Dependency Injection in Swift

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DEPENDENCY INJECTION (DI) IN SWIFT ILYA PUCHKA (@ILYAPUCHKA) IOS DEVELOPER @HELLOFRESH

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FUNCTIONAL OR OBJECT ORIENTED?

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SOLID KISS DRY YAGNI RAP CQS DECORATOR FACADE ABSTRACT FACTORY STRATEGY ...

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DI SOLID

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WHAT IS DEPENDENCY INJECTION?

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In software engineering, dependency injection is a software design pattern that implements inversion of control for resolving dependencies. — Wikipedia

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“Dependency injection is really just passing in an instance variable. — James Shore

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VOODO MAGIC SLOW FRAMEWORKS ONLY FOR TESTING OVERENGINEERING

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> What is Dependency Injection? > How to do it? > How not to do it?

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WHY DEPENDENCY INJECTION?

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ABSTRACTIONS EVERYWHERE

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LOOSE COUPLING

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TEST EXTEND AND REUSE DEVELOP IN PARALLER MAINTAIN

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NOT JUST UNIT TESTS BUT LOOSE COUPLING

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FIRST STEP - PASSING INSTANCE VARIABLES

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SECOND STEP - ? THIRD STEP - ?

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PATTERNS CUSTRUCTOR INJECTION PROPERTY INJECTION METHOD INJECTION AMBIENT CONTEXT

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CONSTRUCTOR INJECTION class NSPersistentStore : NSObject { init(persistentStoreCoordinator root: NSPersistentStoreCoordinator?, configurationName name: String?, URL url: NSURL, options: [NSObject: AnyObject]?) var persistentStoreCoordinator: NSPersistentStoreCoordinator? { get } }

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EASY TO IMPLEMENT IMMUTABILITY

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PROPERTY INJECTION extension UIViewController { weak public var transitioningDelegate: UIViewControllerTransitioningDelegate? }

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LOCAL DEFAULT FOREIGN DEFAULT

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OPTIONALS NOT IMMUTABLE THREAD SAFETY

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METHOD INJECTION public protocol NSCoding { public func encodeWithCoder(aCoder: NSCoder) }

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AMBIENT CONTEXT public class NSURLCache : NSObject { public class func setSharedURLCache(cache: NSURLCache) public class func sharedURLCache() -> NSURLCache }

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CROSS-CUTTING CONCERNS > logging > analitycs > time/date > etc.

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PROS: > does not pollute API > dependency always available CONS: > implicit dependency > global mutable state

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SEPARATION OF CONCERNS > what concrete implementations to use > configure dependencies > manage dependencies' lifetime

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WHERE DEPENDENCIES ARE CREATED?

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COMPOSITION ROOT

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No content

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VIPER EXAMPLE class AppDependencies { init() { configureDependencies() } func configureDependencies() { // Root Level Classes let coreDataStore = CoreDataStore() let clock = DeviceClock() let rootWireframe = RootWireframe() // List Module Classes let listPresenter = ListPresenter() let listDataManager = ListDataManager() let listInteractor = ListInteractor(dataManager: listDataManager, clock: clock) ... listInteractor.output = listPresenter listPresenter.listInteractor = listInteractor listPresenter.listWireframe = listWireframe listWireframe.addWireframe = addWireframe ... } }

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VIPER EXAMPLE @UIApplicationMain class AppDelegate: UIResponder, UIApplicationDelegate { var window: UIWindow? let appDependencies = AppDependencies() func application( application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject : AnyObject]?) -> Bool { appDependencies.installRootViewControllerIntoWindow(window!) return true } }

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The biggest challange of properly implementing DI is getting all classes with dependencies moved to Composition Root — Mark Seeman

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ANTI- PATTERNS

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CONTROL FREAK class RecipesService { let repository: RecipesRepository init() { self.repository = CoreDataRecipesRepository() } }

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init()

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STABLE OR VOLATILE

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VOLATILE DEPENDENCIES > dependency requires environment configuration (data base, networking, file system) > nondetermenistic behavior (dates, cryptography) > expected to be replaced > dependency is still in development

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VOLATILE DEPENDENCIES DISABLE LOOSE COUPLING

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BASTARD INJECTION class RecipesService { let repository: RecipesRepository init(repository: RecipesRepository = CoreDataRecipesRepository()) { self.repository = repository } }

