checkall/uncheck all in gridview

i have got two issues

1) check all checkboxes with javascript like yahoo

2) check all check boxes on button click



now i am able to do the 1st part but i am not able to uncheck headertemplate chekbox when any of the checkbox in itemtemplate is check like yahoo

here is my code

function check_uncheck(Val)
{
var ValChecked = Val.checked;
var ValId = Val.id;
var frm = document.MainForm;
// Loop through all elements
for (i = 0; i < frm.elements.length; i++)
{
// Look for Header Template's Checkbox
//As we have not other control other than checkbox we just check following statement
if (this != null)
{
if (ValId.indexOf('chkSelectAll') != - 1)
{
// Check if main checkbox is checked,
// then select or deselect datagrid checkboxes
if (ValChecked)
frm.elements[i].checked = true;
else
frm.elements[i].checked = false; Go to the complete details ...