code: public void addCustomEventCallback(
              final CrawlEvent event,
              final PatternMatchingCallback callback) {
          customCallbacks.computeIfAbsent(event, key -> new ArrayList<>()).add(callback);
      }
docs: Associates a pattern matching callback with the specific event.
  
  @param event    the event for which the callback should be invoked
  @param callback the pattern matching callback to invoke