Unidirectional Data Flow in Swift

WHAT IS THE STATE OF
YOUR APP?

STATE
struct Player {
var name: String
var characterClass: CharacterClass
var maxHP: Int
var currentHP: Int
}

STATE COMPOSITION
struct RPGState {
var players: Player
var monsters: Monsters
// ...
}

(STATE) -> VIEW
UI AS A PURE FUNCTION OF STATE

REDRAW VIEW ON EVERY STATE
CHANGE?!?!?

(STATE) -> VOID
(WITH SIDE EFFECTS)

extension ViewController {
func update(with state: State) {
nameLabel.text = state.name
}
}

DATA FLOW
> Delegates
> Target-Actions
> Completion Blocks
> Notifications
> Segues
> didSet

WHAT DIRECTION IS THE
DATA FLOWING?

SPOT THE BUG!
class ViewController: UIViewController {
@IBOutlet var titleLabel: UILabel!
var name: String? {
didSet {
titleLabel.text = name
}
}
}

SPOT THE NEW BUG!
@IBOutlet var titleLabel: UILabel?
var name: String? {
didSet {
titleLabel?.text = name
}
}
}

THE IMPLICITLY UNWRAPPED OPTIONAL DANCE
@IBOutlet var titleLabel: UILabel?
var name: String? {
didSet {
configureView()
}
}
func configureView() {
titleLabel?.text = name
}
override func viewDidLoad() {
super.viewDidLoad()
configure()
}
}

REACTOR
> Holds the Single Source of Truth™
> Only the reactor can change the state
> Notifies all subscribers with state changes

Observing State
class ViewController: Reactor.Subscriber {
var reactor = App.sharedReactor
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
reactor.add(subscriber: self)
}
override func viewDidDisappear(_ animated: Bool) {
super.viewDidDisappear(animated)
reactor.remove(subscriber: self)
}
func update(with state: State) {
nameLabel.text = state.name
}
}

UPDATING STATE
struct Increment: Event {}
@IBAction func didPressIncrement() {
reactor.perform(event: Increment())
}
}

ASYNC EVENTS
struct Update<T>: Event {
var value: T
}
class UserService {
// this type of function is also knows as an `EventEmitter`
func getUsers(state: State, reactor: Reactor<State>) -> Event? {
myNetworkThing.get("users") { json in
// transform to user object, using Marshal, of course
reactor.perform(event: Update(value: users))
}
}
}

ASYNC EVENTS
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
reactor.add(subscriber: self)
reactor.perform(event: userService.getUsers)
}
}

HOW DO EVENTS UPDATE STATE?
extension State: StateType {
mutating func handle(event: Event) {
switch event {
case _ as LevelUp:
player.level += 1
default:
break
}
// also call `handle` on any substates
}
}

ROLL YOUR OWN
> Don't like the API?
> Don't like 3rd Party? !
It's a straightforward pattern.

HOT RELOADINGUSING MARSHAL + KZFILEWATCHER

OTHER RESOURCES
> A composable pattern for pure state machines with
effects by Andy Matuschak
> Unidirectional Data Flow in Swift by Benjamin Encz
> Elmification of Swift
> MCV-N Swift Talk by Marcus Zarra
> Backend-driven Native UIs by John Sundell

Unidirectional Data Flow in Swift

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