Mercari Atte and Automaton Vu Nhat Minh / @orakaro Souzoh iOS Talk

Mercari Atte Scala / Swift Engineer / @orakaro

Mercari Atte iOS development • Full RxSwift • Aggressive use of Driver • Define input/output type for ViewModel • Composition over Inheritance • Automaton

State Machine variations • Finite state machine • Transition function (s, a) -> s • Mealy Machine • Transition function (s, a) -> s • Output function (s, a) -> b • Moore Machine • Transition function (s, a) -> s • Output function s -> b

Finite State Machine • Finite state machine • Transition function (s, a) -> s • Mealy Machine • Transition function (s, a) -> s • Output function (s, a) -> b • Moore Machine • Transition function (s, a) -> s • Output function s -> b design pattern for state machines

Binary Gap A binary gap within a positive integer N is any maximal sequence of consecutive zeros that is surrounded by ones at both ends in the binary representation of N. Ex: 9 = 1001 → 2

A binary gap within a positive integer N is any maximal sequence of consecutive zeros that is surrounded by ones at both ends in the binary representation of N. Ex: 9 = 1001 → 2 Started Zero One Binary Gap

Visual Format Language class func constraints(withVisualFormat format: String, options opts: NSLayoutFormatOptions = [], metrics: [String : Any]?, views: [String : Any]) -> [NSLayoutConstraint] "H:|-15-[iconImageView(30)]-[appNameLabel]-[skipButton]-15-|"

Visual Format Language class func constraints(withVisualFormat format: String, options opts: NSLayoutFormatOptions = [], metrics: [String : Any]?, views: [String : Any]) -> [NSLayoutConstraint] "H:|-15-[iconImageView(30)]-[appNameLabel]-[skipButton]-15-|" Direction Start Line Number Label [ ] ( ) End

For a “healthy” codebase Try to avoid large if-else block Try to avoid global variables Try to write pure functions

Signup Flow ʁ ʁ ʁ ʁ ʁ

Signup Flow Signup by email Facebook login One click signup by second party app Reset Password Login by Email

Signup Flow

Signup condition

Facebook Login

Login by email

Signup Flow Graph Root Register Root Profile Register Start With Mercari AtteLogin Facebook Email Login Register With Mercari Confirm SMS Home PassCode Email SignUp Reset Password

Root Register Root Profile Register Start With Mercari AtteLogin Facebook Email Login Register With Mercari Confirm SMS Home PassCode Email SignUp Reset Password

Transitions is Domain knowledge and should be placed at Model layer MVC / MVP / MVVM

Each screen represents State

Transition by event Root Register Root Atte Login Start With Mercari Atte Resource Mercari Resource

RegistrationStateViewController protocol RegistrationStateViewController { var state: RegistrationState { get } func nextViewController(event: RegistrationEvent) -> UIViewController? } extension RegistrationStateViewController { func nextViewController(event: RegistrationEvent) -> UIViewController? { let machine = AutomatonManager.registrationMachine guard let state = machine.transition(from: state, by: event) else { return nil } switch state { //... } } Automaton

Define Transition struct Transition { let from: S let to: S let by: E } from to by State State Event

Implement State Machine class Automaton { var routes: [S: [E: S]] = [:] init(initialState: S, transitions: [Transition]) { for t in transitions { addRoute(t) } } private func addRoute(_ t: Transition) { var dict = routes[t.from] ?? [:] dict[t.by] = t.to routes[t.from] = dict } func transition(from: S, by: E) -> S? { guard let next = routes[from].flatMap({ $0[by] }) else { return nil } return next } }

Implement State Machine class Automaton { var routes: [S: [E: S]] = [:] init(initialState: S, transitions: [Transition]) { for t in transitions { addRoute(t) } } private func addRoute(_ t: Transition) { var dict = routes[t.from] ?? [:] dict[t.by] = t.to routes[t.from] = dict } func transition(from: S, by: E) -> S? { guard let next = routes[from].flatMap({ $0[by] }) else { return nil } return next } } use dictionary for storing transitions

Define State and Event enum RegistrationState { case root case registerRoot case profileRegister case startWithMercari case emailLogin case atteLogin case home } enum RegistrationEvent: Hashable { case showRegisterRoot(loginMode: RootViewController.LoginMode?) case registerWithFacebook(profile: FacebookProfile?) case startWithMercari(resource: MercariIdResource?, tokenPair: TokenPair?, showsCloseButton: Bool) case loginWithEmail(showsCancelButton: Bool) case loginWithAtte(resource: MercariIdResource?, tokenPair: TokenPair?) case loginAndShowHome(tokenPair: TokenPair?, userId: Int64, hasMercariItem: Bool) }

