easyui combotree 禁止选中父节点

$('#userid').combotree({
			url : '${basePath}role/getRoleUser.htm',
			parentField : 'parent_id',
			cascadeCheck:false,
			multiple:true,
			panelHeight : 180,
			formatter : function(node) {
				node.text = node.name;
				return node.name;
			}, 
			//只能选择子节点
			onBeforeCheck: function (node) {
                if (!$("#userid").combotree("tree").tree('isLeaf', node.target)) {
                    return false;
                }
            }
		});

关键代码

//只能选择子节点
onBeforeCheck: function (node) {
   if (!$("#userid").combotree("tree").tree('isLeaf', node.target)) {
        return false;
     }
 }