discourse/app/assets/javascripts/discourse/initializers/message-bus.js.es6

// Initialize the message bus to receive messages.
import pageVisible from 'discourse/lib/page-visible';

export default {
  name: "message-bus",
  after: 'inject-objects',

  initialize(container) {
    // We don't use the message bus in testing
    if (Discourse.testing) { return; }

    const messageBus = container.lookup('message-bus:main'),
      user = container.lookup('current-user:main'),
      siteSettings = container.lookup('site-settings:main');

    messageBus.alwaysLongPoll = Discourse.Environment === "development";

    // we do not want to start anything till document is complete
    messageBus.stop();
    // jQuery ready is called on "interactive" we want "complete"
    // Possibly change to document.addEventListener('readystatechange',...
    // but would only stop a handful of interval, message bus being delayed by
    // 500ms on load is fine. stuff that needs to catch up correctly should
    // pass in a position
    const interval = setInterval(()=>{
      if (document.readyState === "complete") {
        clearInterval(interval);
        messageBus.start();
      }
    },500);

    messageBus.callbackInterval = siteSettings.anon_polling_interval;
    messageBus.backgroundCallbackInterval = siteSettings.background_polling_interval;
    messageBus.baseUrl = siteSettings.long_polling_base_url.replace(/\/$/, '') + '/';

    if (messageBus.baseUrl !== '/') {
      // zepto compatible, 1 param only
      messageBus.ajax = function(opts) {
        opts.headers = opts.headers || {};
        opts.headers['X-Shared-Session-Key'] = $('meta[name=shared_session_key]').attr('content');
        if (pageVisible()) {
          opts.headers['Discourse-Visible'] = "true";
        }
        return $.ajax(opts);
      };
    } else {

      messageBus.ajax = function(opts) {
        opts.headers = opts.headers || {};
        if (pageVisible()) {
          opts.headers['Discourse-Visible'] = "true";
        }
        return $.ajax(opts);
      };

      messageBus.baseUrl = Discourse.getURL('/');
    }

    if (user) {
      messageBus.callbackInterval = siteSettings.polling_interval;
      messageBus.enableLongPolling = true;
    }
  }
};