/**
* Manager responsible for the communication between the experiment running in the participant's browser and the remote PsychoJS manager running on the remote https://pavlovia.org server.
*
* @author Alain Pitiot
* @version 2021.1.0
* @copyright (c) 2017-2020 Ilixa Ltd. (http://ilixa.com) (c) 2020 Open Science Tools Ltd. (https://opensciencetools.org)
* @license Distributed under the terms of the MIT License
*/
import {PsychoJS} from './PsychoJS';
import {PsychObject} from '../util/PsychObject';
import * as util from '../util/Util';
import {ExperimentHandler} from "../data/ExperimentHandler";
import {MonotonicClock} from "../util/Clock";
// import { Howl } from 'howler';
/**
* <p>This manager handles all communications between the experiment running in the participant's browser and the remote PsychoJS manager running on the [pavlovia.org]{@link http://pavlovia.org} server, <em>in an asynchronous manner</em>.</p>
* <p>It is responsible for reading the configuration file of an experiment, for opening and closing a session, for listing and downloading resources, and for uploading results and log.</p>
* <p>Note: The Server Manager uses [Promises]{@link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise} to deal with asynchronicity, is mostly called by {@link PsychoJS}, and is not exposed to the experiment code.</p>
*
* @name module:core.ServerManager
* @class
* @extends PsychObject
* @param {Object} options
* @param {module:core.PsychoJS} options.psychoJS - the PsychoJS instance
* @param {boolean} [options.autoLog= false] - whether or not to log
*/
export class ServerManager extends PsychObject
{
constructor({
psychoJS,
autoLog = false
} = {})
{
super(psychoJS);
// session:
this._session = {};
// resources is a map of {name: string, path: string} -> data: any
this._resources = new Map();
this._nbResources = -1;
this._addAttribute('autoLog', autoLog);
this._addAttribute('status', ServerManager.Status.READY);
}
/**
* @typedef ServerManager.GetConfigurationPromise
* @property {string} origin the calling method
* @property {string} context the context
* @property {Object.<string, *>} [config] the configuration
* @property {Object.<string, *>} [error] an error message if we could not read the configuration file
*/
/**
* Read the configuration file for the experiment.
*
* @name module:core.ServerManager#getConfiguration
* @function
* @public
* @param {string} configURL - the URL of the configuration file
*
* @returns {Promise<ServerManager.GetConfigurationPromise>} the response
*/
getConfiguration(configURL)
{
const response = {
origin: 'ServerManager.getConfiguration',
context: 'when reading the configuration file: ' + configURL
};
this._psychoJS.logger.debug('reading the configuration file: ' + configURL);
const self = this;
return new Promise((resolve, reject) =>
{
$.get(configURL, 'json')
.done((config, textStatus) =>
{
// resolve({ ...response, config });
resolve(Object.assign(response, {config}));
})
.fail((jqXHR, textStatus, errorThrown) =>
{
self.setStatus(ServerManager.Status.ERROR);
const errorMsg = util.getRequestError(jqXHR, textStatus, errorThrown);
console.error('error:', errorMsg);
reject(Object.assign(response, {error: errorMsg}));
});
});
}
/**
* @typedef ServerManager.OpenSessionPromise
* @property {string} origin the calling method
* @property {string} context the context
* @property {string} [token] the session token
* @property {Object.<string, *>} [error] an error message if we could not open the session
*/
/**
* Open a session for this experiment on the remote PsychoJS manager.
