Understanding Scopes¶

The scope of a service controls how long an instance of a service is used by the container. The Dependency Injection 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.

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 Using a synchronized Service).
• 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 Changing the Scope of your Service);
• 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 Passing the Container as a Dependency of your Service);

Each scenario is detailed in the following sections.

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

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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
    }
}