Method then

src/motion.js:178–183

* Chains to the motion's promise that will resolve when the motion has * completed or will reject if motion has failed or was interrupted. * @override

(opt_resolve, opt_reject)

   * @override
   */
  then(opt_resolve, opt_reject) {
    if (!opt_resolve && !opt_reject) {
      return this.promise_;
    }
    return this.promise_.then(opt_resolve, opt_reject);
  }

  /**
   * Callback for regardless whether the motion succeeds or fails.