Adding sort ability to a field

To add sorting ability to an field on result set on the web_ui I have made changes in parameter method of the attribute and could see sorting option on the display.

  WHEN IF_BSP_WD_MODEL_SETTER_GETTER=>FP_SORTABLE.

       RV_VALUE = 'TRUE'.     

But when I click on ascending or descending there is no change on the display based on the given sort order.

I could see there is no eh_onsort method in the context node class of the view and when I tried creating eh_onsort with the needed logics, its not getting triggered when ever I click on sort on web_ui.

I hope upon getting the sort field and order in eh_onsort I could sort in eh_on sort or do_prepare_output. But I couldn't trigger eh_onsort.

How could I solve this..? TIA

Thanks,

Karthik