Testable design example for iOS Apps

This is a sample App to learn testable design.

You can learn the following things by reading this implementation:

• How to make loose coupling for testing
• How to decouple global variables
• How to use type-checking as a test

Architecture

This App adopt Smalltalk flavored MVC (it is not Apple MVC). Smalltalk flavored MVC is a architecture that can test easily. You may know major architectures such as MVVM, MVP, Flux and VIPER, but also Smalltalk MVC can make loose coupling.

While there are a lot of architectures, but they share a common important things that we should do. So, learning this implementation is still worth the candle if you choose other architectures.

Sample Code

In our approach, we create a Xib file per UIViewController. And all UIViewControllers have a initializer that require models.

And we should create ViewBindings and Controllers and connect them to the given Model when UIViewController#loadView() is called.

Concrete implementation is below:

class FooViewController: UIViewController {
    private var model: FooModelProtocol
    private var viewBinding: FooViewBindingProtocol?
    private var controller: FooControllerProtocol?

    init(model: FooModelProtocol) {
        self.model = model
        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder aDecoder: NSCoder) {
        // NOTE: In this project, we do not want to restore the VC.
        return nil
    }

    // Connect Model and ViewBinding, Controller.
    override func loadView() {
        let rootView = FooRootView()
        self.view = rootView

        let controller = FooController(
            observing: rootView.barView,
            willNotifyTo: self.model
        )
        self.controller = controller

        self.viewBinding = FooViewBinding(
            observing: self.model,
            handling: (
                bar: rootView.barView,
                baz: rootView.bazView
            )
        )
        self.viewBinding.delegate = controller
    }
}

// FooModel is a state-machine that can transit to FooModelState.
// Notify change events to others via an observable `didChange` when
// API was successfully done or failed.
class FooModel: FooModelProtocol {
    private let repository: FooRepositoryProtocol
    private let stateVariable: RxSwift.Variable<FooModelState>

    /// An Observable that will notify events when the internal state is changed.
    var didChange: RxSwift.Observable<FooModelState> {
        return self.stateVariable.asObservable()
    }

    /// The current state of the model.
    var currentState: FooModelState {
        get { return self.stateVariable.value }
        set { self.stateVariable.value = newValue }
    }

    init(
        startingWith initialState: FooModelState,
        fetchingVia repository: FooRepositoryProtocol
    ) {
        self.stateVariable = RxSwift.Variable<FooModelState>(initialState)
        self.repository = repository
    }

    func doSomething() {
        switch self.currentState {
        case .preparing:
            // NOTE: Prevent duplicated calls.
            return

        case .success, .failure:
            self.currentState = .preparing

            self.repository
                .doSomething()
                .then { entity in 
                    self.currentState = .success(entity)
                }
                .catch { error in
                    self.currentState = .failure(
                        because: .unspecified(debugInfo: "\(error)")
                    )
                }
        }
    }
}


// States that FooModel can transit to.
enum FooModelState {
    case preparing
    case success(Entity)
    case failure(because: Reason)

    enum Reason {
        case unspecified(debugInfo: String)
    }
}

class FooViewBinding: FooViewBindingProtocol {
    typealias Views = (bar: BarView, baz: BuzzView)
    private let views: Views
    private let model: FooModelProtocol
    private let disposeBag = RxSwift.DisposeBag()

    init(observing model: FooModelProtocol, handling views: Views) {
        self.model = model
        self.views = views

        // NOTE: Change visual by observing model's state transitions.
        self.model
            .didChange
            .subscribe(onNext: { [weak self] state in
                guard let this = self else { return }
                switch state {
                case .preparing:
                    this.views.bar.text = "preparing"
                case let .success(entity):
                    this.views.bar.text = "success \(entity)"
                case let .failure(because: reason):
                    this.views.bar.text = "failure \(reason)"
                }
            })
            .disposed(by: self.disposeBag)
    }
}

class FooController: FooControllerProtocol {
    private let model: FooModelProtocol
    private let view: BarView
    private let disposeBag = RxSwift.DisposeBag()

    init(
        observing view: BarView,
        willNotifyTo model: FooModelProtocol
    ) {
        self.model = model

        // NOTE: Observe UI events from BarView and notify to the FooModel.
        view.rx.tap
            .asDriver
            .drive(onNext: { [weak self] _ in 
                guard let this = self else { return }

                this.model.doSomething()
            })
            .disposed(by: self.disposeBag)
    }
}

How to Connect among UIViewControllers

In this project, use Navigator class for connecting betweren 2 UIViewControllers.

class FooViewController: UIViewController {
    private let navigator: NavigatorProtocol
    private let sharedModel: FooBarModelProtocol

    init(
        representing sharedModel: FooBarModelProtocol,
        navigatingBy navigator: NavigatorProtocol
    ) {
        self.sharedModel = sharedModel
        self.navigator = navigator
        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder aDecoder: NSCoder) {
        // NOTE: We should not instantiate the ViewController by using UINibs to
        // eliminate fields that have force unwrapping types.
        return nil
    }

    @IBAction func buttonDidTap(sender: Any) {
        let nextViewController = BarViewController(
            representing: sharedModel
        )
        self.navigator.navigate(to: nextViewController)
    }
}

And also you can use UIStoryboardSegue, but using the Navigator class have two advantages:

• We can implement easily and simply common behavior (eg. sending logs for analysis)
• We can assert necessary objects at once

Navigator Implementation

/**
 A protocol for wrapper class of `UINavigationController#pushViewController(_:UIViewController, animated:Bool)`.
 */
protocol NavigatorProtocol {
    /**
     Push the specified UIViewController to the held UINavigationController.
     */
    func navigate(to viewController: UIViewController, animated: Bool)
}



class Navigator: NavigatorProtocol {
    private let navigationController: UINavigationController


    init (for navigationController: UINavigationController) {
        self.navigationController = navigationController
    }


    func navigate(to viewController: UIViewController, animated: Bool) {
        self.navigationController.pushViewController(
            viewController,
            animated: animated
        )
    }
}

How to Control Global Variables

In this project, we control global variables by using test doubles; Stub and Spy.