*
* @name module:core.ServerManager#openSession
* @function
* @public
* @returns {Promise<ServerManager.OpenSessionPromise>} the response
*/
openSession()
{
const response = {
origin: 'ServerManager.openSession',
context: 'when opening a session for experiment: ' + this._psychoJS.config.experiment.fullpath
};
this._psychoJS.logger.debug('opening a session for experiment: ' + this._psychoJS.config.experiment.fullpath);
this.setStatus(ServerManager.Status.BUSY);
// prepare POST query:
let data = {};
if (this._psychoJS._serverMsg.has('__pilotToken'))
{
data.pilotToken = this._psychoJS._serverMsg.get('__pilotToken');
}
// query pavlovia server:
const self = this;
return new Promise((resolve, reject) =>
{
const url = this._psychoJS.config.pavlovia.URL + '/api/v2/experiments/' + encodeURIComponent(self._psychoJS.config.experiment.fullpath) + '/sessions';
$.post(url, data, null, 'json')
.done((data, textStatus) =>
{
if (!('token' in data))
{
self.setStatus(ServerManager.Status.ERROR);
reject(Object.assign(response, {error: 'unexpected answer from server: no token'}));
// reject({...response, error: 'unexpected answer from server: no token'});
}
if (!('experiment' in data))
{
self.setStatus(ServerManager.Status.ERROR);
// reject({...response, error: 'unexpected answer from server: no experiment'});
reject(Object.assign(response, {error: 'unexpected answer from server: no experiment'}));
}
self._psychoJS.config.session = {
token: data.token,
status: 'OPEN'
};
self._psychoJS.config.experiment.status = data.experiment.status2;
self._psychoJS.config.experiment.saveFormat = Symbol.for(data.experiment.saveFormat);
self._psychoJS.config.experiment.saveIncompleteResults = data.experiment.saveIncompleteResults;
self._psychoJS.config.experiment.license = data.experiment.license;
self._psychoJS.config.experiment.runMode = data.experiment.runMode;
self.setStatus(ServerManager.Status.READY);
// resolve({ ...response, token: data.token, status: data.status });
resolve(Object.assign(response, {token: data.token, status: data.status}));
})
.fail((jqXHR, textStatus, errorThrown) =>
{
self.setStatus(ServerManager.Status.ERROR);
const errorMsg = util.getRequestError(jqXHR, textStatus, errorThrown);
console.error('error:', errorMsg);
reject(Object.assign(response, {error: errorMsg}));
});
});
}
/**
* @typedef ServerManager.CloseSessionPromise
* @property {string} origin the calling method
* @property {string} context the context
* @property {Object.<string, *>} [error] an error message if we could not close the session (e.g. if it has not previously been opened)
*/
/**
* Close the session for this experiment on the remote PsychoJS manager.
*
* @name module:core.ServerManager#closeSession
* @function
* @public
* @param {boolean} [isCompleted= false] - whether or not the experiment was completed
* @param {boolean} [sync= false] - whether or not to communicate with the server in a synchronous manner
* @returns {Promise<ServerManager.CloseSessionPromise> | void} the response
*/
async closeSession(isCompleted = false, sync = false)
{
const response = {
origin: 'ServerManager.closeSession',
context: 'when closing the session for experiment: ' + this._psychoJS.config.experiment.fullpath
};
this._psychoJS.logger.debug('closing the session for experiment: ' + this._psychoJS.config.experiment.name);
this.setStatus(ServerManager.Status.BUSY);
// prepare DELETE query:
const url = this._psychoJS.config.pavlovia.URL + '/api/v2/experiments/' + encodeURIComponent(this._psychoJS.config.experiment.fullpath) + '/sessions/' + this._psychoJS.config.session.token;
// synchronous query the pavlovia server:
if (sync)
{
/* This is now deprecated in most browsers.
const request = new XMLHttpRequest();
request.open("DELETE", url, false);
request.setRequestHeader("Content-Type", "application/json;charset=UTF-8");
request.send(JSON.stringify(data));
*/
/* This does not work in Chrome before of a CORS bug
await fetch(url, {
method: 'DELETE',
headers: { 'Content-Type': 'application/json;charset=UTF-8' },
body: JSON.stringify(data),
// keepalive makes it possible for the request to outlive the page (e.g. when the participant closes the tab)
keepalive: true
});
*/
const formData = new FormData();
formData.append('isCompleted', isCompleted);
navigator.sendBeacon(url + '/delete', formData);
this._psychoJS.config.session.status = 'CLOSED';
}
// asynchronously query the pavlovia server:
else
{
const self = this;
return new Promise((resolve, reject) =>
{
$.ajax({
url,
type: 'delete',
data: {isCompleted},
dataType: 'json'
})
.done((data, textStatus) =>
{
self.setStatus(ServerManager.Status.READY);
self._psychoJS.config.session.status = 'CLOSED';
// resolve({ ...response, data });
resolve(Object.assign(response, {data}));
})
.fail((jqXHR, textStatus, errorThrown) =>
{
self.setStatus(ServerManager.Status.ERROR);
const errorMsg = util.getRequestError(jqXHR, textStatus, errorThrown);
console.error('error:', errorMsg);
reject(Object.assign(response, {error: errorMsg}));
});
});
}
}
/**
* Get the value of a resource.
