// Keeps the page fresh the way the TUI does: refresh on an interval, but only
// while the page is visible, and immediately when it becomes visible again —
// which is the moment a phone is picked up after a page.

const INTERVAL = 20 * 1000;

let refreshFn = null;
let timer = 0;
let running = false;
let inFlight = null;

export function start(fn) {
  refreshFn = fn;
  running = true;
  schedule();
}

export function stop() {
  running = false;
  clearInterval(timer);
}

// now refreshes straight away and restarts the interval, after an action.
export function now() {
  if (!running) return Promise.resolve();
  schedule();
  return tick();
}

function tick() {
  if (!refreshFn) return Promise.resolve();
  // Collapse overlapping refreshes into the one already under way.
  if (!inFlight) {
    inFlight = Promise.resolve()
      .then(refreshFn)
      .catch(() => {})
      .finally(() => {
        inFlight = null;
      });
  }
  return inFlight;
}

function schedule() {
  clearInterval(timer);
  if (running && !document.hidden) timer = setInterval(tick, INTERVAL);
}

document.addEventListener('visibilitychange', () => {
  if (!running) return;
  if (!document.hidden) tick();
  schedule();
});
window.addEventListener('online', () => {
  if (running) tick();
});