Passing apex page items via button click

Question

I'm baffled by this and hoping someone can explain this behavior to me.

I have a sample apex app here https://apex.oracle.com/pls/apex/f?p=126734:2 (admin credentials: username: guest pw: cubsarechamps).

I just want to enter a value on page 2 in text field P2_SOURCE and then click the button and be redirected to page 3 and have the value from P2_SOURCE populate P3_TARGET. I know that I need to set the value of P2_SOURCE in session and so I created a 'lose focus' dynamic action with a plsql process of null that also submits P2_SOURCE.

After I enter '123' in P2_SOURCE I tab out of the field to trigger the DA, and then I click session in my developer toolbar and I can see that the value of P2_SOURCE is indeed '123'. Then I click my button to go to page 3 but nothing is passed to P3_TARGET (unless I actually submit my page first, which I shouldn't have to do, right?).

So my question is why P2_SOURCE isn't being passed to P3_TARGET. The item is set in session state so why is &P2_SOURCE. passing a null value when there is a value set in session? Can I not use page item substitutions unless I actually submit my page? The documentation seems to indicate this is supported functionality.

My button setup is like this:

Answer 1

Substitution Strings are evaluated when the page is loaded and so, it is NULL in your case as the page item contains no value in the beginning. I would either use Dynamic Action with JavaScript Expression or Branch Process.