The getFlashBag() Issue

One constantly recurring issue, is the missing method of getFlashBag() in the SessionInterface. This leads to being unable to get the flash bag in a clean way.

• Injection the SessionInterface is better than Session, but no getFlashBag() available.
• You can get the Session from the Request, but officially this is a SessionInterface, so no getFlashBag().
• You can inject the FlashBagInterface, but this service is private and in theory shouldn't be used like this afaik.

Just to name a few issues and pull requests:

• adding getFlashBag to SessionInterface in later 3.1.x, or 3.2.x ? #20258 adding getFlashBag to SessionInterface in later 3.1.x, or 3.2.x ?
• [Session] Changed session.flash_bag service to publicly available #11957 [Session] Changed session.flash_bag service to publicly available
• [Session] "getFlashBag" interfacing issue #11279 [Session] "getFlashBag" interfacing issue
• Update SessionInterface.php #11035 Update SessionInterface.php
• Update Request.php #11032 Update Request.php
• getFlashBag missing in SessionInterface #10036 getFlashBag missing in SessionInterface
• [HttpFoundation] [Session] SessionInterface is missing "getFlashBag" method: correct? #5568 [HttpFoundation] [Session] SessionInterface is missing "getFlashBag" method: correct?

Possible Solution

There are 2 solutions that might be worth looking into now that we have ArgumentValueResolvers. Either the FlashBag or Session could be requested via the action.

class fooController
{
    public function barAction(Session $session)
    {
        $flashBag = $session->getFlashBag(); // this is now valid
    }

    public function bazAction(FlashBag $session)
    {
        // still need to do $request->getSession(); sadly
    }
}

Personally I'm leaning towards the Session type-hint. I think this would be a nice addition as the session has no real business in the Request object, but interface wise still is stateful (hence the no-constructor injection).