How to write code for sorting field in a grid

Assuming you are trying to sort field CustGroup on datasource CustTable:

1) add ComboBox on form, set Name=ComboSortOrder, AutoDeclaration=Yes and EnumType=SortOrder

2) override modified() method on ComboBox form control to call executeQuery() of the datasource

public boolean modified()
{
   boolean ret;
   ;
   ret = super();

   CustTable_ds.executeQuery();

   return ret;
}

3) override executeQuery() method on datasource to change sortorder before actual fetch of records

public void executeQuery()
{;
    CustTable_ds.query().dataSourceNo(1).sortClear();
    CustTable_ds.query().dataSourceNo(1).addSortField(fieldNum(CustTable,
    CustGroup), ComboSortOrder.selection());

   // you can change that to  

    //CustTable_ds.query().dataSourceNo(1).addSortField(fieldNum(CustTable,
    //CustGroup), SortOrder::Ascending); 

    //If you want to display fields in an ascending order. 

    super();
}
 

Notice  

There is a sort method on each control, overriding which you can influence the actual actions

 which happed when you click on the header of a column