Backingfile:prerender() and action method in page flow

Question

I am using backing file in a pageflow based portlet in WLP8.1sp5, and implemented its prerender method. After navigating to the middle of this pageflow, the session became expired after 15 mins of inactivity (this is expected). However, when a user click an action button in the browser which sent the request to the stale session, I expected my backing file prerender is invoked and I can check for expired session and route user to a timeout page. But this didn't happen. Instead, I see an ugly error page complaining that action couldn't be found in the pageflow. Of course, the session is no longer there.
My questions are:
1. Why WLP framework behave this way? Why backing wasn't called prior to action method of the pageflow?
2. In WLP (8.1sp5), is there a pattern to handle expired session and render/display a "session timed out" page.
Thanks in advance!

Answer 1

I tried it. However, I encountered two problems:
1. I placed the backing file at desktop level. And the page backing contexts I got back are null:
try {   
DesktopBackingContext ctx = DesktopBackingContext.getDesktopBackingContext(request);
BookBackingContext bbc = ctx.getBookBackingContext();
List children = bbc.getPageBackingContexts();
for (Iterator i = children.iterator(); i.hasNext();)
PageBackingContext pbc = (PageBackingContext) i.next();
String label = pbc.getLabel();
boolean bv = lableInArrayList(label, al);
pbc.setVisible(bv);
pbc.setActive(bv);
} catch (Exception e) {
where list "children" is returned as null, thus the null pointer exception when I tried to get an iterator.
How come?
2. When using SPNEGO single sign on, there is no straightforward and sure way to tell whether the request is (a) first time hit (i.e. session is new) or (b) is a hit after the session expires (i.e. the session is new again but I lost all pageflow state etc.). I could use request.getQueryString() as an indicator for (b) because the query string wouldn't be null vs. first hit http://.../abc.portal (without any query string). My question is: is this method request.getQueryString() a reliable way to indicate (b)?
Thanks in advance!