Define transition graph let registrationGraph: [Transition] = [ Transition(from: .root, to: .registerRoot, by: DefaultEvent.showRegisterRoot), Transition(from: .registerRoot, to: .profileRegister, by: DefaultEvent.registerWithFacebook), Transition(from: .registerRoot, to: .startWithMercari, by: DefaultEvent.startWithMercari), Transition(from: .registerRoot, to: .emailLogin, by: DefaultEvent.loginWithEmail), Transition(from: .registerRoot, to: .atteLogin, by: DefaultEvent.loginWithAtte), Transition(from: .registerRoot, to: .home, by: DefaultEvent.loginAndShowHome), ] let registrationMachine = Automaton( initialState: .root, transitions: registrationGraph )

Test for transition logic only describe("ΞϓϦىಈ࣌τʔΫϯ͕ଘࡏ͠ͳ͍") { it("ϝʔϧͰ৽نొ࿥͕Ͱ͖Δ") { self.tryRoute(events: [ DefaultEvent.showRegisterRoot, DefaultEvent.registerWithEmail, DefaultEvent.showProfileRegister, DefaultEvent.confirmSMS, DefaultEvent.loginAndShowHome ]) XCTAssert(self.currentState == .home) } } //... private func tryRoute(events: [RegistrationEvent]) { for event in events { self.currentState.map { state in self.currentState = self.machine.transition(from: state, by: event) } } }

No content

still have problems

RegistrationStateViewController protocol RegistrationStateViewController { var state: RegistrationState { get } func nextViewController(event: RegistrationEvent) -> UIViewController? } extension RegistrationStateViewController { func nextViewController(event: RegistrationEvent) -> UIViewController? { let machine = AutomatonManager.registrationMachine guard let state = machine.transition(from: state, by: event) else { return nil } switch state { //... } } Should I create ViewController here?

RegistrationStateViewController switch state { case .root: return RootViewController.make(withDependency: ()) case .registerRoot: return RegisterRootViewController.make(withDependency: ()) case .profileRegister : switch event { case .registerWithFacebook(.some(let profile)): return ProfileRegisterViewController.make(withDependency: .init(facebookProfile: profile)) default: return nil } case .startWithMercari: switch event { case .startWithMercari(.some(let resource), .some(let token)): return StartWithMercariViewController.make(withDependency: .init(resource: resource, token: token)) default: return nil } case .emailLogin: return EmailLoginViewController.make(withDependency: ()) case .atteLogin: switch event { case .loginWithAtte(.some(let resource), .some(let token)): return AtteLoginViewController.make(withDependency: .init(resource: resource, token: token)) default: return nil } case .home: switch event { case .loginAndShowHome(.some(let token), let userId): return HomeViewController.make(withDependency: .init(token: token, userId: userId)) default: return nil } default: return nil } Not so much different from large if-else block

Define transition graph let registrationGraph: [Transition] = [ Transition(from: .root, to: .registerRoot, by: DefaultEvent.showRegisterRoot), Transition(from: .registerRoot, to: .profileRegister, by: DefaultEvent.registerWithFacebook), Transition(from: .registerRoot, to: .startWithMercari, by: DefaultEvent.startWithMercari), Transition(from: .registerRoot, to: .emailLogin, by: DefaultEvent.loginWithEmail), Transition(from: .registerRoot, to: .atteLogin, by: DefaultEvent.loginWithAtte), Transition(from: .registerRoot, to: .home, by: DefaultEvent.loginAndShowHome), ] let registrationMachine = Automaton( initialState: .root, transitions: registrationGraph ) DefaultEvent !?

