[DI] New container.service_subscriber & ServiceSubscriberInterface

[weaverryan]

See symfony/symfony#21708

[weaverryan]

@chalasr This is pretty closely related to #7458, right? Would it make sense to put them all in the same article?

[chalasr]

@weaverryan This is a whole concept (built on top of #7458) allowing to request some entries from the DIC at the PHP level. I think it deserves its own article, the need for this feature should be different that the one for #7458 (right now the class must know which services they want exactly, #7458 is more about collecting tagged services).

@nicolas-grekas pov would be good!

[nicolas-grekas]

It's like event listeners vs subscribers.
Personally, I'd advocate to promote subscribers over listeners, for events and for services.

[chalasr]

I agree and it's already the case for events, that doesn't make listeners useless though. But creating locators by hand remain the way to go for collecting services for a given tag, that is especially useful for third party libs/bundles and the core. Opened symfony/symfony#22649 for a related proposal.

[codedmonkey]

I've created a concept page in #9223. It still needs some improvements though, especially properly describing getSubscribedServices(). It's probably also worth mentioning on the page that you're probably already using it through AbstractController. Any thoughts?