enum StateEnum {
    INIT, IN_PROGRESS, COMPLETE, ERROR
}

class EmailSender {
    // interface
    fun send(email : Email) : EmailSendingProgress
    fun cancel(emailSendingProgress: EmailSendingProgress)
    val senderState : BehaviorRelay<EmailSendingProgress> // relay, cannot be closed

    // impl
    private val senderScopedDisposable = CompositeDisposable()

    fun send(email : Email) : EmailSendingProgress {
        val newProgressSubject = BehaviorSubject.default<StateEnum>(StateEnum.INIT)

        senderScopedDisposable += emailApi.send(...) // .send je Single/Completable
                                        .toObservable()
                                        .map { StateEnum.COMPLETE }
                                        .onErrorReturnItem(StateEnum.ERROR)
                                        .onStart(StateEnum.IN_PROGRESS)
                                        .doOnComplete { newProgressSubject.onComplete() }
                                        .subscribe { newProgressSubject.onNext(it) }

        val newProgress = EmailSendingProgress(newProgressSubject)
        senderState.accept(newProgress)

        return newProgress
    }

    // retry by som asi implementoval vytvorenim noveho sendu s rovnakym payloadom + ?id?
}

class EmailSendingProgress(val progress : Observable<StateEnum>)