Dependency Injection in Practice Yoichi Tagaya Appdevcon 2017, Amsterdam

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I'm an Author of Swinject • Dependency Injection framework for Swift • Maintained by 5 members • 1,3k+ stars in GitHub • Supporting iOS, macOS, tvOS, watchOS and Linux • 4 extensions provided • github.com/Swinject/Swinject

Dependency Injection in Practice 1. Basic Concepts 2. Example Program Applying Dependency Injection 3. Advanced Features 5PEBZT5BML

Introduction • Reduce Technical Debt • Asset Catalogs • Dependency Injection • Live Playgrounds "Improving Existing Apps with Modern Best Practices"
 by Woody L. at WWDC 2016

What's Dependency Injection Inversion of Control for resolving dependencies I'm talking to you. ↺ You're talking to me. You call libraries. ↺ Frameworks call you. You create what you use. ↺ What you use are created and passed.

Inversion of Control class Person { let pet = Cat() } You create what you use

Inversion of Control class Person { let pet = Cat() } You create what you use What you use are created and passed class Person1 { let pet: Cat init(pet: Cat) { self.pet = pet } } let p1 = Person1(pet: Cat()) class Person2 { var pet: Cat? } let p2 = Person2() p2.pet = Cat() class Person3 { func play(pet: Cat) { // Play with pet } } let p3 = Person3() p3.play(pet: Cat()) *OJUJBMJ[FS*OKFDUJPO $POTUSVDUPS*OKFDUJPO 1SPQFSUZ*OKFDUJPO .FUIPE*OKFDUJPO

Coupling of Dependency Tight coupling Loose coupling

Coupling of Dependency class Person { let pet = Cat() } Tight coupling Loose coupling • Only a cat can be a pet • Not flexible

Coupling of Dependency class Person { let pet = Cat() } protocol Animal { } class Cat: Animal { } class Dog: Animal { } class Person4 { let pet: Animal init(pet: Animal) { self.pet = pet } } let catPerson = Person4(pet: Cat()) let dogPerson = Person4(pet: Dog()) Tight coupling Loose coupling • Only a cat can be a pet • Not flexible • Any animal can be a pet • Flexible

Example Program Applying Dependency Injection

Main Problems to Develop a Large App

Main Problems to Develop a Large App • Event handling • State management

Main Problems to Develop a Large App • Event handling - Functional Reactive Programming - ReactiveSwift - RxSwift

Main Problems to Develop a Large App • State management - Complicated problem - Caused by dependency often - Dependency injection is a solution (but not the only)

Sample App

Sample App -PHJO -PHJO'BJMVSF -PHPVU -PHJO

Login State Management Singleton account manager named like: • class AccountManager • class LoginManager • class UserAccount Used as: if AccountManager.shared.isLoggedIn { // Do things } else { // Do other things }

Problems of Singleton Login Manager • Tightly coupled with external login service. • Difficult to write unit tests. • Log in/out your app again and again during development.

Let's Dive into the Code

Let’s Dive into the Code 1. Implementation with Account Manager singleton. 2. Refactoring with Dependency Injection. 3. Unit testing.

User Model struct User { let username: String let fullname: String }

final class LocalAccountManager { static let shared = LocalAccountManager() private init() { } } LocalAccountManager Singleton 4JOHMFUPO

final class LocalAccountManager { static let shared = LocalAccountManager() private init() { } private (set) var currentUser: User? var isLoggedIn: Bool { return currentUser != nil } } LocalAccountManager Singleton 4JOHMFUPO

final class LocalAccountManager { static let shared = LocalAccountManager() private init() { } private (set) var currentUser: User? var isLoggedIn: Bool { return currentUser != nil } func login(username: String, password: String) -> Bool { guard currentUser == nil else { return false } guard username == "yoichi" && password == "secure_one" else { return false } currentUser = User(username: "yoichi", fullname: "Yoichi Tagaya") return true } func logout() { currentUser = nil } } LocalAccountManager Singleton 4JOHMFUPO

MainViewController final class MainViewController: UIViewController { @IBOutlet private weak var label: UILabel! @IBOutlet private weak var usernameField: UITextField! @IBOutlet private weak var passwordField: UITextField! @IBOutlet private weak var button: UIButton! }

