Tiny implementation of Unidirectional Architecture with Query based SideEffects.

unidirectional.swift

import Combine
import Foundation

func loadNewsTitles() -> AnyPublisher<[String], Never> {
    ["title1", "title2"]
        .publisher
        .delay(for: .microseconds(500), scheduler: DispatchQueue.main)
        .collect()
        .eraseToAnyPublisher()
}

struct SideEffects {
    let loadNewTitles: () -> AnyPublisher<[String], Never>

    func downloadNewTitles() -> AnyPublisher<Event, Never> {
        loadNewTitles()
            .map(Event.dataLoaded)
            .eraseToAnyPublisher()
    }
}

enum State: Equatable {
    case initial
    case loading
    case loaded(data: [String])
}

enum Event {
    case dataLoaded(data: [String])
    case loadData
}

extension State {
    var loadQuery: Bool {
        guard case .loading = self else { return false }
        return true
    }
}

extension State {
    static func reduce(state: State, event: Event) -> State {
        var state = state
        switch event {
        case .dataLoaded(let data):
            state = .loaded(data: data)
        case .loadData:
            state = .loading
        }
        return state
    }
}

typealias Reducer = (State, Event) -> State

class Store {
    @Published private(set) var state: State
    private let reducer: Reducer
    private let sideEffects: SideEffects

    init(
        initialState: State,
        reducer: @escaping Reducer,
        sideEffects: SideEffects
    ) {
        self.state = initialState
        self.reducer = reducer
        self.sideEffects = sideEffects
    }

    func accept(event: Event) {
        state = reducer(state, event)
    }

    func start() -> AnyCancellable {
        $state
            .map(\.loadQuery)
            .removeDuplicates()
            .filter { $0 == true }
            .map { _ in () }
            .flatMap(sideEffects.downloadNewTitles)
            .sink(receiveValue: accept(event:))
    }
}

let store = Store(
    initialState: .initial,
    reducer: State.reduce(state:event:),
    sideEffects: SideEffects(
        loadNewTitles: loadNewsTitles
    )
)
var cancellables: [AnyCancellable] = []

store
    .start()
    .store(in: &cancellables)

store
    .$state
    .removeDuplicates()
    .sink { state in print(state) }
    .store(in: &cancellables)

store.accept(event: .loadData)