how to display animated wiating message while wiating for response

function ExpandAll() {
    var treeviewCell = document.getElementById('cell_treeview');
    var treeview = treeviewCell.childNodes[0];
    if (treeview != null) {
        for (var r = 0; r < treeview.rows.length; r++) {
            var nodeID = treeview.rows[r].id.split('_')[1];
            ExpandBranch(nodeID);
        }
    }
}

function ExpandBranch(nodeID) {
           var data = "<Node Action=\"GetBranch\" ID=\"" + nodeID + "\" />"; //send xmlhttprequest for getting branch
           var xmlDoc = ReadData(SendData(data)); //read the response as xml document
           var nodes = xmlDoc.getElementsByTagName("Node[@HasChilds=\"True\"]");
           for (var n = 0; n < nodes.length; n++) {
               var nodeID = nodes[n].getAttribute("ID");
               if (ShouldBeExpanded(nodeID)) {
                   CollapseExpand(nodeID); //again send xmlhttprequest for getting childs
               }
           }