srecord noeq FollowTickerFile(File tickerFile) is AutoCloseable {

  settable int historyEntriesToGrab;

  settable int interval = 250;

  settable ISleeper sleeper = defaultSleeper();

  // internal, may change  

  TailFile tailFile;

  event historicLine(S line);

  event liveLine(S line);

  gettable volatile long liveTimestamp;

  gettable volatile S lastLiveLine;

  settable transient bool started;

  event fireStarted;

  settable int maxLineLengthForTickerEntry = 200;

  settable Q q = startQ();

  bool isLive() { ret liveTimestamp != 0; }

  void start {

    q.add(r {

      if (started()) ret;

      started(true);

      fireStarted();

      onLiveLine(line -> {

        lastLiveLine = line;

        liveTimestamp = now();

      });

      long fileSize = l(tickerFile);

      long start = max(0, fileSize-maxLineLengthForTickerEntry*historyEntriesToGrab);

      if (historyEntriesToGrab > 0) {

        S text = loadTextFilePart(tickerFile, start, fileSize);

        LS lastTickerLines = takeLast(historyEntriesToGrab, lines(text));

        print("Have historic lines: " + l(lastTickerLines) + "/" + historyEntriesToGrab);

        for (line : lastTickerLines)

          historicLine(line);

      }

      makeTailFile(fileSize);

    });

  }

  void makeTailFile(long fileSize) {

    started(true);

    print("Following " + tickerFile + " (" + n2(l(tickerFile)) + " bytes) from position " + n2(fileSize));

    tailFile = tailFileLinewiseFromPosition(tickerFile, interval, fileSize, l1 liveLine, sleeper);

  }

  close {

    started(false);

    if (tailFile != null) {

      print("Done " + tailFile);

      dispose tailFile;

    }

  }

  void switchToFile(File newFile) {

    if (eq(tickerFile, newFile)) ret;

    q.add(r {

     if (eq(tickerFile, newFile)) ret;

      print("Switching to ticker file: " + newFile);

      if (started()) {

        close();

        tickerFile = newFile;

        started(true);

        makeTailFile(0);

      } else

        tickerFile = newFile;

    });

  }

  selfType onLine(IVF1<S> l) {

    onHistoricLine(l);

    ret onLiveLine(l);

  }

  toString {

    ret commaCombine(

      "FollowTickerFile",

      stringIf(started(), "started"),

      !isLive() ? null : "Last live line at " + formatLocalDateWithSeconds(liveTimestamp),

      tickerFile

    );

  }

}