code: public void addLoggerEventListener(final LoggerEventListener listener) {
      synchronized (loggerEventListeners) {
        if (loggerEventListeners.get(listener) != null) {
          LogLog.warn(
           "Ignoring attempt to add a previously registerd LoggerEventListener.");
        } else {
          HierarchyEventListenerProxy proxy =
                  new HierarchyEventListenerProxy(listener);
          loggerEventListeners.put(listener, proxy);
          repo.addHierarchyEventListener(proxy);
        }
      }
    }
docs: Add a {@link LoggerEventListener} to the repository. The  listener
  will be called when repository events occur.
  @param listener listener