How to Make Service Arguments/References Optional

How to Make Service Arguments/References Optional

Sometimes, one of your services may have an optional dependency, meaning that the dependency is not required for your service to work properly. In the example above, the app.mailer service must exist, otherwise an exception will be thrown. By modifying the app.newsletter_manager service definition, you can make this reference optional, there are two strategies for doing this.

Setting Missing Dependencies to null

You can use the null strategy to explicitly set the argument to null if the service does not exist:

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    <!-- app/config/services.xml -->
    <?xml version="1.0" encoding="UTF-8" ?>
    <container xmlns="http://symfony.com/schema/dic/services"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://symfony.com/schema/dic/services
            http://symfony.com/schema/dic/services/services-1.0.xsd">
    
        <services>
            <service id="app.mailer">
            <!-- ... -->
            </service>
    
            <service id="app.newsletter_manager" class="AppBundle\Newsletter\NewsletterManager">
                <argument type="service" id="app.mailer" on-invalid="null" />
            </service>
        </services>
    </container>
    
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    // app/config/services.php
    use AppBundle\Newsletter\NewsletterManager;
    use Symfony\Component\DependencyInjection\Definition;
    use Symfony\Component\DependencyInjection\Reference;
    use Symfony\Component\DependencyInjection\ContainerInterface;
    
    $container->setDefinition('app.mailer', ...);
    
    $container->setDefinition('app.newsletter_manager', new Definition(
        NewsletterManager::class,
        array(
            new Reference(
                'app.mailer',
                ContainerInterface::NULL_ON_INVALID_REFERENCE
            )
        )
    ));
    

Note

The "null" strategy is not currently supported by the YAML driver.

Ignoring Missing Dependencies

The behavior of ignoring missing dependencies is the same as the "null" behavior except when used within a method call, in which case the method call itself will be removed.

In the following example the container will inject a service using a method call if the service exists and remove the method call if it does not:

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    # app/config/services.yml
    services:
        app.newsletter_manager:
            class: AppBundle\Newsletter\NewsletterManager
            calls:
                - [setMailer, ['@?app.mailer']]
    
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    <!-- app/config/services.xml -->
    <?xml version="1.0" encoding="UTF-8" ?>
    <container xmlns="http://symfony.com/schema/dic/services"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://symfony.com/schema/dic/services
            http://symfony.com/schema/dic/services/services-1.0.xsd">
    
        <services>
            <service id="app.mailer">
            <!-- ... -->
            </service>
    
            <service id="app.newsletter_manager" class="AppBundle\Newsletter\NewsletterManager">
                <call method="setMailer">
                    <argument type="service" id="my_mailer" on-invalid="ignore"/>
                </call>
            </service>
        </services>
    </container>
    
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    // app/config/services.php
    use AppBundle\Newsletter\NewsletterManager;
    use Symfony\Component\DependencyInjection\Reference;
    use Symfony\Component\DependencyInjection\ContainerInterface;
    
    $container->register('app.mailer', ...);
    
    $container
        ->register('app.newsletter_manager', NewsletterManager::class)
        ->addMethodCall('setMailer', array(
            new Reference(
                'my_mailer',
                ContainerInterface::IGNORE_ON_INVALID_REFERENCE
            ),
        ))
    ;
    

In YAML, the special @? syntax tells the service container that the dependency is optional. Of course, the NewsletterManager must also be rewritten by adding a setMailer() method:

public function setMailer(Mailer $mailer)
{
    // ...
}

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