Registering a new Bag (before) starting the session can be done using a CompilerPass.
https://github.com/rollerworks/RollerworksCacheBundle/blob/master/src/Rollerworks/Bundle/CacheBundle/DependencyInjection/Compiler/SessionPass.php Shows this for example.

But initializing a bag automatically on registering when the session is already started should indeed be possible.

/cc @Drak

@imunhatep What you are suggesting is a code smell.

Bags are initialized only when the session is started so you must instantiate your session objects as you require before hand which can be done as suggested as @sstok suggests or just by altering the initial service definitions.

Remember, bags are not aware of the session so any attempt to make bags do things to the session would mean ugly dependencies which is another clear code smell.

@Drak Where did you found any suggestion from me? I just sad it's not obvious that dynamically created bag is not wired to session! And looking at the code, I do not see any limitation to do so, or at least throw an exception.

@sstok thanks I definitely will take a look at this approach.

I agree with @imunhatep

@Drak Indeed bags are not aware of the session neither they should be, but since the bag registration is invoked from Session's side (Session::registerBag) that should be much possible.

@sstok This approach looks very nice :)

Just fell into this pitfall. I used Session::registerBag then didn't found my bad after a page change or a redirect. I must +1 this as it should register then initialize it... ! Or at least point out that it's not the only thing required to register a bag.

I just ran into this issue, too. It was really annoying, to understand that the session has to be started after the bag was added. Current, I see two options to fix this:

1. Call the initialize() method of the SessionBag from within the registerBag() method of the SessionStorage if the session was already started (similar to the code in loadSession()). Something like this:

       public function registerBag(SessionBagInterface $bag)
       {
           $this->bags[$bag->getName()] = $bag;
   
           if ($this->isStarted()) {
               $session = &$_SESSION;
               $key = $bag->getStorageKey();
               $session[$key] = isset($session[$key]) ? $session[$key] : array();
               $bag->initialize($session[$key]);
           }
       }

   The loadSession() methods accepts an optional session variable. I do not see how and where this parameter is used, so I am not sure what to do here.

2. Forbid adding a bag after the session was started. In this case, an exception should be thrown and the documentation needs to be updated.

I'd prefer the first way. What do you think? I can provide a fix for this, if you agree.

I think we should go with option 2.