code: private void putDefaultCallback(CallbackType callbackType, CallbackMetadata metadata) {
      List<CallbackMetadata> metadataList = globalCallbacks.get(callbackType);
      if (metadataList == null) {
        metadataList = new ArrayList<>();
        globalCallbacks.put(callbackType, metadataList);
      }
      metadataList.add(metadata);
    }
docs: Puts/adds the given callback type and its metadata to the list of default listeners.
  
  @param callbackType
  the event type
  @param metadata
  the callback metadata