Unit Testing and Mocking in Swift (MDevCon 2016)

Having a lot of code that depends on iOS UI frameworks can make it difficult to write reliable unit tests. As Swift replaces Objective-C as the iOS language of choice, it becomes more difficult to mock out dependencies in tests. In this talk, I’ll dissect some testing issues and demonstrate fixing an unreliable sample test suite. We’ll discuss what makes unit tests unreliable, how to make them more stable with dependency injection, and some techniques for writing testable code.

Transcript

1. Unit Testing and Mocking in Swift Ben Asher @benasher44 benasher44@gmail.com
2. Yelp’s Mission: Connecting people with great local businesses.
3. Yelp Stats: As of Q4 2015 86M 32 70% 100M (Tuesday)
4. Testing - Confidence - Final product - Your own code
5. Testing - Tools Acceptance Integration Tests Unit Tests Faster Fewer
6. Testing - Yelp.app - ~1000 unit and integration tests - ~400 acceptance tests Acceptance Unit and Integration - 1 failure cascades into many - Slow
7. - ~1000 unit and integration tests - ~10 minutes (~.6s/test) - ~400 acceptance tests - ~ 1 hour (~3.6s/test) Testing - Yelp.app
8. Testing - Yelp.app - # Acceptance tests increasing faster than unit tests - Suite becoming slow quickly - Suite increasingly unreliable
10. Testing - Yelp.app - ~400 acceptance tests - Interacts with UI - ~1000 unit and integration tests - Interacts with code
11. - Isolate testable code - Mock dependencies for unit tests - Reduce Non-Unit Test Footprint Unit Testing
12. Unit Testing - Isolation 1. What behavior do I want to test? 2. What code is it not my job to test? 2 questions:
13. Simple iOS App - Lists some content - User can interact with content Unit Testing - Isolation
14. Content UIImage List Screen Organize List Layout User Interaction UICollectionView + UICollectionViewFlowLayout
15. Demo: Corgi Collection View Unit Testing - Isolation What behavior do I want to test? What code is it not my job to test?
16. Unit Testing - Dependencies 1. What behavior do I want to test? 2. What code is it not my job to test? 2 questions:
17. Important: What code is it not my job to test? Unit Testing - Dependencies Often live in your class’s ivars References to objects not defined: - In your feature - In your app
18. Important: What code is it not my job to test? Unit Testing - Dependencies var collectionView: UICollectionView? private let selectionSaver = CorgiSelectionSaver()
19. Important: What code is it not my job to test? Unit Testing - Dependencies More difficult: - NSDate() - References to globals buried in your code
20. Problem: Logic has dependency - Your business logic relies on it Unit Testing - Dependencies
21. Problem: Logic has dependency Solution: Mocks! - Object that looks like your dependency, but doesn’t do that much Unit Testing - Dependencies
22. Objective-C (OCMock) Unit Testing - Dependencies - 3rd party library - Open source - Used heavily at Yelp
23. Objective-C (OCMock) Unit Testing - Dependencies id userDefaultsMock = OCMPartialMock([NSUserDefaults standardUserDefaults]); OCMStub([userDefaultsMock integerForKey:”someKey”]).andReturn(@2) Read this as: - Create an object that looks like NSUserDefaults - When something calls integerForKey:”someKey”, return 2
24. Objective-C - Reflection (inspect and modify the runtime) - Slow - Dangerous Unit Testing - Dependencies
25. Objective-C - [UIApplication sharedApplication] - Parts of UIApplication are thread safe - Background task APIs - Once you mock it, no longer the case Unit Testing - Dependencies
26. Unit Testing - Dependencies Swift - Can’t modify the runtime - Pros: - Perf: mock lifecycle eating CPU - Crashes: modifying the runtime is dangerous - Con: No more super convenient mocks
28. - Protocol: “I don’t care what kind of object this is, as long as I can do <x> with it” - Make your dependencies conform to protocols Unit Testing - Dependencies Swift Strategy: Protocols
