Patterns & practices for unit-testing Swift-ly

Jakub Turek18th June, 2018

About me

Jakub Turek

https://jakubturek.com @KubaTurek turekj EL Passion

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Agenda

1. Introduction to unit-testing.▶ Test Driven Development.▶ Reasons (not) to use TDD approach.

2. Engineering for testability.3. Common obstacles:

▶ view controllers,▶ global functions.

4. Auto-generating test code.5. Metrics.

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Introduction to testing

Unit testing

A unit test is an automated piece of code that invokesa unit of work in the system and then checks a singleassumption about the behavior of that unit of work.

— Roy Osherove, The art of unit testing

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Good unit test

Traits of a good unit test:

▶ automated,▶ fast,▶ tests a single logical concept in the system,▶ trustworthy,▶ readable,▶ maintanable.

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TDD

TDD is a programming technique which combines wri-ting a test before writing just enough production codeto fulfill that test and refactoring.

— Kent Beck, Test-Driven Development by example

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Typical response to TDD

We don’t do TDD because:

1. We are not allowed to.2. We don’t have enough time.3. It didn’t work for us*.

* Might indicate feedback loop problems.

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TDD workflow

1. Red - write a failing test.2. Green - write minimal amount of code to pass.3. Refactor.

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3 laws of TDD

1. You can’t write any production code until you writea failing unit test.

2. You can’t write more of a unit test than is sufficient to fail.Not compiling is failing.

3. You can’t write more production code than is sufficient topass currently failing unit test.

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Demo

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TDD research

Research carrier Effort QualityN. Nagappan¹

+15..35%-40..90%

Microsoft & IBM less defectsB. George, L. Williams²

+16%+18%

NCSU test cases passed

¹ Realizing quality improvement through test driven development² An Initial Investigation of Test Driven Development in Industry

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Engineering for testability

Pure functions

How to write testable code?

1. Pass values to functions.2. Return values from functions.

— @mdiep, Matt Diephouse, Twitter

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Boundaries

Boundaries by Gary Bernhardt

This talk is about using simple values (as opposed to complexobjects) not just for holding data, but also as the boundariesbetween components and subsystems.

https://destroyallsoftware.com/talks/boundaries

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Imperative shell, functional core

Imperative shell:

▶ real worlddependencies,

▶ side-effects,▶ stateful operations.

Functional core:

▶ decisions,▶ purely functional

transformations.

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Demo

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Other appliances

Developers use this approach already:

▶ Redux-like architecture:▶ ReSwift,▶ RxFeedback.

▶ View models and data bindings:▶ RxCocoa,▶ ReactiveCocoa.

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Resources

Controlling Complexity in Swifthttps://academy.realm.io/posts/andy-matuschak-controlling-complexity/

Enemy of the Statehttps://speakerdeck.com/jspahrsummers/enemy-of-the-state

Imperative Shell, Functional Core Gisthttps://tinyurl.com/y9cxblm8

Imperative Shell, Functional Core on iOShttps://jakubturek.com/imperative-shell-functional-core/

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Dealing with massive controllers

Metrics (1/5)

Examine the largest files:

find Sources -name '*.swift' \| xargs wc -l \| sort -r

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Metrics (2/5)

Output:

??? AllAssemblies.generated.swift??? ContentController.swift??? PickerController.swift??? MapController.swift??? ClassCell.swift??? CalendarController.swift??? ContactController.swift??? AuthorizeController.swift

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Metrics (3/5)

Actual line counts:

148 AllAssemblies.generated.swift140 ContentController.swift139 PickerController.swift138 MapController.swift138 ClassCell.swift138 CalendarController.swift137 ContactController.swift135 AuthorizeController.swift

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Metrics (4/5)

Examine the average controller size:

cloc Modules \--include-lang=Swift \--match-f='.+Controller\.swift'

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Metrics (5/5)

▶ Controllers in total: 164.▶ Screens in total: 38.▶ Average 4.32 controller per screen.▶ Lines in total: 12327.▶ Average 75.16 lines of code per controller.

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More View Controllers

How to decouple controllers?

Never Subclass as a last resort. Add isolated child viewcontrollers inside a parent:

1. Create a protocol for child controller boundaries.2. Inside a parent refer to a child with UIViewController& ChildProtocol type.

