The first screen in the app is called PhotoListViewController,
and this is its the data flow.
• We are going to test:
1. Does APIService work and return valid photos?
2. Does photoListCellViewModel correctly created from a photo
object?
3. Does PhotoListViewModel fetch data from the APIService?
4. Does PhotoListViewModel display an error if the request failed?
5. Does PhotoListViewModel allow the segue if the user presses a “for
sale” photo?

The First Test Case
2 February 202016
Does photoListCellViewModel correctly created from a photo object?
• Create a test class for PhotoListViewModel.
• Let’s give the unit tests an access to the internal types and functions in
MVVMPlayground.
@testable import MVVMPlayground.
• Create a placeholder for a PhotoListViewModel, which is the System Under Test, or
the object this test case class is concerned with testing.
var sut: PhotoListViewModel!

Does photoListCellViewModel correctly
created from a photo object?
2 February 202017
• Create a PhotoListViewModel object at the class level, so all the tests in this test
class can access the SUT object’s properties and methods.
super.setUp()
sut = PhotoListViewModel()
• Before we forget, release the SUT object in tearDown().
sut = nil
super.tearDown()
• Setup and teardown prepare initial state before tests run and perform cleanup after
tests complete.

2 February 202018
// A test method’s name always begins with test, followed by a description of what it tests.
func test_cell_view_model() {
// It’s good practice to format the test into given, when and then sections
// Given: Here, you set up any values needed.
// In this example, today’s date and a dummy photo object are created.
let today = Date()
let photo = Photo(id: 1,
name: "Name",
description: "desc",
created_at: today,
image_url: "url",
for_sale: true,
camera: "camera")
// When: In this section, you’ll execute the code being tested: Call createCellViewModel(photo:).
let cellViewModel = sut!.createCellViewModel(photo: photo)
// Then: This is the section where you’ll assert the result you expect with a message that prints if the test fails.
XCTAssertEqual(cellViewModel.descText, "(photo.camera!) - (photo.description!)")
}

Debugging a Test
2 February 202019
• To cause a test failure, let’s change desc join character to +
• In the Breakpoint navigator, add a Test Failure Breakpoint.
– It will stop the test run when a test method posts a failure assertion.
• Run the test, and it should stop at the XCTAssertEqual line with a test failure.

Test Asynchronous Operations
2 February 202020
Does APIService work and return valid photos?
• URLSession methods are asynchronous:
– They return right away, but don’t finish running until later.
– use XCTestExpectation to make your test wait for the asynchronous
operation to complete.

2 February 202021
@testable import MVVMPlayground
class APIServiceTests: XCTestCase {
var sut: APIService!
override func setUp() {
super.setUp()
sut = APIService()
}
override func tearDown() {
sut = nil
super.tearDown()
}
}

2 February 202022
func test_fetch_popular_photos() {
// Given: Returns an XCTestExpectation object, stored in promise.
// The description parameter describes what you expect to happen.
let promise = XCTestExpectation(description: "Fetch popular photos
completed")
// When
sut.fetchPopularPhoto(complete: { (photos, error) in
// Then
guard error == nil,
let photos = photos else {
if let errorDesc = error?.rawValue {
XCTFail("Error: (errorDesc)")
}
return
}
XCTAssertEqual(photos.count, 20)
for photo in photos {
XCTAssertNotNil(photo.id)
}
// Call this in the success condition closure of the
asynchronous method’s completion handler to flag that the expectation
has been met.
promise.fulfill()
})
// Keeps the test running until all expectations are fulfilled, or
the timeout interval ends, whichever happens first.
wait(for: [promise], timeout: 3.1)
}

Fail Fast
2 February 202023
• Failure hurts, but it doesn’t have to
take forever.
• Let’s update the file name to test a
failure and run the test.
• Now, Let’s improve it and make the
test fail faster by changing the
assumption.
• Run the test. It should now take about
a second to fail.
func test_fetch_popular_photos_completes() {
// Given
let promise = XCTestExpectation(description: "Fetch popular
photos completed")
var responseError: Error?
var responsePhotos: [Photo]?
// When
sut.fetchPopularPhoto(complete: { (photos, error) in
responseError = error
responsePhotos = photos
promise.fulfill()
})
wait(for: [promise], timeout: 3.1)
// Then
XCTAssertNil(responseError)
XCTAssertEqual(responsePhotos?.count, 20)
}

Faking Objects and Interactions
2 February 202024
• Asynchronous tests give you confidence that your code generates correct input to
an asynchronous API.
• Most apps interact with system or library objects — objects you don’t control such
as Alamofire, NotificationCenter or UserDefaults. Tests that interact with these
objects can be slow and unrepeatable, violating two of the FIRST principles.
• Instead, you can fake the interactions by getting input from stubs or by
updating mock objects.
• We should mock all mentioned Black Boxes because unit testing is White Box
testing.

Mocking - Fake Update to Mock Object
2 February 202025
Does PhotoListViewModel fetch data from the APIService?
• We use a technique named Dependency Injection (DI) to design
our PhotoListViewModel.
• The property apiService is a dependency of the PhotoListViewModel.
• In the production environment we assign an APIService object, which connects to a
real server, to the PhotoListViewModel.
• In the test environment, we inject a mock APIService object instead.

