Tone.js/Tone/core/context/OfflineContext.js

import Tone from "../core/Tone";
import "../core/Context";

/**
 *  @class Wrapper around the OfflineAudioContext
 *  @extends {Tone.Context}
 *  @param  {Number}  channels  The number of channels to render
 *  @param  {Number}  duration  The duration to render in samples
 *  @param {Number} sampleRate the sample rate to render at
 */
const OfflineContext = function(channels, duration, sampleRate) {

	/**
	 *  The offline context
	 *  @private
	 *  @type  {OfflineAudioContext}
	 */
	var offlineContext = new OfflineAudioContext(channels, duration * sampleRate, sampleRate);

	//wrap the methods/members
	Tone.Context.call(this, {
		"context" : offlineContext,
		"clockSource" : "offline",
		"lookAhead" : 0,
		"updateInterval" : 128 / sampleRate,
	});

	/**
	 *  A private reference to the duration
	 *  @private
	 *  @type  {Number}
	 */
	this._duration = duration;

	/**
	 *  An artificial clock source
	 *  @type  {Number}
	 *  @private
	 */
	this._currentTime = 0;
};

// Tone.extend(OfflineContext, Tone.Context);

/**
 *  Override the now method to point to the internal clock time
 *  @return  {Number}
 */
OfflineContext.prototype.now = function() {
	return this._currentTime;
};

/**
 *  Overwrite resume, should not do anything in the OfflineAudioContext.
 *  @return {Promise}
 */
OfflineContext.prototype.resume = function() {
	return Promise.resolve();
};

/**
 *  Render the output of the OfflineContext
 *  @return  {Promise}
 */
OfflineContext.prototype.render = function() {
	while (this._duration - this._currentTime >= 0) {
		//invoke all the callbacks on that time
		this.emit("tick");
		//increment the clock in 5ms chunks
		this._currentTime += 0.005;
	}

	return this._context.startRendering();
};

/**
 *  Close the context
 *  @return  {Promise}
 */
OfflineContext.prototype.close = function() {
	this._context = null;
	return Promise.resolve();
};

export default OfflineContext;