What is DefaultEvent? • In Swift 3 we still can not use default value for associated value in enum • Proposal: SE-0155 Accepted status but not included in Swift 4 struct DefaultEvent { static let showRegisterRoot: RegistrationEvent = .showRegisterRoot() static let registerWithFacebook: RegistrationEvent = .registerWithFacebook(profile: nil) static let startWithMercari: RegistrationEvent = .startWithMercari(resource: nil, token: nil) static let loginWithEmail: RegistrationEvent = .loginWithEmail() static let loginWithAtte: RegistrationEvent = .loginWithAtte(resource: nil, token: nil) static let loginAndShowHome: RegistrationEvent = .loginAndShowHome(token: nil, userId: 0) }

Optional type associated value enum RegistrationEvent: Hashable { case showRegisterRoot(loginMode: RootViewController.LoginMode?) case registerWithFacebook(profile: FacebookProfile?) case startWithMercari(resource: MercariIdResource?, tokenPair: TokenPair?, showsCloseButton: Bool) case loginWithEmail(showsCancelButton: Bool) case loginWithAtte(resource: MercariIdResource?, tokenPair: TokenPair?) case loginAndShowHome(tokenPair: TokenPair?, userId: Int64, hasMercariItem: Bool) } Optional

Automaton + FP

Transition function (aka Reducer) • Finite State Machine • Transition function (s, a) -> s • Mealy Machine • Transition function (s, a) -> s • Output function (s, a) -> b • Moore Machine • Transition function (s, a) -> s • Output function s -> b

Transition (s, a) -> s Output (s, a) -> b Mealy Machine / Moore Machine Reducer (s, a) -> (s, b) Transition (s, a) -> s Output s -> b Reducer s -> (a -> s, b)

Mealy Machine vs Elm Elm Architecture fun update(state: State, action: Action): Pair Transition (s, a) -> s Output (s, a) -> b Reducer (s, a) -> (s, b)

Mealy Machine Reducer a -> s -> (s, b) Transition (s, a) -> s Output (s, a) -> b Reducer (s, a) -> (s, b)

Mealy Machine Reducer a -> s -> (s, b) Transition (s, a) -> s Output (s, a) -> b Reducer (s, a) -> (s, b) Stateful Computation !!

Stateful Computation A stateful computation is a function that takes some state and returns a value along with some new state: s -> (s, a) class State { private let run: (S) -> (S, A) init(f: @escaping (S) -> (S, A)) { self.run = f } func run(s: S) -> (S, A) { return self.run(s) } }

State Monad extension State { func map(g: @escaping (A) -> B) -> State { return State { s in let (s1, val) = self.run(s: s) return (s1, g(val)) } } func flatMap(g: @escaping (A) -> State) -> State { return State { s in let (s1, val) = self.run(s: s) return g(val).run(s: s1) } } } A stateful computation is a function that takes some state and returns a value along with some new state: s -> (s, a)

Monad = flatmappable type Image credit: Functors, Applicatives, And Monads In Pictures

Swift Monads Maybe Monad Either Monad Reader Monad Observable Monad State Monad I/O Monad … Swift Optional antitypical/Result Statically typed Dependency Injection Signal / Observable Mealy Machine

Mealy Machine Reducer a -> s -> (s, b) Transition (s, a) -> s Output (s, a) -> b Reducer (s, a) -> (s, b) Reducer a -> State[s, b] State Monad

MonadicAutomaton class MonadicAutomaton { typealias T = (A) -> State private var f : T init(f: @escaping T) { self.f = f } func transition(from: S, by: A) -> (S, B) { return f(by).run(s: from) } } Reducer A -> State[S, B] A:InputɺB: Output A = Event enumɺB = UIViewController

Define States and Events enum RState { case any case root case registerRoot case profileRegister case startWithMercari case emailLogin case atteLogin case home } enum REvent { case showRegisterRoot case registerWithFacebook(profile: FacebookProfile) case startWithMercari(resource: MercariIdResource, token: String) case loginWithEmail case loginWithAtte(resource: MercariIdResource, token: String) case loginAndShowHome(token: String, userId: Int64) } custom type

Define transition graph let transitionFunc: (REvent) -> State = { event in switch event { case .registerWithFacebook(let profile): let vc = ProfileRegisterViewController.make(withDependency: .init(facebookProfile: profile)) return State { s in let s1: RState = s == .registerRoot ? .profileRegister : .any return (s1, vc) //... } } let registrationMachine = MonadicAutomaton(f : transitionFunc) A S B A S B

Automaton + FRP

Automaton + FRP • State Machine for UI inside one screen • Use stream to represent events • ReactiveSwift • inamiy/ReactiveAutomaton • RxSwift • kzaher/RxFeedback

Recap • State Machine • Finite State Machine • Mealy Machine • Moore Machine • Design pattern for state management • Can use FSM for simple transitions • Mealy Machine = State Monad

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