web-vitals/src/lib/onHidden.ts

/*
 * 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 {
  (event: Event): void;
}

let beforeUnloadFixAdded = false;

export const onHidden = (cb: OnHiddenCallback, once?: boolean) => {
  // Adding a `beforeunload` listener is needed to fix this bug:
  // https://bugs.chromium.org/p/chromium/issues/detail?id=987409
  if (!beforeUnloadFixAdded &&
      // Avoid adding this in Firefox as it'll break bfcache:
      // https://stackoverflow.com/questions/9847580/how-to-detect-safari-chrome-ie-firefox-and-opera-browser
      // @ts-ignore
      typeof InstallTrigger === 'undefined') {
    // eslint-disable-next-line @typescript-eslint/no-empty-function
    addEventListener('beforeunload', () => {});
    beforeUnloadFixAdded = true;
  }

  const onVisibilityChange = (event: Event) => {
    if (document.visibilityState === 'hidden') {
      cb(event);
      if (once) {
        removeEventListener('visibilitychange', onVisibilityChange, true);
      }
    }
  }
  addEventListener('visibilitychange', onVisibilityChange, true);
};