import { addEventListener } from "./listeners";
import { doc, win } from "./globals";

let isUnloading = false;

/**
 * Triggers the `fn` when the HTML document is getting unloaded
 * (or when it looks like it might be). Useful to flush batch activities.
 */
export function onLastChance(fn: () => void) {
  if (isUnloading) {
    fn();
  }

  if (!doc || !win) return;

  addEventListener(doc, "visibilitychange", function () {
    if (doc!.visibilityState !== "visible") {
      fn();
    }
  });

  addEventListener(win, "pagehide", function () {
    isUnloading = true;
    fn();
  });

  // According to the spec visibilitychange should be a replacement for
  // beforeunload, but the reality is different (as of 2019-04-17). Chrome will
  // close tabs without firing visibilitychange. beforeunload on the other hand
  // is fired.
  addEventListener(win, "beforeunload", function () {
    isUnloading = true;
    fn();
  });
}