https://stackoverflow.com/questions/72074315/is-there-any-situation-where-i-have-to-use-the-public-subject-instead-of-the-pub

MenuViewModel.swift

struct MenuViewModelInput {
    let fetchMenus: Observable<Void>
    let clearSelections: Observable<Void>
    let makeOrder: Observable<Void>
    let increaseMenuCount: Observable<(menu: ViewMenu, inc: Int)>
}

struct MenuViewModelOutput {
    let activated: Observable<Bool>
    let errorMessage: Observable<Error>
    let allMenus: Observable<[ViewMenu]>
    let totalSelectedCountText: Observable<String>
    let totalPriceText: Observable<String>
    let showOrderPage: Observable<[ViewMenu]>
}

func menuViewModel(fetchMenus: Observable<[Thing]>) -> (MenuViewModelInput) -> MenuViewModelOutput {
    { input in
        let errorSubject = PublishSubject<Error>()

        enum Action {
            case reset([ViewMenu])
            case adjustCount(menu: ViewMenu, inc: Int)
        }

        let resetMenus = input.fetchMenus
            .flatMap {
                fetchMenus
                    .catch { error in
                        errorSubject.onNext(error)
                        return .empty()
                    }
            }
            .map { Action.reset($0.map { ViewMenu($0) }) }

        let allMenus = Observable.merge(
            resetMenus,
            input.increaseMenuCount.map(Action.adjustCount(menu:inc:))
        )
            .scan(into: [ViewMenu]()) { state, action in
                switch action {
                case let .reset(viewMenus):
                    state = viewMenus
                case .adjustCount(menu: let menu, inc: let inc):
                    guard let index = state.firstIndex(where: { $0.name == menu.name }) else { break }
                    state[index] = state[index].countUpdated(inc)
                }
            }
            .share(replay: 1)

        return MenuViewModelOutput(
            activated: Observable.merge(
                input.fetchMenus.map { true },
                resetMenus.map { _ in false }
            )
            .startWith(false),
            errorMessage: errorSubject.asObservable(),
            allMenus: allMenus,
            totalSelectedCountText: allMenus
                .map { $0.map { $0.count }.reduce(0, +) }
                .map { "\($0)" },
            totalPriceText: allMenus
                .map { $0.map { $0.price * $0.count }.reduce(0, +) }
                .map { $0.currencyKR() },
            showOrderPage: input.makeOrder
                .withLatestFrom(allMenus)
                .map { $0.filter { $0.count > 0 } }
                .do(onNext: { items in
                    if items.count == 0 {
                        let err = NSError(domain: "No Orders", code: -1, userInfo: nil)
                        errorSubject.onNext(err)
                    }
                })
                .filter { $0.count > 0 }
        )
    }
}