static void onNewFileInDir(final File dir, int interval, final VF1<File> onNewFile) {
  doEvery(interval, new Runnable {
    Set<File> files = asHashSet(listFiles(dir));
    
    public void run {
      L<File> newFiles = listFiles(dir);
      for (File f : newFiles)
        if (!files.contains(f))
          pcallF(onNewFile, f);
      files = newFiles;
    }
  });
}