Dependency Injection in Practice (Appdevcon)

Appdevcon 2017 in Amsterdam, March 16-17, 2017
http://appdevcon.nl/

Transcript

1. Dependency Injection in Practice Yoichi Tagaya Appdevcon 2017, Amsterdam
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3. 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
4. Dependency Injection in Practice 1. Basic Concepts 2. Example Program Applying Dependency Injection 3. Advanced Features 5PEBZT
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5. Introduction • Reduce Technical Debt • Asset Catalogs • Dependency Injection • Live Playgrounds "Improving Existing Apps with Modern Best Practices"
 by Woody L. at WWDC 2016
6. 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.
7. Inversion of Control class Person { let pet = Cat() } You create what you use
8. 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
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9. Coupling of Dependency Tight coupling Loose coupling
10. Coupling of Dependency class Person { let pet = Cat() } Tight coupling Loose coupling • Only a cat can be a pet • Not flexible
11. 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
12. Example Program Applying Dependency Injection
13. Main Problems to Develop a Large App
14. Main Problems to Develop a Large App • Event handling • State management
15. Main Problems to Develop a Large App • Event handling - Functional Reactive Programming - ReactiveSwift - RxSwift
16. Main Problems to Develop a Large App • State management - Complicated problem - Caused by dependency often - Dependency injection is a solution (but not the only)
17. Sample App
18. Sample App -PH
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19. 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 }
20. 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.
21. Let's Dive into the Code
22. Let’s Dive into the Code 1. Implementation with Account Manager singleton. 2. Refactoring with Dependency Injection. 3. Unit testing.
23. User Model struct User { let username: String let fullname: String }
24. final class LocalAccountManager { static let shared = LocalAccountManager() private init() { } } LocalAccountManager Singleton 4JOHMFUPO
25. final class LocalAccountManager { static let shared = LocalAccountManager() private init() { } private (set) var currentUser: User? var isLoggedIn: Bool { return currentUser != nil } } LocalAccountManager Singleton 4JOHMFUPO
26. 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
27. 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! }
28. 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) } } 6TJOH
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29. Implemented
30. But Singleton is Used LocalAccountManager.shared.isLoggedIn LocalAccountManager.shared.currentUser LocalAccountManager.shared.login(...) LocalAccountManager.shared.logout()
31. Refactoring to Inject Dependency final class MainViewController: UIViewController { // Set this property before you use this class. var accountManager: LocalAccountManager! // Omit } 1SPQFSUZ
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32. 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) } } 1SPQFSUZ
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33. 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 } } *OKFDU
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34. Improved by Refactoring • Removed dependency to LocalAccountManager singleton from the view controller. • Injected an LocalAccountManager instance to the view controller.
35. But Still Tightly-Coupled with LocalAccountManager final class MainViewController: UIViewController { var accountManager: LocalAccountManager! }
36. 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 } } &YUSBDUFE
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37. 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 } } $POGPSNFE
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38. Depend on the Protocol final class MainViewController: UIViewController { // Set this property before you use this class. var accountManager: AccountManager! // Omit } 3FQMBDFE
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39. 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 } } *OKFDU
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40. Decoupled Dependency • Loosely-coupled the view controller with AccountManager protocol. • AccountManager can be replaced with any concrete type conforming to the protocol.
41. Let's Add Unit Tests
42. 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 } } 5FYUT
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43. 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" } } .PWFE
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44. 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) } } *OKFDU
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45. Moved the View Logic • View logic was moved from View Controller to View Model. • It’s easier to test View Model.
46. 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 } } .PDL
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47. 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!") } .PDL
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48. 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.
49. 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.") } } *OKFDU
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50. Testable by Refactoring✅
51. Summary of the Example App 1. Tightly-coupled dependency to singleton LocalAccountManager 2. Refactoring by dependency injection 3. Made the program testable✅
52. Advanced Features
53. Dependency Injection Container (DI Container)
54. Problem of Dependency Injection At the beginning... Dependency Injection Code let a = A(b: B()) " Dependency #
55. Problem of Dependency Injection Soon... " Dependency # let a = A(b: B(c: C())) $ Dependency Dependency Injection Code
56. 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
57. Dependency Injection Container $ ' * & # ) " % ( %*
    $POUBJOFS Dependency Graph Register the dependency graph let a = ... Resolve dependencies upon your request
58. // Program entry point. let container = Container() Swinject Example $ ' & # " How to use a container
59. // 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 #
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60. // 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 #
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61. // 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 #
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62. // 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 #
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63. Dependency Injection Container • Pros - It's easy to use. - Definitions are unified. • Cons - Error at runtime for ! 1SPCMFN
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64. 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
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65. 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)