onBeforeSorting

fires before data sorting is started

void onBeforeSorting(number ind,string type,string direction);

ind	number	the index of a column
type	string	the sorting type ("str","int","date")
direction	string	the direction of sorting ("asc","des")

Available only in PRO Edition

grid.attachEvent("onBeforeSorting", function(ind,type,direction){
    //your code here
    return false;
});