GenEvent.handle_call

You're seeing just the callback handle_call, go back to GenEvent module for more information.
Link to this callback

handle_call(request, state)

View Source

Specs

handle_call(request :: term(), state :: term()) ::
  {:ok, reply, new_state}
  | {:ok, reply, new_state, :hibernate}
  | {:remove_handler, reply}
when reply: term(), new_state: term()