Simple (Naive) Rx Cart

Cart.swift

struct Cart {
  private let action = PublishRelay<Action>()
  public let items: Observable<[Item]>

  init() {
    items = action
      .scan([Item]()) { items, action in
        switch action {
        case .add(let item):
          return items + [item]
        case .remove(let item):
          return items.filter { $0.id != item.id }
        }
      }
  }
}

extension Cart {
  enum Action {
    case add(Item)
    case remove(Item)
  }
}

extension Cart: ObserverType {
  func on(_ event: Event<Cart.Action>) {
    switch event {
    case .next(let newAction):
      action.accept(newAction)
    default:
      break // ignore
    }
  }

  typealias E = Cart.Action
}

/// ### USAGE

let cart = Cart()
let items = [
  "Coffee",
  "Tea",
  "Biscuit",
  "Something"
].map(Item.init)

cart.items
    .debug("cart")
    .subscribe()
    .disposed(by: disposeBag)

// Some stream of actions coming from outside
Observable<Cart.Action>
  .of (.add(items[0]),
       .add(items[1]),
       .add(items[3]),
       .add(items[2]),
       .remove(items[3]),
       .remove(items[1])
  )
  .bind(to: cart)
  .disposed(by: disposeBag)