/*
 * Copyright 2020 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */


export interface OnHiddenCallback {
  // TODO(philipwalton): add `isPersisted` if needed for bfcache.
  ({timeStamp, isUnloading}: {timeStamp: number; isUnloading: boolean}): void;
}

let isUnloading = false;
let listenersAdded = false;

const onPageHide = (event: PageTransitionEvent) => {
  isUnloading = !event.persisted;
};

const addListeners = () => {
  addEventListener('pagehide', onPageHide);

  // `beforeunload` is needed to fix this bug:
  // https://bugs.chromium.org/p/chromium/issues/detail?id=987409
  // eslint-disable-next-line @typescript-eslint/no-empty-function
  addEventListener('beforeunload', () => {});
}

export const onHidden = (cb: OnHiddenCallback, once = false) => {
  if (!listenersAdded) {
    addListeners();
    listenersAdded = true;
  }

  addEventListener('visibilitychange', ({timeStamp}) => {
    if (document.visibilityState === 'hidden') {
      cb({timeStamp, isUnloading});
    }
  }, {capture: true, once});
};