3. Inject child factory into a parent controller.

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Adding a child controller (1/2)

func embed(child: UIViewController,inside view: UIView) {

addChildViewController(child)view.addSubview(child.view)

child.snp.makeConstraints {$0.edges.equalToSuperview()

}

child.didMove(toParentViewController: self)}

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Adding a child controller (2/2)

func unembed(child: UIViewController) {child.willMove(toParentViewController: nil)child.view.removeFromSuperview()child.removeFromParentViewController()

}

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Decoupling controllers (1/2)

protocol ChildControllerType: class {var onProductSelected: ((String) -> Void)?

}

class ChildController: UIViewController,ChildControllerType {

init(/* dependencies */) { /* ... */ }

var onProductSelected: ((String) -> Void)?

override func viewDidLoad() {/* implementation */

}}

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Decoupling controllers (2/2)

typealias ChildControlling =UIViewController & ChildControllerType

class ParentController: UIViewController {init(factory: () -> ChildControlling) {self.factory = factory

}

override func viewDidLoad() {super.viewDidLoad()embed(child: child, inside: view)

}

private lazy var child = factory()private let factory: () -> ChildControlling

} 26

Stubbing a child controller (1/2)

class ChildControllerStub: UIViewController,ChildControllerType {

var onProductSelected: ((String) -> Void)?}

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Stubbing a child controller (2/2)

class ParentControllerTests: XCTestCase {var childStub: ChildControllerStub!var sut: ParentController!

override func setUp() {super.setUp()childStub = ChildControllerStub()sut = ParentController(factory: { childStub })

}

func testThatSelectedProductNameIsDisplayed() {childStub.onProductSelected?("Water")XCTAssertEqual(sut.view.label.text, "Water")

}}

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Child view controller examples

▶ Buttons:▶ perform side-effects, expose results,▶ error handling,▶ disable interaction during asynchronous operations.

▶ Forms:▶ user input validation,▶ complex presentation,▶ error handling.

▶ API data coordination. Immutable children for presenting:▶ data,▶ empty state,▶ error.

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Global methods

Global method example (1/2)

extension UIImage {static func with(color: UIColor) -> UIImage? {let origin = CGPoint(x: 0, y: 0)let size = CGSize(width: 1, height: 1)let rect = CGRect(origin: origin, size: size)

return CGContext.drawImage(of: size) { ctx inctx.setFillColor(color.cgColor)ctx.fill(rect)

}}

}

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Global method example (2/2)

static func drawImage(of size: CGSize,using drawer: (CGContext) -> Void)-> UIImage? {

UIGraphicsBeginImage...(size, false, 0.0)

defer { UIGraphicsEndImageContext() }

guard let ctx = UIGraphicsGetCurrentContext() else {return nil

}

drawer(ctx)

return UIGraphicsGetImage...()}

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Swift namespace resolution (1/2)

Shipping your own controller type:

class UIViewController {func theTimeHasComeToLoadAView() {myOwnPersonalView = SomeView()

}}

let controller = UIViewController()controller.theTimeHasComeToLoadAView() // works

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Swift namespace resolution (2/2)

Shipping your own controller library:

# Podfilepod 'MyOwnController', '~> 0.1'

// SourceFile.swiftimport MyOwnControllerimport UIKit

UIViewController() // compilation error

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Testing global methods (1/2)

import UIKit

var imageFromContext: () -> UIImage? =UIKit.UIGraphicsGetImageFromCurrentImageContext

func UIGraphicsGetImageFromCurrentImageContext()-> UIImage? {

return imageFromContext()}

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Testing global methods (2/2)

override func setUp() {imageFromContext = { UIImage.catMeme }

}

override func tearDown() {imageFromContext = UIKit.UIGraphicsGetImage...

}

func testThatDrawImageReturnsImageFromContext() {let image = CGContext.drawImage(of: .zero) { _ in }

XCTAssertEqual(UIImagePNGRepresentation(image),UIImagePNGRepresentation(imageFromContext())

)} 35

Test code generation

Sourcery

Sourcery

Sourcery is a code generator for Swift language, built on top ofApple’s own SourceKit. It extends the language abstractions toallow you to generate boilerplate code automatically.

https://github.com/krzysztofzablocki/Sourcery

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Problem: equality check

var sut: Account!

override func setUp() {super.setUp()sut = try! JSONDecoder.default.decode(

Account.self,from: Bundle.jsonData(file: "account.json")

