iOS Architecture: A State Container based approach

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iOS Architecture A State Container based approach The Movie

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iOS Mobile Apps Architecture A State Container based approach The Movie

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Luis Recuenco @luisrecuenco

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1. Motivation 2. Implementation 3. Problems

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Motivation

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What is bad architecture?

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And what is good architecture?

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What about software?

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Three traits of good design

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1. Balanced distribution of responsibilities

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2. Testability

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3. Easy to use

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Good code is the one that you can easily delete

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Good code is the one that you can easily delete — Luis Recuenco

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pod install

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pod remove

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Good code depends on context

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Buildings vs Software

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Good software is more subjective

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Difﬁcult to know that bad code is actually bad code

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Simple vs Easy

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Software changes a lot more

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Greenﬁeld

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Brownﬁeld

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Most of us are in an industry of changing existing software

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A good design is easier to change than a bad design — Dave Thomas

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I'm here to talk about my book

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I'm here to talk about my book blog post

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Who actually read it?

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This talk is like the movie

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Books are better than the movie

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Books and movies are like apples and oranges. They both are fruit, but taste completely different — Stephen King

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Mobile Apps Architecture A state container based approach

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The Movie

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First things ﬁrst We need a name

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Luis ---------------------- Swift

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Luis ------------ Swift

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Luis ------ Swift

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Luis - Swift

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Lwift

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Current state of architectures

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It all started with MVC

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View Controller has too many responsibilities

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View Controller has too many responsibilities • View lifecycle • Business logic • Data transformation • Network requests • Delegate and data source for UI components • And so on and so forth...

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Massive View Controller

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Testing issues View Controller is tightly coupled to the view

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MVP

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Better testing Presenter is a view independent mediator

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But we have to mock the view...

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The assembly "problem"

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let viewController = UIViewController() let presenter = Presenter(viewController, model) viewController.presenter = presenter

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let viewController = UIViewController() let presenter = Presenter(viewController, model) viewController.presenter = presenter

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MVVM

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Better testing No view mocks

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Better assembly let viewModel = ViewModel(model) let viewController = UIViewController(viewModel)

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VIPER

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Router

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Too many pieces

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Problems?

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Broad concepts Not concrete enough

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The problem about multiple, scattered, out-of-sync outputs

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What's an output?

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MVP Command from presenter to view

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MVVM An observable property

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class ProcessListViewModel { private(set) var loading: Bool private(set) var processes: [Process] private(set) var error: Error func fetchProcesses() { loading = true service.loadProcesses { processes, error in if error { self.error = error } else { self.processes = processes } } } }

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Conﬂicting states

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Invisible outputs

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No traceability

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Outputs consistency

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View State

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struct ProcessListViewState { let loading: Bool let processes: [ProcessViewState]? let error: Error? static func loading() -> ProcessListViewState { return ProcessListViewState(loading: true, processes: nil, error: nil) } static func loaded(with processes: [Process]) -> ProcessListViewState { return ProcessListViewState(loading: false, processes: processes, error: nil) } static func error(with error: Error) -> ProcessListViewState { return ProcessListViewState(loading: false, processes: nil, error: error) } }

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class ProcessListViewModel { private(set) var state: ProcessListViewState { didSet { queue.sync { // Save new state and notify subscribers } } } func fetchProcesses() { state = ProcessListViewState.loading() service.loadProcesses { processes, error in if error { state = ProcessListViewState.error(with: error) } else { state = ProcessListViewState.loaded(with: process) } } } }

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class ProcessListViewModel { private(set) var state: ProcessListViewState { didSet { queue.sync { // Save new state, log it, serialize it and notify subscribers } } } func fetchProcesses() { state = ProcessListViewState.loading() service.loadProcesses { processes, error in if error { state = ProcessListViewState.error(with: error) } else { state = ProcessListViewState.loaded(with: process) } } } }

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Deterministic view renders

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The problem about exclusive states

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class ProcessListViewController { init(viewModel: ProcessListViewModel) { viewModel.subscribe(from: self) } func render(state: ProcessListViewState) { if let error = state.error { ... } else if state.loading { ... } else if let processes = state.processes { ... } } }

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Can our APIs convey proper usage?

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Sum types

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Product type

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enum PushNotification { case showPrivateInfo case showProcessDetail(Process.Identifier) case showChatConversation(Chat.Identifier, Candidate.Identifier) }

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Null References

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Null References The billion dollar mistake

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enum Optional { case none case some(Value) }

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Maybe monad

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enum Result { case failure(Error) case success(Value) }

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Result monad

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Pattern matching service.doNetworkRequest { result in switch result { case .failure(let error): handleError(with: error) case .success(let value): renderScreen(with: value) } }

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enum ProcessListViewState { case loading case loaded([ProcessViewState]) case error(Error) } class ProcessListViewController { func render(state: ProcessListViewState) { switch state { case .loading: renderLoading() case .loaded(let processes): render(processes) case .error(let error): render(error) } } }

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UI as a State Machine

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enum ProcessListViewState { case loading case loaded([ProcessViewState]) case error(Error) mutating func toLoading() { switch self { case .loading: fatalError("Wrong state transition") case .loaded, .idle, .error: self = .loading } } }

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Exhaustive Switch statements

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State is now buried in the view layer

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View state vs domain state

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App = UI(state)

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UI(viewState(domainState))

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Domain State

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Single source of truth

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The value of values

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COW semantics

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===

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90% of our actors are reference objects

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90% of live instances are values

