openmct/src/utils/clock/Ticker.js

/*****************************************************************************
 * Open MCT, Copyright (c) 2014-2023, 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;