FileSystemEvent.java

import java.nio.file.WatchEvent;

public class FileSystemEvent {
    WatchEvent<?> watchEvent;

    FileSystemEvent(WatchEvent<?> watchEvent) {
        this.watchEvent = watchEvent;
    }

}

FileSystemWatcher.java

import rx.Observable;
import rx.Scheduler;
import rx.schedulers.Schedulers;

import java.io.IOException;
import java.nio.file.*;


public class FileSystemWatcher {

    public static Observable<FileSystemEvent> watch(String dir, WatchEvent.Kind<?>[] events) {
        return Observable.<FileSystemEvent>create(subscriber -> {

            FileSystem fileSystem = FileSystems.getDefault();
            try (WatchService watcher = fileSystem.newWatchService()) {

                Path path = fileSystem.getPath(dir);
                path.register(watcher, events);

                while (true) {
                    WatchKey key = watcher.take();
                    Observable.from(key.pollEvents()).forEach(event -> subscriber.onNext(new FileSystemEvent(event)));
                    key.reset();
                }

            } catch (Throwable e) {
                subscriber.onError(e);
            } finally {
                subscriber.onCompleted();
            }
        });
    }
}