29. Swift Strategy: Protocols public protocol DefaultsStore { func setObject(value: AnyObject?, forKey defaultName: String) func objectForKey(defaultName: String) -> AnyObject? } extension NSUserDefaults: DefaultsStore {} Unit Testing - Dependencies
30. Protocol Extensions extension protocol DefaultsStore { subscript(key: String) -> AnyObject? { get { return objectForKey(key) } set(value) { setObject(value, forKey: key) } } } Unit Testing - Dependencies userDefaults[key] = obj
31. Demo: Dependency Injection Unit Testing - Dependencies
32. Testing - Big Picture Acceptance Integration Tests Unit Tests Reduce non-unit testing footprint
33. Unit Testing - Big Picture override func viewDidAppear(animated: Bool) { super.viewDidAppear() collectionView?.reloadData() }
34. Unit Testing - Big Picture override func viewDidAppear(animated: Bool) { super.viewDidAppear() collectionView?.reloadData() // Lots of other things we MUST do } Integration test: When event A happens, verify that event B happens in response.
35. Unit Testing - Big Picture override func viewDidAppear(animated: Bool) { super.viewDidAppear() controller.performViewDidAppearActions() } - controller contains logic that’s easy to unit test - Dependencies are made explicit - Doesn’t depend on lifecycle events
36. Unit Testing - Big Picture override func viewDidAppear(animated: Bool) { super.viewDidAppear() controller.performViewDidAppearActions() } - View controller contains minimal configuration - Super readable view controller - Small integration test suite
37. import Foundation import UIKit class CorgiViewController: UIViewController { let controller = CorgiCollectionController() override func loadView() { view = controller.collectionView } override func viewDidAppear(animated: Bool) { super.viewDidAppear(animated) controller.loadCorgis() controller.loadRecentCorgiSelection() } } import Foundation import UIKit let kNumTiles = 100 let kNumCorgis = 4 class CorgiViewController: UIViewController { var collectionView: UICollectionView? private let selectionSaver = CorgiSelectionSaver() override func loadView() { collectionView = newCorgiCollectionView() view = collectionView } override func viewDidAppear(animated: Bool) { super.viewDidAppear(animated) collectionView?.reloadData() if let selection = selectionSaver.loadCorgiSelection() { collectionView?.selectItemAtIndexPath(selection, animated: true, scrollPosition: . CenteredVertically) } } } extension CorgiViewController: UICollectionViewDelegate { func collectionView(collectionView: UICollectionView, didSelectItemAtIndexPath indexPath: NSIndexPath) { selectionSaver.saveCorgiSelection(indexPath) } } extension CorgiViewController: UICollectionViewDataSource { func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell { let cell = collectionView.dequeueReusableCellWithReuseIdentifier(String (CorgiCollectionViewCell), forIndexPath: indexPath) as! CorgiCollectionViewCell let imageIndex = indexPath.item % kNumCorgis let imageName = "corgi\(imageIndex).jpg" cell.image = UIImage(named: imageName) return cell } func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int { return 1; } func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) - > Int { return kNumTiles; } } Before After
38. Unit Testing - Recap How to make your unit test suite great again 1. Isolate the code you want to test Identify the code that isn’t your job to test func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell { // Get The cell let imageIndex = indexPath.item % kNumCorgis let imageName = "corgi\(imageIndex).jpg" // Set the cell’s image, and return the cell }
39. Unit Testing - Recap How to make your unit test suite great again 2. Use dependency injection - Make protocols, as you identify things you don’t want to test in your code
40. Unit Testing - Recap How to make your unit test suite great again 3. Minimize your non-unit test footprint - Create areas in your code that only contain testable logic
41. Unit Testing - References - Demo on Github: https://github. com/benasher44/MDevCon2016Demo - OCMock - http://ocmock.org - Google paper about testing: http://misko.hevery. com/attachments/Guide-Writing%20Testable%20Code. pdf