Method on

third_party/subscriptions-project/swg.js:3984–3988

* @param {!function(new: T)} message * @param {function(?)} callback * @template T

(message, callback)

   * @template T
   */
  on(message, callback) {
    this.getPortPromise_().then((port) => {
      port.on(message, callback);
    });
  }

  /**
   * @param {!../proto/api_messages.Message} request