)}

func testThatAccountIsDeserialized() {XCTAssertEqual(sut.id, 32)XCTAssertEqual(sut.email, "hello@gmail.com")/* and so on... */

}37

Account+AutoEquatable.swift

Equatable conformance can be automatically synthesized ina test target.

protocol AutoEquatable {}

extension Account: AutoEquatable {}

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AutoEquatable.generated.swift

public func == (lhs: Account,rhs: Account) -> Bool {

guard lhs.id == rhs.id else { return false }guard lhs.email == rhs.email else {return false

}guard compareOptionals(lhs: lhs.firstName,rhs: rhs.firstName,compare: ==

) else { return false }/* and so on... */return true

}39

AutoEquatable vs. auto-synthesized Equatable

Sourcery’sAutoEquatable:▶ Can be declared as an

extension.▶ Required additional

setup to work.▶ Conformance can be

asserted.

Automatically synthesizedEquatable:▶ Introduced in Swift 4.1

(Xcode 9.3).▶ Cannot be declared as

an extension.▶ Featuresassociatedtype.

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Problem: required initializers

0% code coverage when not using inteface builder

required init?(coder aDecoder: NSCoder) {return nil

}

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AllViews.stencil

extension UIView {

static var allInitializers: [(NSCoder) -> UIView?] {return [{% for view in types.classes where view.based.UIView %}{% set spacer %}{% if not forloop.last %},{% endif %}{% endset %}{% for initializer in view.initializers %}{% if initializer.selectorName == "init(coder:)" %}{{ view.name }}.init(coder:){{ spacer }}{% endif %}{% endfor %}{% endfor %}]

}

}

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AllViews.generated.swift

extension UIView {

static var allInitializers: [(NSCoder) -> UIView?] {return [

ActionBarView.init(coder:),AuthorizeErrorView.init(coder:),BlurView.init(coder:),BorderedButtonView.init(coder:),FilterView.init(coder:),HeaderView.init(coder:),/* ... */

]}

}

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AllViewsTests.swift

func testThatAllViewsAreNonCodable() {UIView.allInitializers.forEach { initializer inXCTAssertNil(initializer(NSCoder()))

}}

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Other usages

▶ Mock object generation.▶ Complex assertions:

▶ There is a factory class for every Route.▶ There is an integration test for every request.

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Metrics

Metrics

Measured on every pull request:

▶ Project size:▶ lines of code,▶ files count,▶ average file size.

▶ Static analysis:▶ SwiftLint: consistent coding style,▶ jscpd: automatic copy-paste detection.

▶ Code coverage.

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Metrics - visualization

Danger

Danger runs during your CI process, and gives teams thechance to automate common code review chores.

http://danger.systems/js/swift.html

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Automatic copy-paste detection

JSCPD

jscpd is a tool for detect copy/paste ”design pattern” inprogramming source code.

https://github.com/kucherenko/jscpd

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JSCPD + Danger = (1/3)

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JSCPD + Danger = (2/3)

languages:- swift

files:- "Sources/**"

exclude:- "**/*.generated.swift"

reporter: jsonoutput: jscpd_report.json

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JSCPD + Danger = (3/3)

def find_duplicates`jscpd`

rep = JSON.parse(File.read('jscpd_report.json'))clones = rep["statistics"]["clones"]

if clones > 0warn("JSCPD found #{clones} clone(s)")dups = rep["duplicates"]out = dups.map { |d| format_duplicate d }.join("\n")markdown("## JSCPD issues\n#{out}")

endend

Full version: https://tinyurl.com/yc23t4mb

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Code coverage

Code coverage is a percentage of code which is covered byautomated tests.

Test coverage is a useful tool for finding untested partsof a codebase. Test coverage is of little use as a numericstatement of how good your tests are.

— Martin Fowler, TestCoverage

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danger-xcov (1/2)

danger-xcov

danger-xcov is the Danger plugin of xcov, a friendlyvisualizer for Xcode’s code coverage files.

https://github.com/nakiostudio/danger-xcov

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danger-xcov (2/2)

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Thank you!

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Additional resources

Jakub TurekPatterns & practices for unit-testing Swift-lyhttps://jakubturek.com/talks/

Jon ReidQuality Coding - Bloghttps://qualitycoding.org/blog/

Kasper B. GraversenGist: functional core, imperative shellhttps://tinyurl.com/y9cxblm8

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