*
* @name module:core.ServerManager#getResource
* @function
* @public
* @param {string} name of the requested resource
* @return {Object} value of the resource
* @throws {Object.<string, *>} exception if no resource with that name has previously been registered
*/
getResource(name)
{
const response = {
origin: 'ServerManager.getResource',
context: 'when getting the value of resource: ' + name
};
const path_data = this._resources.get(name);
if (typeof path_data === 'undefined')
// throw { ...response, error: 'unknown resource' };
{
throw Object.assign(response, {error: 'unknown resource'});
}
return path_data.data;
}
/**
* Set the resource manager status.
*
* @name module:core.ServerManager#setStatus
* @function
* @public
*/
setStatus(status)
{
const response = {
origin: 'ServerManager.setStatus',
context: 'when changing the status of the server manager to: ' + util.toString(status)
};
// check status:
const statusKey = (typeof status === 'symbol') ? Symbol.keyFor(status) : null;
if (!statusKey)
// throw { ...response, error: 'status must be a symbol' };
{
throw Object.assign(response, {error: 'status must be a symbol'});
}
if (!ServerManager.Status.hasOwnProperty(statusKey))
// throw { ...response, error: 'unknown status' };
{
throw Object.assign(response, {error: 'unknown status'});
}
this._status = status;
// inform status listeners:
this.emit(ServerManager.Event.STATUS, this._status);
return this._status;
}
/**
* Reset the resource manager status to ServerManager.Status.READY.
*
* @name module:core.ServerManager#resetStatus
* @function
* @public
* @return {ServerManager.Status.READY} the new status
*/
resetStatus()
{
return this.setStatus(ServerManager.Status.READY);
}
/**
* Asynchronously download resources for the experiment and register them with the server manager.
*
* <ul>
* <li>For an experiment running locally: the root directory for the specified resources is that of index.html
* unless they are prepended with a protocol, such as http:// or https://.</li>
* <li>For an experiment running on the server: if no resources are specified, all files in the resources directory
* of the experiment are downloaded, otherwise we only download the specified resources. All resources are assumed
* local to index.html unless they are prepended with a protocol.</li>
* </ul>
*
* @name module:core.ServerManager#downloadResources
* @param {Array.<{name: string, path: string}>} [resources=[]] - the list of resources
* @function
* @public
*/
downloadResources(resources = [])
{
const response = {
origin: 'ServerManager.downloadResources',
context: 'when downloading the resources for experiment: ' + this._psychoJS.config.experiment.name
};
this._psychoJS.logger.debug('downloading resources for experiment: ' + this._psychoJS.config.experiment.name);
// we use an anonymous async function here since downloadResources is non-blocking (asynchronous)
// but we want to run the asynchronous _listResources and _downloadResources in sequence
const self = this;
const newResources = new Map();
let download = async () =>
{
try
{
if (self._psychoJS.config.environment === ExperimentHandler.Environment.SERVER)
{
// no resources specified, we register them all:
if (resources.length === 0)
{
// list the resources from the resources directory of the experiment on the server:
const serverResponse = await self._listResources();
for (const name of serverResponse.resources)
{
self._resources.set(name, {path: serverResponse.resourceDirectory + '/' + name});
}
}
else
{
// only registered the specified resources:
for (const {name, path} of resources)
{
self._resources.set(name, {path});
newResources.set(name, {path});
}
}
}
else
{
// register the specified resources:
for (const {name, path} of resources)
{
self._resources.set(name, {path});
newResources.set(name, {path});
}
}
self._nbResources = self._resources.size;
for (const name of self._resources.keys())
{
this._psychoJS.logger.debug('resource:', name, self._resources.get(name).path);
}
self.emit(ServerManager.Event.RESOURCE, {
message: ServerManager.Event.RESOURCES_REGISTERED,
count: self._nbResources
});
// download the registered resources:
await self._downloadRegisteredResources(newResources);
}
catch (error)
{
console.log('error', error);
// throw { ...response, error: error };
throw Object.assign(response, {error});
}
};
download();
}
/**
* @typedef ServerManager.UploadDataPromise
* @property {string} origin the calling method
* @property {string} context the context
* @property {Object.<string, *>} [error] an error message if we could not upload the data
*/
/**
* Asynchronously upload experiment data to the remote PsychoJS manager.
