title: Programmatically refresh a Gwt CellTree
body: <p>I want to fire the "open root node" event on my current working CellTree, which now has the following behaviour: </p>
  
  <pre><code>@Override
      public &lt;T&gt; NodeInfo&lt;?&gt; getNodeInfo(final T value) {
          return new DefaultNodeInfo&lt;Categoria&gt;(
                  (value instanceof Categoria) ? 
                          createBranchDataProvider((Categoria)value) :
                          rootDataProvider, 
                  new CategoriaCell()
          );
      }
  
  private AsyncDataProvider&lt;Categoria&gt; createRootDataProvider() {
          AsyncDataProvider&lt;Categoria&gt; dataProvider = new AsyncDataProvider&lt;Categoria&gt;() {
              @Override
              protected void onRangeChanged(HasData&lt;Categoria&gt; display) {
                  AsyncCallback&lt;Categoria[]&gt; cb = new AsyncCallback&lt;Categoria[]&gt;() {
                      @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;
      }
  </code></pre>
  
  <p>How can I fire that "onRangeChanged" event, to refresh my level-1 nodes?</p>
  
  <p>What is my convenience method missing? </p>
  
  <pre><code>private void updateTree() {     
          TreeNode rootTreeNode = cellTree.getRootTreeNode();
          for (int i = 0; i &lt; rootTreeNode.getChildCount(); i++) {
              rootTreeNode.setChildOpen(i, false);
          }
          // HOW TO REFRESH LEVEL-1 NODES?
      }
  </code></pre>