code: protected final void registerCustomEventCallback(
              final CrawlEvent event,
              final PatternMatchingCallback callback) {
          Validate.notNull(event, "The event cannot be null.");
          Validate.notNull(callback, "The callback cannot be null.");
  
          callbackManager.addCustomEventCallback(event, callback);
      }
docs: Registers an operation which is invoked when the specific event occurs and the provided
  pattern matches the request URL.
  
  @param event    the event for which the callback should be triggered
  @param callback the pattern matching callback to invoke