function OnServer($params,$result,$keys) $keys is

I have added a button to an Edit Page. The default code runs fine. According to the documentation of the onServer method:
Description

PHP code executed on the server side.
Parameters

$params - array of parameters sent from the client

$result - array of values to return from the server

$keys - array of keys values for selected records on the List page and key values

for view and edit pages
However when the $keys array is empty, count($keys); always returns 0. How do I access the values of the row being edited in the onServer method?