code: public synchronized void register(@NonNull EventHandlerWrapper eventHandler) {
          String type = eventHandler.type;
          List<EventHandlerWrapper> handlers = subscribers.get(eventHandler.type);
          if (handlers == null) {
              handlers = new ArrayList<>();
              subscribers.put(type, handlers);
          }
          handlers.add(eventHandler);
      }
docs: Register an event subscriber which is wrapped by {@link EventHandlerWrapper}
  @param eventHandler event subscriber wrapper