Sample code

Bad Design (fragile tests)

// BAD DESIGN
class UserDefaultsCalculator {
    func read10TimesValue() {
        return UserDefaults.standard.integer(forKey: "foo") * 10
    }


    func write10TimesValue(_ value: Int) {
        UserDefaults.standard.set(value * 10, forKey: "foo")
    }
}

// In production code:
let calc = UserDefaultsCalculator()
let value = calc.read10TimesValue()
calc.write10TimesValue(value)


// In the unit-test A, it is fragile :-(
let calc = UserDefaultsCalculator()
UserDefaults.standard.set(1, forKey: "foo")
XCTAssertEqual(calc.read10TimesValue(), 10)


// In the unit-test B, it is also fragile :-(
let calc = UserDefaultsCalculator()
calc.write10TimesValue(1)
XCTAssertEqual(UserDefaults.standard.integer(forKey: "foo"), 10)

Good Design (robust tests)

// GOOD DESIGN
class UserDefaultsCalculator {
    private let readableRepository: ReadableRepositoryProtocol
    private let writableRepository: WritableRepositoryProtocol


    init(
        reading readableRepository: ReadableRepositoryProtocol,
        writing writableRepository: WritableRepositoryProtocol
    ) {
        self.readableRepository = readableRepository
        self.writableRepository = writableRepository
    }


    func read10TimesValue() {
        return self.readableRepository.read() * 10
    }


    func write10TimesValue(value: Int) {
        self.writableRepository.write(value * 10)
    }
}


protocol ReadableRepositoryProtocol {
    func read() -> Int
}


class ReadableRepository: ReadableRepositoryProtocol {
    private let userDefaults: UserDefaults


    init(reading userDefaults: UserDefaults) {
        self.userDefaults = userDefaults
    }


    func read() -> Int {
        return self.userDefaults.integer(forKey: "foo")
    }
}


protocol WritableRepositoryProtocol {
    func write(_ value: Int)
}


class WritableRepository: WritableRepositoryProtocol {
    private let userDefaults: UserDefaults


    init(reading userDefaults: UserDefaults) {
        self.userDefaults = userDefaults
    }


    func write(_ value: Int) {
        self.userDefaults.set(value, forKey: "foo")
    }
}

// In production code:
let calc = UserDefaultsCalculator(
    reading: ReadableRepository(UserDefaults.standard),
    writing: WirtableRepository(UserDefaults.standard)
)
let value = calc.read10TimesValue()
calc.write10TimesValue(value)


// In the unit-test A, it is robust, because
// we don't touch actual UserDefaults :-D
let calc = UserDefaultsCalculator(
    reading: ReadableRepositoryStub(firstValue: 1),
    writing: WritableRepositorySpy()
)
XCTAssertEqual(calc.read10TimesValue(), 10)


// In the unit-test B, it is also robust :-D
let spy = WritableRepositorySpy()
let calc = UserDefaultsCalculator(
    reading: ReadableRepositoryStub(firstValue: 0),
    writing: spy
)
calc.write10TimesValue(1)
XCTAssertEqual(spy.callArgs.last!, 10)

// TestDoubles definitions

class ReadableRepositoryStub: ReadableRepositoryProtocol {
    var nextValue: Int

    init(firstValue: Int) {
        self.nextValue = firstValue
    }

    func read() {
        return self.nextValue
    }
}


class WritableRepositorySpy: WritableRepositoryProtocol {
    private(set) var callArgs = [Int]()

    func write(_ value: Int) {
        self.callArgs.append(value)
    }
}

Testing strategy

We stronlgy agree the blog entry; "Just Say No to More End-to-End Tests".

In this project, we use type-checking instead of other tests (unit tests and integration tests and UI tests) to get feedbacks from tests rapidly. Because type-checking is higher effictiveness than other tests.

For example, we can check registering UITableViewCell to UITableVIew before dequeueing by using type-checking:

class MyCell: UITableViewCell {
    /**
     A class for registration token that will create after registering the cell to the specified UITableView.
     */
    struct RegistrationToken {
        // Hide initializer to other objects.
        fileprivate init() {}
    }


    /**
     Registers the cell class to the specified UITableView and returns a registration token.
     */
    static func register(to tableView: UITableView) -> RegistrationToken {
        tableView.register(R.nib.myCell)
        return RegistrationToken()
    }


    /**
     Dequeues the cell by the specified UITableView.
     You must have a registration token (it means you must register the cell class before dequeueing).
     */
    static func dequeue(
        by tableView: UITableView,
        for indexPath: IndexPath,
        andMustHave token: RegistrationToken
    ) -> MyCell {
        guard let cell = tableView.dequeueReusableCell(
            withIdentifier: R.reuseIdentifier.myCell.identifier,
            for: indexPath
        ) as? MyCell else {
            // > dequeueReusableCell(withIdentifier:for:)
            // >
            // > A UITableViewCell object with the associated reuse identifier.
            // > This method always returns a valid cell.
            // >
            // > https://developer.apple.com/reference/uikit/uitableview/1614878-dequeuereusablecell
            fatalError("This case must be success")
        }

        // Configuring the cell.

        return cell
    }
}

Taken together, we should follow the Test Pyramid:

References

1. xUnit Test Patterns: http://xunitpatterns.com/index.html