how to enable grid geader sorting context menu and catch the click event on it ?

hi,

I have a grid panel and there is a header context menu, but the sorting menu gets disabled.
ow can I enable the sorting ASC/DES menu and handle the clicking event on it ?

besides, the sorting is on grid's datastore ?

here is the code for the grid.
///////////////////////////////////////////////////////////////////

function snpGrid_universal (divId, JsonData)
{

var len = JsonData.col_headers.length;

var dataStore = new Ext.data.Store ({
reader: new Ext.data.JsonReader ({
root: 'snpRows',
totalProperty: 'totalSnp',
id: 'snpId'
},
JsonData.snpRows_index

)

});
dataStore.loadData (JsonData);

var colModelArray = [];
for (var i=0; i < len; i++){
colModelArray.push(
{header: JsonData.col_headers[i], dataIndex: JsonData.snpRows_index[i], tooltip: JsonData.col_header_desc[i], width: JsonData.col_widths[i], align:JsonData.col_aligns[i], sortable: true }
);
}

var columnModel = new Ext.grid.ColumnModel (colModelArray );

var snpTable = new Ext.grid.GridPanel ({
ds: dataStore,
cm: columnModel,
id: divId,
applyTo: divId,
height: 140,
width: 720,
enableHdMenu: true,
loadMask: true

});
snpTable.render;
// hide sort menus
snpTable.view.hmenu.on('beforeshow',function() {
//snpTable.view.hmenu.items.get(0).hide();
//snpTable.view.hmenu.items.get(1).hide();
snpTable.view.hmenu.items.get(2).hide();
//var colsubMenu = snpTable.view.hmenu.items.get(3);
//if(colsubMenu)
// colsubMenu.???;
});

}

//////////////////////////////////////////////////////////////////////