public WatchEventsNonBlockingBuilder kinds(Kind<?>... kinds) {
            Preconditions.checkNotNull(kinds);
            for (Kind<?> kind : kinds) {
                this.kinds.add(kind);
            }
            return this;
        }

        public Observable<WatchEvent<?>> build() {
            List<Kind<?>> kindsCopy = new ArrayList<>(kinds);
            if (kindsCopy.isEmpty()) {
                kindsCopy.add(StandardWatchEventKinds.ENTRY_CREATE);
                kindsCopy.add(StandardWatchEventKinds.ENTRY_DELETE);
                kindsCopy.add(StandardWatchEventKinds.ENTRY_MODIFY);
                kindsCopy.add(StandardWatchEventKinds.OVERFLOW);
            }
            return Observable.using( //
                    () -> watchService(file, kindsCopy, modifiers), //
                    ws -> Files.events(ws, scheduler.orElse(Schedulers.io()), pollIntervalUnit.toMillis(pollInterval)), //