Programmatically refresh a Gwt CellTree

I want to fire the "open root node" event on my current working CellTree, which now has the following behaviour:

@Override
    public <T> NodeInfo<?> getNodeInfo(final T value) {
        return new DefaultNodeInfo<Categoria>(
                (value instanceof Categoria) ? 
                        createBranchDataProvider((Categoria)value) :
                        rootDataProvider, 
                new CategoriaCell()
        );
    }

private AsyncDataProvider<Categoria> createRootDataProvider() {
        AsyncDataProvider<Categoria> dataProvider = new AsyncDataProvider<Categoria>() {
            @Override
            protected void onRangeChanged(HasData<Categoria> display) {
                AsyncCallback<Categoria[]> cb = new AsyncCallback<Categoria[]>() {
                    @Override
                    public void onSuccess(Categoria[] result) {
                        updateRowCount(result.length, true);
                        updateRowData(0, Arrays.asList(result));
                    }
                    @Override
                    public void onFailure(Throwable caught) {
                        Window.alert(caught.toString());
                    }
                };  
                rpcService.getCategorie(cb);
            }
        };
        return dataProvider;
    }

How can I fire that "onRangeChanged" event, to refresh my level-1 nodes?

What is my convenience method missing?

private void updateTree() {     
        TreeNode rootTreeNode = cellTree.getRootTreeNode();
        for (int i = 0; i < rootTreeNode.getChildCount(); i++) {
            rootTreeNode.setChildOpen(i, false);
        }
        // HOW TO REFRESH LEVEL-1 NODES?
    }

The Level-1 nodes (I suppose you the mean below the root node) can not be refreshed the way you are doing it.

You have to store the instance of your dataProvider for the level-1 nodes somewhere. Later when you refresh your list you have to update your stored dataProvider for your level-1 nodes.
The nodes below the level-1 can be refreshed the way you are doing it. Because as soon as you close the level 1 nodes (that is what you are doing in the updateTree method) and the next time you open it getNodeInfo will be called and the updated Subcategories will be retrieved and displayed in the CellTree.

UPDATE

For refreshing the CellWidgets which is attached to AsyncDataProvider you will probably have to extend the AsyncDataProvider and either extract the RPC call to a getData() method which is called in the onRangeChanged() method or create an interface with a refresh method and implement it in your custom AsyncDataProvider which calls the protected onRangeChanged() method.