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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:
- XML
- PHP
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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>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:
- YAML
- XML
- PHP
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# app/config/services.yml
services:
app.newsletter_manager:
class: AppBundle\Newsletter\NewsletterManager
calls:
- [setMailer, ['@?app.mailer']]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:
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public function setMailer(Mailer $mailer)
{
// ...
}