service-expressions.md

Symfony Service Expressions: Do things you thought Impossible

Did you know you can do this with Symfony's service container?

# app/config/services.yml
services:
    app_user_manager:
        class: AppBundle\Service\UserManager
        arguments:
            - "@=service('doctrine').getRepository('AppBundle:User')"
            # ... other arguments

The @= means that you're using Symfony's Expression Language, which let's you mix dynamic logic into your normally-static service definitions.

Normally, if you want to inject a repository, you need to register it as a service first, using a factory. And while that's fine (and probably better if you're injecting the factory a lot), using the expression language is well, kinda cool.

In the compiled container, it beautifully generates with exactly the code you would've used directly::

// app/cache/appDevDebugProjectContainer.php
protected function getAppUserManagerService()
{
    return $this->services['app_user_manager'] = new AppBundle\Service\UserManager(
        $this->get('doctrine')->getRepository('AppBundle:User')
    );
}

And from an object-oriented, dependency-injection perspective, that feels good, and makes up for the slightly-wonky service expression syntax. The expression language isn't new, but I think a lot of people don't really know its power.

Passing Database Configuration as a Service Argument

How about reading configuration from the database and passing those as arguments? Normally, you'd need create a service that reads the configuration and inject the whole thing in:

services:
    # some service that can read configuration values from the database
    app_configuration_reader:
        class: AppBundle\Config\ConfigurationReader
        arguments: ["@doctrine.orm_entity_manager"]

    # some service that helps email things
    app_mailer:
        class: AppBundle\Service\Mailer:
        arguments:
            - "@mailer"
            - "@app_configuration_reader"

Suppose our app_mailer service needs some database configuration value called email_from_username, which it uses as the from address when sending emails. To accomplish this, you'd usually need inject the entire app_configuration_reader service (the service you create to read config values). But with expressions, you can inject only what you need:

    services:
        # some service that can read configuration values from the database
        app_configuration_reader:
            class: AppBundle\Config\ConfigurationReader
            arguments: ["@doctrine.orm_entity_manager"]

        # some service that helps email things
        app_mailer:
            class: AppBundle\Service\Mailer:
            arguments:
                - "@mailer"
                - "@=service('app_configuration_reader').get('email_from_username')"

And in addition to the service() function you also have a parameter function, and all the normal syntax (including if statement logic) from the Expression Language. See Using the Expression Language for a few more details about using it with services.

Now, go do something cool with this :).