Estimated Tax model

EstimatedTaxProtocol.swift

protocol EstimatedTaxProtocol {
    associatedtype ItemType: PurchaseItem
    var items: BehaviorSubject<[ItemType]> { get set }
    var sumData: Observable<Int> { get }
    var estimatedTax: Observable<Int> { get }
    var estimatedTaxItems: Observable<[ItemType]> { get }
}

extension EstimatedTaxProtocol {
    var sumData: Observable<Int> {
        return self.items.flatMap({ items -> Observable<Int> in
            let sum = items.reduce(0, { (result, item) -> Int in
                return result + item.dollar
            })
            return Observable.just(sum)
        })
    }
    //...
}