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 ContainerAwareHttpKernel 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 service, you will 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. There are several reasons why this presents a problem:

  • When requesting my_mailer, an instance of my_mailer (called MailerA) is created and the request service (called 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.

Using a Service from a narrower Scope

If your service has a dependency on a scoped service (like the request), you have three ways to deal with it:

  • Use setter injection if the dependency is "synchronized"; this is the recommended way and the best solution for the request instance as it is synchronized with the request scope (see How to Work with Scopes);
  • Put your service in the same scope as the dependency (or a narrower one). If you depend on the request service, this means putting your new service in the request scope (see How to Work with Scopes);
  • Pass the entire container to your service and retrieve your dependency from the container each time you need it to be sure you have the right instance -- your service can live in the default container scope (see How to Work with Scopes).

Each scenario is detailed in the following sections.

Using a Synchronized Service

2.3

Synchronized services were introduced in Symfony 2.3.

Injecting the container or setting your service to a narrower scope have drawbacks. For synchronized services (like the request), using setter injection is the best option as it has no drawbacks and everything works without any special code in your service or in your definition:

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// src/AppBundle/Mail/Mailer.php
namespace AppBundle\Mail;

use Symfony\Component\HttpFoundation\Request;

class Mailer
{
    protected $request;

    public function setRequest(Request $request = null)
    {
        $this->request = $request;
    }

    public function sendEmail()
    {
        if (null === $this->request) {
            // throw an error?
        }

        // ... do something using the request here
    }
}

Whenever the request scope is entered or left, the service container will automatically call the setRequest() method with the current request instance.

You might have noticed that the setRequest() method accepts null as a valid value for the request argument. That's because when leaving the request scope, the request instance can be null (for the master request for instance). Of course, you should take care of this possibility in your code. This should also be taken into account when declaring your service:

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# app/config/services.yml
services:
    greeting_card_manager:
        class: AppBundle\Mail\GreetingCardManager
        calls:
            - [setRequest, ['@?request=']]

Tip

You can declare your own synchronized services very easily; here is the declaration of the request service for reference:

  • YAML
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services:
    request:
        scope: request
        synthetic: true
        synchronized: true

Caution

The service using the synchronized service will need to be public in order to have its setter called when the scope changes.

Changing the Scope of your Service

Changing the scope of a service should be done in its definition:

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# app/config/services.yml
services:
    greeting_card_manager:
        class: AppBundle\Mail\GreetingCardManager
        scope: request
        arguments: ['@request']

Passing the Container as a Dependency of your Service

Setting the scope to a narrower one 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 can pass the entire container into your service:

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// src/AppBundle/Mail/Mailer.php
namespace AppBundle\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:

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# app/config/services.yml
services:
    my_mailer:
        class:     AppBundle\Mail\Mailer
        arguments: ['@service_container']
        # scope: container can be omitted as it is the default

Note

Injecting the whole container into a service is generally not a good idea (only inject what you need).

Tip

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 Controller for details.

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