[DependencyInjection] Drupal needs ability to register parameterized fallback service definitions

[effulgentsia]

Hi. I'm one of Drupal 8's core developers exploring how we can best use Symfony's ContainerBuilder. In general, I really like it.

One thing I'd like to use it for is to replace our current cache() function. We have other similar use-cases, but this one demonstrates the essentials. The key feature needed here is the ability to return a different service object for each "bin" without needing to define every bin individually.

Here's some work in progress code that demonstrates the idea I have in mind:
https://gist.github.com/2358766

While we can add the above class to Drupal, I'm wondering two things:

1. Is this something that's of interest to Symfony, and if so, is it possible for it to make it into 2.1 (I'm happy to spend time refining it based on any feedback)?
2. If not of interest to Symfony, do you think it's a flawed approach in general, and if so, do you have suggestions for another approach we should consider?

[lsmith77]

you can probably cover your use case with aliases.
note that aliases are only supported for services.

[effulgentsia]

Thanks for the reply. I don't see how aliases help. ContainerBuilder::get() has this code:

if (!$this->hasDefinition($id) && isset($this->aliases[$id])) {
return $this->get($this->aliases[$id]);
}

So getting an alias would return the same $service instance as the original. The key to my use-case is wanting a definition to serve as a template. I see that the Definition class has isPublic() and isAbstract() methods, but I don't see ContainerBuilder using that information in any way. Is there any code or documentation that you can point me to that shows how to do https://gist.github.com/2358766 without having to extend ContainerBuilder?

[lsmith77]

so you can define a service as abstract and then decorate it:
https://github.com/doctrine/DoctrineBundle/blob/master/Resources/config/dbal.xml#L46
https://github.com/doctrine/DoctrineBundle/blob/master/DependencyInjection/DoctrineExtension.php#L97

this is how you define a template service and then configure it with concrete configs.

[stof]

@lsmith77 if I understand properly, he wants to have the fallback being resolved at runtime.
However, I don't think it is a good idea as it would mean adding a bunch of code in the runtime Container to do this, and would require disabling all container optimizations (as they would complain about the missing services).
I would recommend using a compiler pass instead. A good example is probably the one I wrote for MonologBundle as it achieves a similar feature (the way to use things is not the same than your gist but I guess your syntax is not enforced by keeping BC anyway 😄)

[lsmith77]

Right .. there is no sense in doing it at runtime, which is why I initially suggested using an alias instead and yes this alias would likely best be set inside a compiler pass.

However given his reply asking for a template, this is what abstract services and decorators are for.

[effulgentsia]

Thanks guys. You answered my initial question that this type of runtime fallback is not of interest to Symfony. I'm gonna try to figure out the code you linked to (thanks for those links), and how we want to adapt it for Drupal. We're still in the early stages of refactoring Drupal for dependency injection, and haven't gotten to compilation at all yet.