Does PhotoListViewModel fetch data from
the APIService?
2 February 202026
@testable import MVVMPlayground
// The mock APIService (APIServiceMock) object doesn’t connect to the real server,
// it’s an object designed only for the test.
// Both APIService and APIServiceMock conform to APIServiceProtocol,
// so that we are able to inject different dependency in different situation.
class APIServiceMock: APIServiceProtocol {
var isFetchPopularPhotoCalled = false
func fetchPopularPhoto(complete: @escaping ([Photo]?, APIError?) -> ()) {
isFetchPopularPhotoCalled = true
}
}

the APIService?
2 February 202027
• Create an instance of the mock in PhotoListViewModelTests
var apiServiceMock: APIServiceMock!
apiServiceMock = APIServiceMock()
• Inject apiServiceMock to PhotoListViewModel
sut = PhotoListViewModel(apiService: apiServiceMock)
• Add the test case
func test_fetch_photo() {
// When
sut.initFetch()
// Then
XCTAssert(apiServiceMock.isFetchPopularPhotoCalled)
}

the APIService?
2 February 202028
• In addition to the behavior test, we also want to see if
the PhotoListViewModel correctly handles networking states.
• By using the DI technique, we are able to simulate the success and failure
networking states by changing the response of the MockAPIService.
• Let’s take a look at the PhotoListViewModel.initFetch():

Does PhotoListViewModel display an error if the request
failed?
2 February 202029
var completeClosure: (([Photo]?, APIError?) -> ())!
// the callback closure is saved to the completeClosure for the later use.
completeClosure = complete
}
// the function call is finished but the escaping closure is still pending.
//The closure won’t be triggered until the fetchSuccess() or the fetchFail(error:) is called.
// Finally, when we call the fetchSuccess() or the fetchFail(error:),
// the PhotoListViewModel receives the response data and continue to finish its jobs.
func fetchSuccess() {
completeClosure([Photo](), nil)
}
func fetchFail(error: APIError?) {
completeClosure(nil, error)
}
}

Does PhotoListViewModel display an error if the request
failed?
2 February 202030
func test_fetch_photo_fail() {
// Given
let error = APIError.permissionDenied
// When
sut.initFetch()
// here’s a trick: we ask the mock to fail the request by calling the fetchFail(error:) function.
apiServiceMock.fetchFail(error: error)
// Then
// assert the alert message of the PhotoListViewModel to see if it handles the error correctly.
XCTAssertEqual(sut.alertMessage, error.rawValue)
}

Stubbing - Fake Input From Stub
2 February 202031
Does PhotoListViewModel allow the segue if the user presses a “for sale” photo?
• The ViewModel receives the user interaction and changes the presentation with
respect to the interaction.
• In MVVM, the user interactions are abstracted into a set of methods such
as userPressed(), userSwipe(), etc.
• Therefore, testing the user interactions is straightforward: we call a certain method
and assert the corresponding property of the ViewModel.

Does PhotoListViewModel allow the segue if the user
presses a “for sale” photo?
2 February 202032
func test_user_press_for_sale_item() {
// Given
let indexPath = IndexPath(row: 0, section: 0)
// When
sut.userPressed(at: indexPath)
// Then
XCTAssertTrue(sut.isAllowSegue)
}

2 February 202033
// We create an array, completePhotos, to save the stubs.
// And those stubs will be returns to the PhotoListViewModel once the PhotoListViewModel calls
fetchPopularPhoto(complete:).
var completePhotos: [Photo] = [Photo]()
var completeClosure: (([Photo]?, APIError?) -> ())!
completeClosure = complete
}
func fetchSuccess() {
completeClosure(completePhotos, nil)
}
func fetchFail(error: APIError?) {
completeClosure(nil, error)
}
}

2 February 202034
func test_user_press_for_sale_item() {
// Given
let indexPath = IndexPath(row: 0, section: 0)
// The StubGenerator().stubPhotos() generates couple of photo objects.
// Then we assign the photo objects to the mockAPIService.completePhotos.
apiServiceMock.completePhotos = StubGenerator().stubPhotos()
sut.initFetch()
// When the request finished (mockAPIService.fetchSuccess() is called),
// the PhotoListViewModel will receive those photo stubs via the callback closure.
apiServiceMock.fetchSuccess()
// When
sut.userPressed(at: indexPath)
// Then
XCTAssertTrue(sut.isAllowSegue)
}

Code coverage
2 February 202035
• The code coverage tool tells you what app code is actually being run by your tests,
so you know what parts of the app code aren’t (yet) tested.
• To enable code coverage, edit the scheme’s Test action and check the Gather
coverage for check box under the Options tab.
• Run all tests (Command-U), then open the Report navigator (Command-9).
Select Coverage under the top item in that list:
• Research shows that 80% is best cost-efficient code coverage.

Do's and Don'ts
2 February 202036
• Verify One Concern Per Test Method
– A concern is a single end result: a return value or change to app state.
– if the first assert fails, does it matter what happens to the next one?
– If yes, you must split the test into smaller ones.
• Control Side-Effects
– When we change global app data from our test, this indirectly affects the whole test suite.
– To prevent this from happening you must cleanup before and after each test
• Do Not Leak Test Code into Production
– Firstly, do not weaken encapsulation for the purpose of testing.
– It’s either a flaw in the app design or you might be testing too much.
– Secondly, do not put logic into production code, only to support testing.
– Changes in your test code should not affect the production.

2 February 202037
Recap
• We used Unit Test to check:
– If APIService works and returns valid photos.
– If photoListCellViewModel correctly created from a photo object.
– If the SUT correctly interacts with the APIService.
– If the SUT handles the error state correctly.
– If the SUT handles the user interaction correctly.
• Now, you are able to use the knowledge you learned to write more test cases:
– The loading animation should start when the network request starts.
– The loading animation should stop when the request completes.
– The table view should render correctly.
– It should display error message when user press on the photo that is not for sale.

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