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Move business logic to the value layer

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Better testing

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Queries Derived state

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Decouple views from state shape speciﬁcs

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To cache or not to cache

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Staleness vs efﬁciency bugs

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Commands Mutate state

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New truth

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lenses

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mutating

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mutating composition

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Whole tree is rebuilt

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Performance issues

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Persistent Data Structures

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Tries

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Structural sharing

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State enum ProcessStoreState { case idle case loading case loaded([Process]) case error(Error) }

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Queries extension ProcessStoreState { var processes: [Process]? { switch self { case .idle, .loading, .error: return nil case .loaded(let processes): return processes } } var error: Error? { switch self { case .idle, .loaded, .loading: return nil case .error(let error): return error } } }

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Commands extension ProcessStoreState { mutating func enterProcess(id: String) { // create modifiedProcesses... self = .loaded(modifiedProcesses) } mutating func toLoading() { // Check correct state transition if needed self = .loading } }

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Only one missing piece...

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Stores Domain State Containers

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Store

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Responsible for side effects

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View State Pure function of domain state

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enum ProcessListViewState { case loading case loaded([ProcessViewState]) case error(Error) init(domainState: ProcessStoreState) { switch domainState { case .idle, .loading: self = .loading case .loaded(let processes): self = .loaded(processes.map { ... }) case .error: self = .error(error.map { ... }) } } }

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class ProcessListViewModel { private let store: ProcessStore private(set) var state: ProcessListViewState { didSet { print("New state: \(viewState)") } } init(store: ProcessStore) { self.store = store subscribe(to: store) { newDomainState in state = ProcessListViewState(domainState: newDomainState) } } func fetchProcesses() { store.fetchProcesses() } }

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Implementation

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It's just observing a state property That's all, really

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NSNotiﬁcationCenter

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Unsafely typed

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Implicit dependency

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keypath observers

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token = store.observe(\.state) { ... }

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strongly typed

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Problems

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@objc dynamic

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Forgetting about dynamic

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State must be represented in Objc

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State must be represented in Objc No generics, enums, structs or sum types

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Custom strongly typed pub sub implementation

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protocol StoreState: Equatable {} protocol ViewState: StoreState {}

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class Store { init(initialState: State) { state = initialState } ... }

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class Store { ... private lazy var stateTransactionQueue = DispatchQueue(label: "com.jobandtalent.\(type(of: self)).StateTransactionQueue") var state: State { didSet(oldState) { if #available(iOS 10.0, *) { dispatchPrecondition(condition: .onQueue(stateTransactionQueue)) } stateDidChange(oldState: oldState, newState: state) } } ... }

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class Store { ... func stateDidChange(oldState: State, newState: State) { guard oldState != newState else { print("Skip forwarding same state: \(newState)") return } print("State change: \(newState)") subscriptions.allObjects.forEach { $0.fire(state) } } }

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class StateSubscription { private(set) var block: ((State) -> Void)? func fire(_ state: State) { block?(state) } func stop() { block = nil } deinit { stop() } }

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class StateSubscription { private(set) var block: ((State) -> Void)? func fire(_ state: State) { block?(state) } func stop() { block = nil } deinit { stop() } }

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class StateSubscription { private(set) var block: ((State) -> Void)? func fire(_ state: State) { block?(state) } func stop() { block = nil } deinit { stop() } }

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class Store { private var subscriptions = NSHashTable>.weakObjects() func subscribe(_ block: @escaping (State) -> Void) -> StateSubscription { let subscription = StateSubscription(block) subscriptions.add(subscription) subscription.fire(state) return subscription } }

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RAII

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protocol StatefulView { associatedtype State: ViewState func render(state: State) var renderPolicy: RenderPolicy { get } }

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protocol StatefulView { associatedtype State: ViewState func render(state: State) var renderPolicy: RenderPolicy { get } }

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protocol StatefulView { associatedtype State: ViewState func render(state: State) var renderPolicy: RenderPolicy { get } }

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protocol StatefulView { associatedtype State: ViewState func render(state: State) var renderPolicy: RenderPolicy { get } }

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enum RenderPolicy { case possible case notPossible(RenderError) enum RenderError { case viewNotReady case viewDeallocated }

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type erasure

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protocol StatefulView { func render() }

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How to access state?

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View needs to maintain view model

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Not always the case

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class ViewModel: Store { private var views = Set>() override var state: State { didSet(oldState) { views.forEach { stateDidChange(oldState: oldState, newState: state, view: $0) } } } }

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class ViewModel: Store { private var views = Set>() override var state: State { didSet(oldState) { views.forEach { stateDidChange(oldState: oldState, newState: state, view: $0) } } } }

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class ViewModel: Store { private func stateDidChange(oldState: State, newState: State, view: AnyStatefulView) { switch view.renderPolicy { case .possible: if newState == oldState { return } DispatchQueue.main.async { view.render(state: newState) } case .notPossible(let renderError): // Handle error } } }

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Problems

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Problems • Not being able to subscribe to speciﬁc parts of the state.

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Problems • Not being able to subscribe to speciﬁc parts of the state. • Not persistent data structures.

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Problems • Not being able to subscribe to speciﬁc parts of the state. • Not persistent data structures. • No guarantee only subclasses can change the state (no protected modiﬁer in swift).

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Problems • Not being able to subscribe to speciﬁc parts of the state. • Not persistent data structures. • No guarantee only subclasses can change the state (no protected modiﬁer in swift). • Mutating can be cumbersome with big trees -> leverage composition.