MainViewController final class MainViewController: UIViewController { @IBOutlet private weak var label: UILabel! @IBOutlet private weak var usernameField: UITextField! @IBOutlet private weak var passwordField: UITextField! @IBOutlet private weak var button: UIButton! @IBAction private func buttonTapped(_ sender: Any) { if LocalAccountManager.shared.isLoggedIn { logout() } else { login() } } private func login() { let username = usernameField.text ?? "" let password = passwordField.text ?? "" let status = LocalAccountManager.shared.login(username: username, password: password) if status, let user = LocalAccountManager.shared.currentUser { label.text = "Hello \(user.fullname)!" button.setTitle("Log out", for: .normal) } else { label.text = "Failed to log in." } } private func logout() { LocalAccountManager.shared.logout() label.text = "Please log in." button.setTitle("Log in", for: .normal) } } 6TJOHTJOHMFUPO 6TJOHTJOHMFUPO

Implemented

But Singleton is Used LocalAccountManager.shared.isLoggedIn LocalAccountManager.shared.currentUser LocalAccountManager.shared.login(...) LocalAccountManager.shared.logout()

Refactoring to Inject Dependency final class MainViewController: UIViewController { // Set this property before you use this class. var accountManager: LocalAccountManager! // Omit } 1SPQFSUZJOKFDUJPO

Refactoring to Inject Dependency final class MainViewController: UIViewController { // Set this property before you use this class. var accountManager: LocalAccountManager! // Omit @IBAction private func buttonTapped(_ sender: Any) { if accountManager.isLoggedIn { logout() } else { login() } } private func login() { let username = usernameField.text ?? "" let password = passwordField.text ?? "" let status = accountManager.login(username: username, password: password) if status, let user = accountManager.currentUser { label.text = "Hello \(user.fullname)!" button.setTitle("Log out", for: .normal) } else { label.text = "Failed to log in." } } private func logout() { accountManager.logout() label.text = "Please log in." button.setTitle("Log in", for: .normal) } } 1SPQFSUZJOKFDUJPO 3FQMBDFTJOHMFUPO XJUIUIFQSPQFSUZ

Refactoring to Inject Dependency @UIApplicationMain class AppDelegate: UIResponder, UIApplicationDelegate { var window: UIWindow? func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool { // Setup window let window = UIWindow(frame: UIScreen.main.bounds) window.makeKeyAndVisible() self.window = window // Setup root view controller let storyboard = UIStoryboard(name: "Main", bundle: nil) let vc = storyboard.instantiateInitialViewController() as! MainViewController vc.accountManager = LocalAccountManager.shared window.rootViewController = vc return true } } *OKFDUBOJOTUBODF PG"DDPVOU.BOBHFS

Improved by Refactoring • Removed dependency to LocalAccountManager singleton from the view controller. • Injected an LocalAccountManager instance to the view controller.

But Still Tightly-Coupled with LocalAccountManager final class MainViewController: UIViewController { var accountManager: LocalAccountManager! }

Introduce a Protocol to Decouple protocol AccountManager { var currentUser: User? { get } var isLoggedIn: Bool { get } func login(username: String, password: String) -> Bool func logout() } extension AccountManager { var isLoggedIn: Bool { return currentUser != nil } } &YUSBDUFEJOUFSGBDF PG-PDBM"DDPVOU.BOBHFS "EEFEEFGBVMUJNQMFNFOUBUJPO CZQSPUPDPMFYUFOTJPO

Conform to the Protocol final class LocalAccountManager: AccountManager { private (set) var currentUser: User? func login(username: String, password: String) -> Bool { guard currentUser == nil else { return false } guard username == "yoichi" && password == "secure_one" else { return false } currentUser = User(username: "yoichi", fullname: "Yoichi Tagaya") return true } func logout() { currentUser = nil } } $POGPSNFEUP UIFQSPUPDPM *NQMFNFOUBUJPOJTUIFTBNF

Depend on the Protocol final class MainViewController: UIViewController { // Set this property before you use this class. var accountManager: AccountManager! // Omit } 3FQMBDFEXJUIUIFQSPUPDPM

Inject Instance to View Controller @UIApplicationMain class AppDelegate: UIResponder, UIApplicationDelegate { var window: UIWindow? var accountManager: AccountManager = LocalAccountManager() func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool { // Setup window let window = UIWindow(frame: UIScreen.main.bounds) window.makeKeyAndVisible() self.window = window // Setup root view controller let storyboard = UIStoryboard(name: "Main", bundle: nil) let vc = storyboard.instantiateInitialViewController() as! MainViewController vc.accountManager = accountManager window.rootViewController = vc return true } } *OKFDUUIFJOTUBODF $SFBUFBOJOTUBODF

Decoupled Dependency • Loosely-coupled the view controller with AccountManager protocol. • AccountManager can be replaced with any concrete type conforming to the protocol.

