function createContextPoller({
  fetchContext,
  onSnapshot,
  onError,
  isHidden = () => false,
  setTimer = setTimeout,
  clearTimer = clearTimeout,
  intervalMs = 8000,
}) {
  let inFlight = null;
  let timer = null;
  let stopped = false;

  function cancelTimer() {
    if (timer !== null) clearTimer(timer);
    timer = null;
  }

  function schedule() {
    cancelTimer();
    if (stopped || isHidden()) return;
    timer = setTimer(() => {
      timer = null;
      refresh();
    }, intervalMs);
  }

  function refresh() {
    if (stopped || isHidden()) return Promise.resolve(null);
    if (inFlight) return inFlight;

    let request;
    try {
      request = fetchContext();
    } catch (error) {
      request = Promise.reject(error);
    }
    inFlight = Promise.resolve(request)
      .then((snapshot) => {
        onSnapshot(snapshot);
        return snapshot;
      })
      .catch((error) => {
        onError(error);
        return null;
      })
      .finally(() => {
        inFlight = null;
        schedule();
      });
    return inFlight;
  }

  function setVisible(visible) {
    cancelTimer();
    if (!visible) return Promise.resolve(null);
    return refresh();
  }

  return {
    start: refresh,
    refresh,
    setVisible,
    stop() {
      stopped = true;
      cancelTimer();
    },
  };
}

if (typeof module !== 'undefined' && module.exports) {
  module.exports = createContextPoller;
}