How to call form level edit method in form datasource level method in extension

Good Morning all,

As i am trying to call form EditQty() method in form datasource Qty().

Example : PurchCopying form .

In this form methods there is one edit method called EditQty().

In this form Datasource - VendInvoiceTrans there is one edit method QTY() which calls form method using Element.EditQty(). That is fine.

But i want to call same Form level method in form datasource extension class using extension class.

1.i created one extension class and mentioned attribute with formdatasourcestr

2.Formdatasource fds = this;

Note : COC is not possible for this method.

Please show some light on this.

Regards.

Have a great day.