Let's Add Unit Tests

Introduce a View Model final class MainViewModel { private let accountManager: AccountManager private (set) var labelText = "Please log in." private (set) var buttonText = "Log in" init(accountManager: AccountManager) { self.accountManager = accountManager } } 5FYUTUPEJTQMBZJO7JFX *OJUJBMJ[FSJOKFDUJPO

Introduce a View Model final class MainViewModel { // Omit func login(username: String, password: String) { guard !accountManager.isLoggedIn else { labelText = "Log out first." return } let status = accountManager.login(username: username, password: password) if status, let user = accountManager.currentUser { labelText = "Hello \(user.fullname)!" buttonText = "Log out" } else { labelText = "Failed to log in." } } func logout() { guard accountManager.isLoggedIn else { labelText = "Log in first." return } accountManager.logout() labelText = "Please log in." buttonText = "Log in" } } .PWFEMPHJOMPHPVUMPHJD GSPN7JFX$POUSPMMFS

Use the View Model final class MainViewController: UIViewController { // Set this property before you use this class. var accountManager: AccountManager! private lazy var viewModel: MainViewModel = MainViewModel(accountManager: self.accountManager) // Omit @IBAction private func buttonTapped(_ sender: Any) { if accountManager.isLoggedIn { viewModel.logout() } else { let username = usernameField.text ?? "" let password = passwordField.text ?? "" viewModel.login(username: username, password: password) } updateTexts() } private func updateTexts() { label.text = viewModel.labelText button.setTitle(viewModel.buttonText, for: .normal) } } *OKFDUEFQFOEFODZ 6TFUIFWJFXNPEFM 6TFUIFWJFXNPEFM

Moved the View Logic • View logic was moved from View Controller to View Model. • It’s easier to test View Model.

Add Unit Tests // MARK: Successful Login final class MainViewModelTests: XCTestCase { // MARK: Mock private final class SuccessfulAccountManager: AccountManager { private (set) var currentUser: User? @discardableResult func login(username: String, password: String) -> Bool { currentUser = User(username: "test", fullname: "Test Test") return true } func logout() { currentUser = nil } } .PDLTVDDFFEJOH UPMPHJOBMXBZT

Add Unit Tests // MARK: Successful Login final class MainViewModelTests: XCTestCase { // MARK: Mock private final class SuccessfulAccountManager: AccountManager { private (set) var currentUser: User? @discardableResult func login(username: String, password: String) -> Bool { currentUser = User(username: "test", fullname: "Test Test") return true } func logout() { currentUser = nil } } // MARK: Tests func testLogin_saysHelloToTheUserOnSuccess() { let accountManager = SuccessfulAccountManager() let viewModel = MainViewModel(accountManager: accountManager) viewModel.login(username: "", password: "") XCTAssertEqual(viewModel.labelText, "Hello Test Test!") } .PDLTVDDFFEJOH UPMPHJOBMXBZT *OKFDUUIFNPDL The message is checked in this example, but checking
 a state parameter or flag makes a unit test stable.

Add Unit Tests func testLogin_showsErrorIfAlreadyLoggedIn() { let accountManager = SuccessfulAccountManager() accountManager.login(username: "", password: "") let viewModel = MainViewModel(accountManager: accountManager) viewModel.login(username: "", password: "") XCTAssertEqual(viewModel.labelText, "Log out first.") } func testLogout_asksToLoginAgain() { let accountManager = SuccessfulAccountManager() accountManager.login(username: "", password: "") let viewModel = MainViewModel(accountManager: accountManager) viewModel.logout() XCTAssertEqual(viewModel.labelText, "Please log in.") } func testLogout_promptsToLoginIfNotLoggedIn() { let accountManager = SuccessfulAccountManager() let viewModel = MainViewModel(accountManager: accountManager) viewModel.logout() XCTAssertEqual(viewModel.labelText, "Log in first.") } } The message is checked in this example, but checking
 a state parameter or flag makes a unit test stable.

