Application Express :: Interactive Report  Count Selected Rows?

I have a I/R report with a checkbox is column 1 of each row using:

HTMLDB_ITEM.CHECKBOX
(1,R.RESEARCH_ID,
decode(:Pxx_SELECT_ALL_ROWS,'Y','CHECKED',
decode(instr(nvl(:Pxx_SELECTED_IDS,0),R.RESEARCH_ID),0,NULL,'CHECKED'))
)
AS select_flag

I want to use Javscript or a Dynamic Action to "count" how many selected rows there are, when a UPDATE button is pressed. Right now, the UPDATE button "submits" the page, then a page item computation runs the following code to get a count:

DECLARE
v_cnt number(9);
BEGIN
v_cnt := 0;
FOR i in 1..htmldb_application.g_f01.count
LOOP
v_cnt := v_cnt + 1;
END LOOP;
return v_cnt;

This works... but I dont want to submit (and rerun the report) just to get this count.

I have copied this loop into a Dynamic Action using the Click action of the UPDATE button, but it doesnt seem to recognize htmldb_application.g_f01.count .. so my value returns as 0.