*
* @name module:core.ServerManager#uploadData
* @function
* @public
* @param {string} key - the data key (e.g. the name of .csv file)
* @param {string} value - the data value (e.g. a string containing the .csv header and records)
* @param {boolean} [sync= false] - whether or not to communicate with the server in a synchronous manner
*
* @returns {Promise<ServerManager.UploadDataPromise>} the response
*/
uploadData(key, value, sync = false)
{
const response = {
origin: 'ServerManager.uploadData',
context: 'when uploading participant\'s results for experiment: ' + this._psychoJS.config.experiment.fullpath
};
this._psychoJS.logger.debug('uploading data for experiment: ' + this._psychoJS.config.experiment.fullpath);
this.setStatus(ServerManager.Status.BUSY);
const url = this._psychoJS.config.pavlovia.URL +
'/api/v2/experiments/' + encodeURIComponent(this._psychoJS.config.experiment.fullpath) +
'/sessions/' + this._psychoJS.config.session.token +
'/results';
// synchronous query the pavlovia server:
if (sync)
{
const formData = new FormData();
formData.append('key', key);
formData.append('value', value);
navigator.sendBeacon(url, formData);
}
// asynchronously query the pavlovia server:
else
{
const self = this;
return new Promise((resolve, reject) =>
{
const data = {
key,
value
};
$.post(url, data, null, 'json')
.done((serverData, textStatus) =>
{
self.setStatus(ServerManager.Status.READY);
resolve(Object.assign(response, {serverData}));
})
.fail((jqXHR, textStatus, errorThrown) =>
{
self.setStatus(ServerManager.Status.ERROR);
const errorMsg = util.getRequestError(jqXHR, textStatus, errorThrown);
console.error('error:', errorMsg);
reject(Object.assign(response, {error: errorMsg}));
});
});
}
}
/**
* Asynchronously upload experiment logs to the remote PsychoJS manager.
*
* @name module:core.ServerManager#uploadLog
* @function
* @public
* @param {string} logs - the base64 encoded, compressed, formatted logs
* @param {boolean} [compressed=false] - whether or not the logs are compressed
* @returns {Promise<ServerManager.UploadDataPromise>} the response
*/
uploadLog(logs, compressed = false)
{
const response = {
origin: 'ServerManager.uploadLog',
context: 'when uploading participant\'s log for experiment: ' + this._psychoJS.config.experiment.fullpath
};
this._psychoJS.logger.debug('uploading server log for experiment: ' + this._psychoJS.config.experiment.fullpath);
this.setStatus(ServerManager.Status.BUSY);
// prepare the POST query:
const info = this.psychoJS.experiment.extraInfo;
const participant = ((typeof info.participant === 'string' && info.participant.length > 0) ? info.participant : 'PARTICIPANT');
const experimentName = (typeof info.expName !== 'undefined') ? info.expName : this.psychoJS.config.experiment.name;
const datetime = ((typeof info.date !== 'undefined') ? info.date : MonotonicClock.getDateStr());
const filename = participant + '_' + experimentName + '_' + datetime + '.log';
const data = {
filename,
logs,
compressed
};
// query the pavlovia server:
const self = this;
return new Promise((resolve, reject) =>
{
const url = self._psychoJS.config.pavlovia.URL +
'/api/v2/experiments/' + encodeURIComponent(self._psychoJS.config.experiment.fullpath) +
'/sessions/' + self._psychoJS.config.session.token +
'/logs';
$.post(url, data, null, 'json')
.done((serverData, textStatus) =>
{
self.setStatus(ServerManager.Status.READY);
resolve(Object.assign(response, {serverData}));
})
.fail((jqXHR, textStatus, errorThrown) =>
{
self.setStatus(ServerManager.Status.ERROR);
const errorMsg = util.getRequestError(jqXHR, textStatus, errorThrown);
console.error('error:', errorMsg);
reject(Object.assign(response, {error: errorMsg}));
});
});
}
/**
* List the resources available to the experiment.
