Firestore first document retrieving for pagination

FirebaseGetListExtension.kt

fun FirebaseFirestore.getFirstDocument(
    path: String,
    page: FirebasePage
): Maybe<DocumentSnapshot> = Maybe.create { emitter ->
    val collectionReference = collection(path)
        .orderBy(page.orderBy, page.direction)
        .limit(page.size * page.number + 1)

    val listenerRegistration: ListenerRegistration = collectionReference.addSnapshotListener { querySnapshot, exception ->
        querySnapshot?.run {
            if (size() > page.size * page.number) {
                val firstDocument = documents[size() - 1]
                emitter.onSuccess(firstDocument)
            } else {
                emitter.onComplete()
            }
        } ?: exception?.run {
            emitter.onError(this)
        } ?: kotlin.run { emitter.onError(FirebaseUnexpectedException()) }
    }

    emitter.setCancellable { listenerRegistration.remove() }
}