CombineLatest.swift

let firstOperation = Publishers.CombineLatest4($nameValid, $writerValid, $publisherValid, $publishDateValid) { isNameValid, isWriterValid, isPublisherValid, isPublishDateValid in
                return isNameValid && isWriterValid && isPublisherValid && isPublishDateValid
            }

let stream = Publishers.CombineLatest(firstOperation, $buyLinkValid) { isValid, isBuyLinkValid in
                return isValid && isBuyLinkValid }
            .sink { [weak self] isEnabled in
                self?.confirmBarButton.isEnabled = isEnabled }