import java.nio.file.*;
import static java.nio.file.StandardWatchEventKinds.*;

sclass FileWatchService implements AutoCloseable {
  WatchService watchService = FileSystems.getDefault().newWatchService();
  new MultiMap<WatchKey, VIF1<File>> listeners;
  int interval = 50; // larger interval helps show less duplicate events (https://stackoverflow.com/questions/16777869/java-7-watchservice-ignoring-multiple-occurrences-of-the-same-event/34818620#34818620)
  Thread pollThread;
  
  public void close { dispose watchService; }
  
  WatchKey addListener(File path, IVF1<File> listener, WatchEvent.Kind... eventKinds) {
    startPollThread();
    WatchKey key = path.toPath().register(watchService, eventKinds);
    listeners.put(key, listener);
    ret key;
  }

  synchronized void startPollThread() {
    if (pollThread != null) ret;
    pollThread = startThread("File Watch Service", r {
      while licensed { 
        WatchKey key = service.poll(10, TimeUnit.SECONDS);
        if (key != null) {
          Path dir = cast key.watchable();
          sleep(interval);
          L<WatchEvent<?>> events = key.pollEvents();
          new LinkedHashSet<File> changedFiles;
          for (WatchEvent<?> event : reversedIterator(events)) pcall {
            final Path changed = (Path) event.context();
            File full = newFile(dir.toFile(), changed.toFile().getPath());
            changedFiles.add(full);
            //print("WatchService Changed: " + full + ", " + event.kind());
          }
          for (File f : changedFiles)
            pcallFAll(listeners.get(key), f);
          key.reset();
        }
      }
    });
  }
}