Client side event for a custom button in a grid view column, It get which button (if there is more than one) is clicked and send it to a java script function.
<ClientSideEvents CustomButtonClick="function(s, e) {
if (e.buttonID == 'gvColCmdUpload') {
onColAttButtonClick(e.visibleIndex);
}
else if(e.buttonID == 'gvColCmdView')
{
onColAttViewButtonClick(e.visibleIndex);
}
}" />