sclass TailFile implements AutoCloseable {

  File file;

  int interval;

  O onData;

  settable ISleeper sleeper = defaultSleeper();

  long l;

  gettable long currentFileSize;

  new Flag stopped;

  FlexibleRateTimer timer;

  ReliableSingleThread thread;

  volatile bool hasIdled, debug;

  *() {}

  *(final File *file, int *interval, O *onData) {

    thread = new ReliableSingleThread(r { update() });

  }

  void update {

    long l2 = currentFileSize = l(file);

    if (l2 < l) l = 0; // file shrunk (log rotated!) - start over from beginning of file

    if (l2 == l) hasIdled = true;

    else pcall {

      if (debug) print("New data in " + f2s(file) + ": " + n2(l2-l, "byte"));

      for (S s : loadTextFilePart_iterator(file, l, l2))

        pcallF(onData, s);

      l = l2;

    }

  }

  void start {

    timer = new FlexibleRateTimer(msIntervalToHertz(interval), -> {

      if (!stopped.isUp()) thread.trigger();

    });

    timer.sleeper(sleeper);

    timer.start();

  }

  void stop {

    timer.cancel();

    timer = null;

    if (stopped.raise())

      thread.triggerAndWait(); // One last time baby

  }

  public void close { stop(); }

  bool started() {

    ret timer != null;

  }

  void debugOn {

    if (debug) ret;

    debug = true;

    print("Watching file: " + f2s(file));

  }

  toString { ret "Tailing file " + file; }

}