code: @Override
      @Synchronized
      public void addListener(ClusterListener listener) {
          Preconditions.checkNotNull(listener, "listener");
          if (!cache.isPresent()) {
              initializeCache();
          }
          cache.get().getListenable().addListener(pathChildrenCacheListener(listener));
      }
docs: Add Listener to the cluster.
  
  @param listener Cluster event Listener.