Add Unit Tests // MARK: - Failed Login extension MainViewModelTests { // MARK: Mock private final class FailingAccountManager: AccountManager { private (set) var currentUser: User? func login(username: String, password: String) -> Bool { return false } func logout() { } } // MARK: Tests func testLogin_showsErrorOnLoginFailure() { let accountManager = FailingAccountManager() let viewModel = MainViewModel(accountManager: accountManager) viewModel.login(username: "", password: "") XCTAssertEqual(viewModel.labelText, "Failed to log in.") } } *OKFDUUIFNPDL .PDLGBJMJOHUPMPHJOBMXBZT The message is checked in this example, but checking
 a state parameter or flag makes a unit test stable.

Testable by Refactoring✅

Summary of the Example App 1. Tightly-coupled dependency to singleton LocalAccountManager 2. Refactoring by dependency injection 3. Made the program testable✅

Advanced Features

Dependency Injection Container (DI Container)

Problem of Dependency Injection At the beginning... Dependency Injection Code let a = A(b: B()) " Dependency #

Problem of Dependency Injection Soon... " Dependency # let a = A(b: B(c: C())) $ Dependency Dependency Injection Code

let a = A(b: B(c: C(f: F()), e: E(f: F()))) Problem of Dependency Injection Much later... $ ' * & # ) " % ( Hard to manage Dependency Injection Code

Dependency Injection Container $ ' * & # ) " % ( %*$POUBJOFS Dependency Graph Register the dependency graph let a = ... Resolve dependencies upon your request

// Program entry point. let container = Container() Swinject Example $ ' & # " How to use a container

// Program entry point. let container = Container() container.register(AProtocol.self) { r in A(b: r.resolve(BProtocol.self)!) } Swinject Example $ ' & # " How to use a container #JOTUBODFJTNBOBHFE CZUIFDPOUBJOFS

// Program entry point. let container = Container() container.register(AProtocol.self) { r in A(b: r.resolve(BProtocol.self)!) } container.register(BProtocol.self) { r in B(c: r.resolve(CProtocol.self)!, e: r.resolve(EProtocol.self)!) } Swinject Example $ ' & # " How to use a container #JOTUBODFJTNBOBHFE CZUIFDPOUBJOFS

// Program entry point. let container = Container() container.register(AProtocol.self) { r in A(b: r.resolve(BProtocol.self)!) } container.register(BProtocol.self) { r in B(c: r.resolve(CProtocol.self)!, e: r.resolve(EProtocol.self)!) } container.register(CProtocol.self) { r in C(f: r.resolve(FProtocol.self)!) } container.register(EProtocol.self) { r in E(f: r.resolve(FProtocol.self)!) } container.register(FProtocol.self) { _ in F() } Swinject Example $ ' & # " How to use a container #JOTUBODFJTNBOBHFE CZUIFDPOUBJOFS

// Program entry point. let container = Container() container.register(AProtocol.self) { r in A(b: r.resolve(BProtocol.self)!) } container.register(BProtocol.self) { r in B(c: r.resolve(CProtocol.self)!, e: r.resolve(EProtocol.self)!) } container.register(CProtocol.self) { r in C(f: r.resolve(FProtocol.self)!) } container.register(EProtocol.self) { r in E(f: r.resolve(FProtocol.self)!) } container.register(FProtocol.self) { _ in F() } // Anywhere later... let a = container.resolve(AProtocol.self)! Swinject Example $ ' & # " How to use a container #JOTUBODFJTNBOBHFE CZUIFDPOUBJOFS +VTUBTLUPHFU BO"JOTUBODF

Dependency Injection Container • Pros - It's easy to use. - Definitions are unified. • Cons - Error at runtime for ! 1SPCMFNPGEZOBNJDEFQFOEFODZJOKFDUJPO

Static dependency injection in Cake Pattern • Pros - Lack of definitions checked by compiler. - No error at runtime for dependency injection. • Cons - Less easy than DI container. %FQFOEFODZ*OKFDUJPOXJUIUIF$BLF1BUUFSOJO4XJGUCZ#PC$PUUSFMM
 IUUQBDRVJIJSFNFEFQFOEFODZJOKFDUJPOXJUIUIFDBLFQBUUFSOJOTXJGU (SFBUBSUJDMFUPMFBSO$BLF1BUUFSO

Summary • Basic Concepts - Inversion of Control - Tight/loose coupling • Example Program - State management problem - Refactoring by Dependency Injection • Advanced Features - Dependency Injection Container (dynamic) - Cake Pattern (static)