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FOREIGN DEFAULT

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SERVICE LOCATOR let locator = ServiceLocator.sharedInstance locator.register( { CoreDataRecipesRepository() }, forType: RecipesRepository.self) class RecipesService { let repository: RecipesRepository init() { let locator = ServiceLocator.sharedInstance self.repository = locator.resolve(RecipesRepository.self) } }

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Pros: > Extensibility > Testability > Parallel development > Separation of concerns

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Cons: > Implicit dependencies > Hidden complexity > Tight coupling > Not reusable > Less maintainable

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> DI enables loose coupling > 4 patterns, prefer constructor injection > Use local defaults, not foreign > Inject volatile dependencies, not stable > Avoid anti-patterns

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EXPLICIT DEPENDENCIES COMPOSITION ROOT

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SECOND STEP - ABSTRACTIONS

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DEPENDENCY INVERSION PRINCIPLE (DIP)

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DI = DI PATTERNS + DIP

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Program to an interface, not an implementation — Design Patterns: Elements of Reusable Object-Oriented Software

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PROGRAM TO AN INTERFACE ABSTRACTION

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INTERFACES ARE NOT ABSTRACTIONS 1 1 http://blog.ploeh.dk/2010/12/02/Interfacesarenotabstractions/

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DI = DI PATTERNS + DIP

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THIRD STEP - INVERSION OF CONTROL

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DI CONTAINERS

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TYPHOON DIP

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TYPHOON http://typhoonframework.org > a lot of features > good docs > well maintained > continuously improved

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public class APIClientAssembly: TyphoonAssembly { public dynamic func apiClient() -> AnyObject { ... } public dynamic func session() -> AnyObject { ... } public dynamic func logger() -> AnyObject { ... } }

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public dynamic func apiClient() -> AnyObject { return TyphoonDefinition.withClass(APIClientImp.self) { definition in definition.useInitializer(#selector(APIClientImp.init(session:))) { initializer in initializer.injectParameterWith(self.session()) } definition.injectProperty("logger", with: self.logger()) } }

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public dynamic func session() -> AnyObject { return TyphoonDefinition.withClass(NSURLSession.self) { definition in definition.useInitializer(#selector(NSURLSession.sharedSession)) } } public dynamic func logger() -> AnyObject { return TyphoonDefinition.withClass(ConsoleLogger.self) { definition in definition.scope = .Singleton } }

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let assembly = APIClientAssembly().activate() let apiClient = assembly.apiClient() as! APIClient

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TYPHOON + SWIFT ! > Requires to subclass NSObject and define protocols with @objc > Methods called during injection should be dynamic > requires type casting > not all features work in Swift > too wordy API for Swift

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DIP https://github.com/AliSoftware/Dip > pure Swift api > cross-platform > type-safe > small code base

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REGISTER let container = DependencyContainer() container.register { try APIClientImp( session: container.resolve() ) as APIClient } .resolveDependencies { container, client in client.logger = try container.resolve() } container.register { NSURLSession.sharedSession() as NetworkSession } container.register(.Singleton) { ConsoleLogger() as Logger }

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RESOLVE let apiClient = try! container.resolve() as APIClient

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AUTO-WIRING class APIClientImp: APIClient { private let _logger = Injected() var logger: Logger? { return _logger.value } }

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AUTO-WIRING class APIClientImp: APIClient { init(session: NetworkSession) { ... } } container.register { APIClientImp(session: $0) as APIClient }

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Typhoon Dip Constructor, property, method injection ✔ ✔ Lifecycle management ✔ ✔ Circular dependencies ✔ ✔ Runtime arguments ✔ ✔ Named definitions ✔ ✔ Storyboards integration ✔ ✔ ----------------------------------------------------------- Auto-wiring ✔ ✔ Thread safety ✘ ✔ Interception ✔ ✘ Infrastructure ✔ ✘

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WHY SHOULD I BOTHER? > easy integration with storyboards > manage components lifecycle > can simplify configurations > allow interception (in Typhoon using NSProxy) > provides additional features

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DI ≠ DI CONTAINER

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DEPENDENCY INJECTION IS A MEANS TO AN END

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LINKS > "Dependency Injection in .Net" Mark Seeman > Mark Seeman's blog > objc.io Issue 15: Testing. Dependency Injection, by Jon Reid > "DIP in the wild" > Non-DI code == spaghetti code?

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THANK YOU! @ILYAPUCHKA HTTP://ILYA.PUCHKA.ME