work.suroh.tk/node_modules/engine.io-client/lib/transports/polling.js

/**
 * Module dependencies.
 */

var Transport = require('../transport');
var parseqs = require('parseqs');
var parser = require('engine.io-parser');
var inherit = require('component-inherit');
var yeast = require('yeast');
var debug = require('debug')('engine.io-client:polling');

/**
 * Module exports.
 */

module.exports = Polling;

/**
 * Is XHR2 supported?
 */

var hasXHR2 = (function () {
  var XMLHttpRequest = require('xmlhttprequest-ssl');
  var xhr = new XMLHttpRequest({ xdomain: false });
  return null != xhr.responseType;
})();

/**
 * Polling interface.
 *
 * @param {Object} opts
 * @api private
 */

function Polling (opts) {
  var forceBase64 = (opts && opts.forceBase64);
  if (!hasXHR2 || forceBase64) {
    this.supportsBinary = false;
  }
  Transport.call(this, opts);
}

/**
 * Inherits from Transport.
 */

inherit(Polling, Transport);

/**
 * Transport name.
 */

Polling.prototype.name = 'polling';

/**
 * Opens the socket (triggers polling). We write a PING message to determine
 * when the transport is open.
 *
 * @api private
 */

Polling.prototype.doOpen = function () {
  this.poll();
};

/**
 * Pauses polling.
 *
 * @param {Function} callback upon buffers are flushed and transport is paused
 * @api private
 */

Polling.prototype.pause = function (onPause) {
  var self = this;

  this.readyState = 'pausing';

  function pause () {
    debug('paused');
    self.readyState = 'paused';
    onPause();
  }

  if (this.polling || !this.writable) {
    var total = 0;

    if (this.polling) {
      debug('we are currently polling - waiting to pause');
      total++;
      this.once('pollComplete', function () {
        debug('pre-pause polling complete');
        --total || pause();
      });
    }

    if (!this.writable) {
      debug('we are currently writing - waiting to pause');
      total++;
      this.once('drain', function () {
        debug('pre-pause writing complete');
        --total || pause();
      });
    }
  } else {
    pause();
  }
};

/**
 * Starts polling cycle.
 *
 * @api public
 */

Polling.prototype.poll = function () {
  debug('polling');
  this.polling = true;
  this.doPoll();
  this.emit('poll');
};

/**
 * Overloads onData to detect payloads.
 *
 * @api private
 */

Polling.prototype.onData = function (data) {
  var self = this;
  debug('polling got data %s', data);
  var callback = function (packet, index, total) {
    // if its the first message we consider the transport open
    if ('opening' === self.readyState) {
      self.onOpen();
    }

    // if its a close packet, we close the ongoing requests
    if ('close' === packet.type) {
      self.onClose();
      return false;
    }

    // otherwise bypass onData and handle the message
    self.onPacket(packet);
  };

  // decode payload
  parser.decodePayload(data, this.socket.binaryType, callback);

  // if an event did not trigger closing
  if ('closed' !== this.readyState) {
    // if we got data we're not polling
    this.polling = false;
    this.emit('pollComplete');

    if ('open' === this.readyState) {
      this.poll();
    } else {
      debug('ignoring poll - transport state "%s"', this.readyState);
    }
  }
};

/**
 * For polling, send a close packet.
 *
 * @api private
 */

Polling.prototype.doClose = function () {
  var self = this;

  function close () {
    debug('writing close packet');
    self.write([{ type: 'close' }]);
  }

  if ('open' === this.readyState) {
    debug('transport open - closing');
    close();
  } else {
    // in case we're trying to close while
    // handshaking is in progress (GH-164)
    debug('transport not open - deferring close');
    this.once('open', close);
  }
};

/**
 * Writes a packets payload.
 *
 * @param {Array} data packets
 * @param {Function} drain callback
 * @api private
 */

Polling.prototype.write = function (packets) {
  var self = this;
  this.writable = false;
  var callbackfn = function () {
    self.writable = true;
    self.emit('drain');
  };

  parser.encodePayload(packets, this.supportsBinary, function (data) {
    self.doWrite(data, callbackfn);
  });
};

/**
 * Generates uri for connection.
 *
 * @api private
 */

Polling.prototype.uri = function () {
  var query = this.query || {};
  var schema = this.secure ? 'https' : 'http';
  var port = '';

  // cache busting is forced
  if (false !== this.timestampRequests) {
    query[this.timestampParam] = yeast();
  }

  if (!this.supportsBinary && !query.sid) {
    query.b64 = 1;
  }

  query = parseqs.encode(query);

  // avoid port if default for schema
  if (this.port && (('https' === schema && Number(this.port) !== 443) ||
     ('http' === schema && Number(this.port) !== 80))) {
    port = ':' + this.port;
  }

  // prepend ? to query
  if (query.length) {
    query = '?' + query;
  }

  var ipv6 = this.hostname.indexOf(':') !== -1;
  return schema + '://' + (ipv6 ? '[' + this.hostname + ']' : this.hostname) + port + this.path + query;
};