extjs4.0 treePanel中如何用ajax动态更新指定节点的图标

如题

Ext.tree.TreeNodeUI = function(node){
  this.node = node;
  this.rendered = false;
  this.animating = false;
  this.wasLeaf = true;
  this.ecc = 'x-tree-ec-icon x-tree-elbow';
  this.emptyIcon = Ext.BLANK_IMAGE_URL;
};

这是它的源码。node.getUI()获取UI后，看下是否可以修改emptyIcon 的值。