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How to work with Scopes

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How to work with Scopes

This entry is all about scopes, a somewhat advanced topic related to the Service Container. If you've ever gotten an error mentioning "scopes" when creating services, or need to create a service that depends on the request service, then this entry is for you.

Understanding Scopes

The scope of a service controls how long an instance of a service is used by the container. The DependencyInjection component provides two generic scopes:

  • container (the default one): The same instance is used each time you request it from this container.
  • prototype: A new instance is created each time you request the service.

The FrameworkBundle also defines a third scope: request. This scope is tied to the request, meaning a new instance is created for each subrequest and is unavailable outside the request (for instance in the CLI).

Scopes add a constraint on the dependencies of a service: a service cannot depend on services from a narrower scope. For example, if you create a generic my_foo service, but try to inject the request component, you'll receive a ScopeWideningInjectionException when compiling the container. Read the sidebar below for more details.

Scopes and Dependencies

Imagine you've configured a my_mailer service. You haven't configured the scope of the service, so it defaults to container. In other words, every time you ask the container for the my_mailer service, you get the same object back. This is usually how you want your services to work.

Imagine, however, that you need the request service in your my_mailer service, maybe because you're reading the URL of the current request. So, you add it as a constructor argument. Let's look at why this presents a problem:

  • When requesting my_mailer, an instance of my_mailer (let's call it MailerA) is created and the request service (let's call it RequestA) is passed to it. Life is good!

  • You've now made a subrequest in Symfony, which is a fancy way of saying that you've called, for example, the {{ render(...) }} Twig function, which executes another controller. Internally, the old request service (RequestA) is actually replaced by a new request instance (RequestB). This happens in the background, and it's totally normal.

  • In your embedded controller, you once again ask for the my_mailer service. Since your service is in the container scope, the same instance (MailerA) is just re-used. But here's the problem: the MailerA instance still contains the old RequestA object, which is now not the correct request object to have (RequestB is now the current request service). This is subtle, but the mis-match could cause major problems, which is why it's not allowed.

    So, that's the reason why scopes exist, and how they can cause problems. Keep reading to find out the common solutions.

Note

A service can of course depend on a service from a wider scope without any issue.

Setting the Scope in the Definition

The scope of a service is set in the definition of the service:

  • YAML
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    # src/Acme/HelloBundle/Resources/config/services.yml
services:
    greeting_card_manager:
        class: Acme\HelloBundle\Mail\GreetingCardManager
        scope: request
  • XML
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    <!-- src/Acme/HelloBundle/Resources/config/services.xml -->
<services>
    <service id="greeting_card_manager" class="Acme\HelloBundle\Mail\GreetingCardManager" scope="request" />
</services>
  • PHP
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    // src/Acme/HelloBundle/Resources/config/services.php
use Symfony\Component\DependencyInjection\Definition;

$container->setDefinition(
    'greeting_card_manager',
    new Definition('Acme\HelloBundle\Mail\GreetingCardManager')
)->setScope('request');

If you don't specify the scope, it defaults to container, which is what you want most of the time. Unless your service depends on another service that's scoped to a narrower scope (most commonly, the request service), you probably don't need to set the scope.

Using a Service from a narrower Scope

If your service depends on a scoped service, the best solution is to put it in the same scope (or a narrower one). Usually, this means putting your new service in the request scope.

But this is not always possible (for instance, a Twig extension must be in the container scope as the Twig environment needs it as a dependency). In these cases, you should pass the entire container into your service and retrieve your dependency from the container each time you need it to be sure you have the right instance:

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// src/Acme/HelloBundle/Mail/Mailer.php
namespace Acme\HelloBundle\Mail;

use Symfony\Component\DependencyInjection\ContainerInterface;

class Mailer
{
    protected $container;

    public function __construct(ContainerInterface $container)
    {
        $this->container = $container;
    }

    public function sendEmail()
    {
        $request = $this->container->get('request');
        // ... do something using the request here
    }
}

Caution

Take care not to store the request in a property of the object for a future call of the service as it would cause the same issue described in the first section (except that Symfony cannot detect that you are wrong).

The service config for this class would look something like this:

  • YAML
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    # src/Acme/HelloBundle/Resources/config/services.yml
parameters:
    # ...
    my_mailer.class: Acme\HelloBundle\Mail\Mailer
services:
    my_mailer:
        class:     "%my_mailer.class%"
        arguments:
            - "@service_container"
        # scope: container can be omitted as it is the default
  • XML
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    <!-- src/Acme/HelloBundle/Resources/config/services.xml -->
<parameters>
    <!-- ... -->
    <parameter key="my_mailer.class">Acme\HelloBundle\Mail\Mailer</parameter>
</parameters>

<services>
    <service id="my_mailer" class="%my_mailer.class%">
         <argument type="service" id="service_container" />
    </service>
</services>
  • PHP
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    // src/Acme/HelloBundle/Resources/config/services.php
use Symfony\Component\DependencyInjection\Definition;
use Symfony\Component\DependencyInjection\Reference;

// ...
$container->setParameter('my_mailer.class', 'Acme\HelloBundle\Mail\Mailer');

$container->setDefinition('my_mailer', new Definition(
    '%my_mailer.class%',
    array(new Reference('service_container'))
));

Note

Injecting the whole container into a service is generally not a good idea (only inject what you need). In some rare cases, it's necessary when you have a service in the container scope that needs a service in the request scope.

If you define a controller as a service then you can get the Request object without injecting the container by having it passed in as an argument of your action method. See The Request as a Controller Argument for details.

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.