systrace/catapult/third_party/polymer/components/shadycss/src/document-wait.js

/**
@license
Copyright (c) 2017 The Polymer Project Authors. All rights reserved.
This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
Code distributed by Google as part of the polymer project is also
subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
*/

'use strict';

/** @type {Promise<void>} */
let readyPromise = null;

/** @type {?function(?function())} */
let whenReady = window['HTMLImports'] && window['HTMLImports']['whenReady'] || null;

/** @type {function()} */
let resolveFn;

/**
 * @param {?function()} callback
 */
export default function documentWait(callback) {
  requestAnimationFrame(function() {
    if (whenReady) {
      whenReady(callback)
    } else {
      if (!readyPromise) {
        readyPromise = new Promise((resolve) => {resolveFn = resolve});
        if (document.readyState === 'complete') {
          resolveFn();
        } else {
          document.addEventListener('readystatechange', () => {
            if (document.readyState === 'complete') {
              resolveFn();
            }
          });
        }
      }
      readyPromise.then(function(){ callback && callback(); });
    }
  });
}