* @name module:core.ServerManager#_listResources
* @function
* @private
*/
_listResources()
{
const response = {
origin: 'ServerManager._listResourcesSession',
context: 'when listing the resources for experiment: ' + this._psychoJS.config.experiment.fullpath
};
this._psychoJS.logger.debug('listing the resources for experiment: ' +
this._psychoJS.config.experiment.fullpath);
this.setStatus(ServerManager.Status.BUSY);
// prepare GET data:
const data = {
'token': this._psychoJS.config.session.token
};
// query pavlovia server:
const self = this;
return new Promise((resolve, reject) =>
{
const url = this._psychoJS.config.pavlovia.URL +
'/api/v2/experiments/' + encodeURIComponent(this._psychoJS.config.experiment.fullpath) +
'/resources';
$.get(url, data, null, 'json')
.done((data, textStatus) =>
{
if (!('resources' in data))
{
self.setStatus(ServerManager.Status.ERROR);
// reject({ ...response, error: 'unexpected answer from server: no resources' });
reject(Object.assign(response, {error: 'unexpected answer from server: no resources'}));
}
if (!('resourceDirectory' in data))
{
self.setStatus(ServerManager.Status.ERROR);
// reject({ ...response, error: 'unexpected answer from server: no resourceDirectory' });
reject(Object.assign(response, {error: 'unexpected answer from server: no resourceDirectory'}));
}
self.setStatus(ServerManager.Status.READY);
// resolve({ ...response, resources: data.resources, resourceDirectory: data.resourceDirectory });
resolve(Object.assign(response, {
resources: data.resources,
resourceDirectory: data.resourceDirectory
}));
})
.fail((jqXHR, textStatus, errorThrown) =>
{
self.setStatus(ServerManager.Status.ERROR);
const errorMsg = util.getRequestError(jqXHR, textStatus, errorThrown);
console.error('error:', errorMsg);
reject(Object.assign(response, {error: errorMsg}));
});
});
}
/**
* Download the resources previously registered.
*
* <p>Note: we use the [preloadjs library]{@link https://www.createjs.com/preloadjs}.</p>
*
* @name module:core.ServerManager#_downloadRegisteredResources
* @function
* @private
*/
_downloadRegisteredResources(resources = new Map())
{
const response = {
origin: 'ServerManager._downloadResources',
context: 'when downloading the resources for experiment: ' + this._psychoJS.config.experiment.name
};
this._psychoJS.logger.debug('downloading the registered resources for experiment: ' + this._psychoJS.config.experiment.name);
this.setStatus(ServerManager.Status.BUSY);
this._nbLoadedResources = 0;
// (*) set-up preload.js:
this._resourceQueue = new createjs.LoadQueue(true); //, this._psychoJS.config.experiment.resourceDirectory);
const self = this;
const filesToDownload = resources.size ? resources : this._resources;
this._resourceQueue.addEventListener("filestart", event =>
{
self.emit(ServerManager.Event.RESOURCE, {
message: ServerManager.Event.DOWNLOADING_RESOURCE,
resource: event.item.id
});
});
this._resourceQueue.addEventListener("fileload", event =>
{
++self._nbLoadedResources;
let path_data = self._resources.get(event.item.id);
path_data.data = event.result;
self.emit(ServerManager.Event.RESOURCE, {
message: ServerManager.Event.RESOURCE_DOWNLOADED,
resource: event.item.id
});
});
// loading completed:
this._resourceQueue.addEventListener("complete", event =>
{
self._resourceQueue.close();
if (self._nbLoadedResources === filesToDownload.size)
{
self.setStatus(ServerManager.Status.READY);
self.emit(ServerManager.Event.RESOURCE, {message: ServerManager.Event.DOWNLOAD_COMPLETED});
}
});
// error: we throw an exception
this._resourceQueue.addEventListener("error", event =>
{
self.setStatus(ServerManager.Status.ERROR);
const resourceId = (typeof event.data !== 'undefined') ? event.data.id : 'UNKNOWN RESOURCE';
// throw { ...response, error: 'unable to download resource: ' + resourceId + ' (' + event.title + ')' };
throw Object.assign(response, {error: 'unable to download resource: ' + resourceId + ' (' + event.title + ')'});
});
// (*) dispatch resources to preload.js or howler.js based on extension:
