Extending Redux with an effect system

Redux.swift

/* Credits to @gsampaio for coming up with the Effect<S, A> type.
 * Even though this flavor of reducer also describes side-effects,
 * testing is still trivial since it's sufficient to check if the
 * side-effects are of the right type. The actual execution of the
 * effect can be tested separately using integration tests. */

import RxSwift

class Effect<S: Equatable, A: Action> {
    open func execute(state: () -> S) -> Observable<A> {
        fatalError("not implemented")
    }

    static func empty<S, A>() -> Effect<S, A> {
        return EmptyEffect()
    }

    private class EmptyEffect<S: Equatable, A: Action>: Effect<S, A> {
        override func execute(state: () -> S) -> Observable<A> {
            return .empty()
        }
    }
}

protocol Action {}

struct Reducer<S: Equatable, A: Action> {
    let reduce: (S, A) -> (S, Effect<S, A>)

    init (_ reduce: @escaping (S, A) -> (S, Effect<S, A>)) {
        self.reduce = reduce
    }
}

class Store<S: Equatable, A: Action> {
    private let actionSink: AnyObserver<A>
    private let state: Observable<S>

    fileprivate init (actionSink: AnyObserver<A>, state: Observable<S>) {
        self.actionSink = actionSink
        self.state = state
    }

    static func create<S, A>(reducer: Reducer<S, A>, initialState: S) -> Store<S, A> {
        let actionSink: AnyObserver<A>
        let stateSubject = BehaviorSubject<S>(value: initialState)
        var dispatch: (A) -> Void = { _ in }
        var interpret: (Effect<S, A>) -> Void = { _ in }

        var currentState = initialState {
            didSet { stateSubject.onNext(currentState) }
        }

        actionSink = AnyObserver<A> { event in
            switch event {
            case let .next(action): dispatch(action)
            default: break
            }
        }

        dispatch = { action in
            let (nextState, nextEffect) = reducer.reduce(currentState, action)
            currentState = nextState
            interpret(nextEffect)
        }

        interpret = { effect in
            _ = effect.execute(state: { currentState }).subscribe(onNext: dispatch)
        }

        return Store<S, A>(actionSink: actionSink, state: stateSubject)
    }

    func asObserver() -> AnyObserver<A> {
        return actionSink
    }

    func asObservable() -> Observable<S> {
        return state
    }
}

//

class PrintEffect: Effect<PrintState, PrintAction> {
    override func execute(state: () -> PrintState) -> Observable<PrintAction> {
        print("State before effect:", state())
        return .just(.finishPrinting)
    }
}

enum PrintAction: Action {
    case startPrinting
    case finishPrinting
}

struct PrintState: Equatable {
    let isPrinting: Bool

    static func == (lhs: PrintState, rhs: PrintState) -> Bool {
        return lhs.isPrinting == rhs.isPrinting
    }
}

func PrintReducer() -> Reducer<PrintState, PrintAction> {
    return Reducer { _, action in
        switch action {
        case .startPrinting:
            return (PrintState(isPrinting: true), PrintEffect())
        case .finishPrinting:
            return (PrintState(isPrinting: false), .empty())
        }
    }
}

let store = Store<PrintState, PrintAction>.create(reducer: PrintReducer(), initialState: PrintState(isPrinting: false))
_ = store.asObservable().subscribe(onNext: { state in print("Subscribed state:", state) })
store.asObserver().onNext(.startPrinting)