Method registerCallback

3p/iframe-messaging-client.js:105–110

* Register callback function for message with type messageType. * @param {string} messageType The type of the message. * @param {function(?JsonObject)} callback The callback function to call * when a message with type messageType is received. * @return {function()}

(messageType, callback)

   * @return {function()}
   */
  registerCallback(messageType, callback) {
    // NOTE : no validation done here. any callback can be register
    // for any callback, and then if that message is received, this
    // class *will execute* that callback
    return this.getOrCreateObservableFor_(messageType).add(callback);
  }

  /**
   * Send a postMessage to Host Window, or all parent windows if host window is