let manifest = [];
let soundResources = [];
for (const [name, path_data] of filesToDownload)
{
const nameParts = name.toLowerCase().split('.');
const extension = (nameParts.length > 1) ? nameParts.pop() : undefined;
// warn the user if the resource does not have any extension:
if (typeof extension === 'undefined')
{
this.psychoJS.logger.warn(`"${name}" does not appear to have an extension, which may negatively impact its loading. We highly recommend you add an extension.`);
}
// preload.js with forced binary for xls and xlsx:
if (['csv', 'odp', 'xls', 'xlsx'].indexOf(extension) > -1)
{
manifest.push({id: name, src: path_data.path, type: createjs.Types.BINARY});
}/* ascii .csv are adequately handled in binary format
// forced text for .csv:
else if (['csv'].indexOf(resourceExtension) > -1)
manifest.push({ id: resourceName, src: resourceName, type: createjs.Types.TEXT });
*/
// sound files are loaded through howler.js:
else if (['mp3', 'mpeg', 'opus', 'ogg', 'oga', 'wav', 'aac', 'caf', 'm4a', 'weba', 'dolby', 'flac'].indexOf(extension) > -1)
{
soundResources.push(name);
if (extension === 'wav')
{
this.psychoJS.logger.warn(`wav files are not supported by all browsers. We recommend you convert "${name}" to another format, e.g. mp3`);
}
}
// preload.js for the other extensions (download type decided by preload.js):
else
{
manifest.push({id: name, src: path_data.path});
}
}
// (*) start loading non-sound resources:
if (manifest.length > 0)
{
this._resourceQueue.loadManifest(manifest);
}
else
{
if (this._nbLoadedResources === filesToDownload.size)
{
this.setStatus(ServerManager.Status.READY);
this.emit(ServerManager.Event.RESOURCE, {message: ServerManager.Event.DOWNLOAD_COMPLETED});
}
}
// (*) prepare and start loading sound resources:
for (const name of soundResources)
{
self.emit(ServerManager.Event.RESOURCE, {
message: ServerManager.Event.DOWNLOADING_RESOURCE,
resource: name
});
const path_data = self._resources.get(name);
const howl = new Howl({
src: path_data.path,
preload: false,
autoplay: false
});
howl.on('load', (event) =>
{
++self._nbLoadedResources;
path_data.data = howl;
// self._resources.set(resource.name, howl);
self.emit(ServerManager.Event.RESOURCE, {
message: ServerManager.Event.RESOURCE_DOWNLOADED,
resource: name
});
if (self._nbLoadedResources === filesToDownload.size)
{
self.setStatus(ServerManager.Status.READY);
self.emit(ServerManager.Event.RESOURCE, {message: ServerManager.Event.DOWNLOAD_COMPLETED});
}
});
howl.on('loaderror', (id, error) =>
{
// throw { ...response, error: 'unable to download resource: ' + name + ' (' + util.toString(error) + ')' };
throw Object.assign(response, {error: 'unable to download resource: ' + name + ' (' + util.toString(error) + ')'});
});
howl.load();
}
}
}
/**
* Server event
*
* <p>A server event is emitted by the manager to inform its listeners of either a change of status, or of a resource related event (e.g. download started, download is completed).</p>
*
* @name module:core.ServerManager#Event
* @enum {Symbol}
* @readonly
* @public
*/
ServerManager.Event = {
/**
* Event type: resource event
*/
RESOURCE: Symbol.for('RESOURCE'),
/**
* Event: resources all registered
*/
RESOURCES_REGISTERED: Symbol.for('RESOURCES_REGISTERED'),
/**
* Event: resource download has started
*/
DOWNLOADING_RESOURCE: Symbol.for('DOWNLOADING_RESOURCE'),
/**
* Event: resource has been downloaded
*/
RESOURCE_DOWNLOADED: Symbol.for('RESOURCE_DOWNLOADED'),
/**
* Event: resources all downloaded
*/
DOWNLOAD_COMPLETED: Symbol.for('DOWNLOAD_COMPLETED'),
/**
* Event type: status event
*/
STATUS: Symbol.for('STATUS')
};
/**
* Server status
*
* @name module:core.ServerManager#Status
* @enum {Symbol}
* @readonly
* @public
*/
ServerManager.Status = {
/**
* The manager is ready.
*/
READY: Symbol.for('READY'),
/**
* The manager is busy, e.g. it is downloaded resources.
*/
BUSY: Symbol.for('BUSY'),
/**
* The manager has encountered an error, e.g. it was unable to download a resource.
*/
ERROR: Symbol.for('ERROR')
};