Update primefaces datatable from actionListener

Question

I have a primefaces datatable, where I added an empty line to the end. I would like to add a new empty line automatically when the last line was modified. I put the logic to the onRowEdit listener, but the datatable is not updated with the new empty line. (with a separate commandbutton it works, but I would like to do it automatically)

<p:dataTable id="categoryTable" var="category" value="#{categoryController.categories}" editable="true">

  <p:ajax event="rowEdit" listener="#{categoryController.onRowEdit}" />
  <p:ajax event="rowEditCancel" listener="#{categoryController.onRowCancel}" />

  <p:column headerText="Category" sortBy="#{category.name}">
    <p:cellEditor>
       <f:facet name="output">
         <h:outputText value="#{category.name}" />
       </f:facet>
       <f:facet name="input">
          <p:inputText id="categoryName" value="#{category.name}" style="width:96%" />
       </f:facet>
     </p:cellEditor>
   </p:column>
</p:dataTable>

.

@Named("categoryController")
@ViewScoped
public class CategoryController {
  private List<Category> categories;
  @EJB
  CategoryEjb categoryEjb;


  @PostConstruct
  public void init() {

    categories = categoryEjb.getAllCategory();
    //add new row
    categories.add(new Category());
  }

  public void onRowEdit(RowEditEvent event) {
    Category category = (Category) event.getObject();
    if (categories.indexOf(category) == categories.size()-1) {

      categoryEjb.addCategory(category);
      //add new row
      categories.add(new Category());
    } else {
      categoryDao.editCategory(category);
    }
  }

 ...
}

I tried to add update="categoryForm:categoryTable" to <p:ajax event="rowEdit"> or

RequestContext context = RequestContext.getCurrentInstance();
context.update("categoryForm:categoryTable");

to the onRowEdit listener without success.