code: public synchronized EventBus once(Class<? extends EventObject> eventType, OnceEventListenerBase listener) {
          if (null != onceBus) {
              onceBus.bind(eventType, listener);
          } else {
              bind(eventType, listener);
          }
          return this;
      }
docs: Bind an {@link OnceEventListenerBase once event listener} to an {@link EventObject event object type}
  
  @param eventType
  the event object type
  @param listener
  the listener
  @return
  this event bus instance