text: /*****************************************************************************
   * Open MCT, Copyright (c) 2009-2021, United States Government
   * as represented by the Administrator of the National Aeronautics and Space
   * Administration. All rights reserved.
   *
   * Open MCT is 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
   * http://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.
   *
   * Open MCT includes source code licensed under additional open source
   * licenses. See the Open Source Licenses file (LICENSES.md) included with
   * this source code distribution or the Licensing information page available
   * at runtime from the About dialog for additional information.
   *****************************************************************************/
  
  class Ticker {
      constructor() {
          this.callbacks = [];
          this.last = new Date() - 1000;
      }
  
      /**
       * Calls functions every second, as close to the actual second
       * tick as is feasible.
       * @constructor
       * @memberof utils/clock
       */
      tick() {
          const timestamp = new Date();
          const millis = timestamp % 1000;
  
          // Only update callbacks if a second has actually passed.
          if (timestamp >= this.last + 1000) {
              this.callbacks.forEach(function (callback) {
                  callback(timestamp);
              });
              this.last = timestamp - millis;
          }
  
          // Try to update at exactly the next second
          this.timeoutHandle = setTimeout(() => {
              this.tick();
          }, 1000 - millis, true);
      }
  
      /**
       * Listen for clock ticks. The provided callback will
       * be invoked with the current timestamp (in milliseconds
       * since Jan 1 1970) at regular intervals, as near to the
       * second boundary as possible.
       *
       * @param {Function} callback callback to invoke
       * @returns {Function} a function to unregister this listener
       */
      listen(callback) {
          if (this.callbacks.length === 0) {
              this.tick();
          }
  
          this.callbacks.push(callback);
  
          // Provide immediate feedback
          callback(this.last);
  
          // Provide a deregistration function
          return () => {
              this.callbacks = this.callbacks.filter(function (cb) {
                  return cb !== callback;
              });
  
              if (this.callbacks.length === 0) {
                  clearTimeout(this.timeoutHandle);
              }
          };
      }
  }
  
  let ticker = new Ticker();
  
  export default ticker;
  