How to Manage Common Dependencies with Parent Services

As you add more functionality to your application, you may well start to have related classes that share some of the same dependencies. For example you may have a Newsletter Manager which uses setter injection to set its dependencies:

namespace Acme\HelloBundle\Mail;

use Acme\HelloBundle\Mailer;
use Acme\HelloBundle\EmailFormatter;

class NewsletterManager
{
    protected $mailer;
    protected $emailFormatter;

    public function setMailer(Mailer $mailer)
    {
        $this->mailer = $mailer;
    }

    public function setEmailFormatter(EmailFormatter $emailFormatter)
    {
        $this->emailFormatter = $emailFormatter;
    }
    // ...
}

and also a Greeting Card class which shares the same dependencies:

namespace Acme\HelloBundle\Mail;

use Acme\HelloBundle\Mailer;
use Acme\HelloBundle\EmailFormatter;

class GreetingCardManager
{
    protected $mailer;
    protected $emailFormatter;

    public function setMailer(Mailer $mailer)
    {
        $this->mailer = $mailer;
    }

    public function setEmailFormatter(EmailFormatter $emailFormatter)
    {
        $this->emailFormatter = $emailFormatter;
    }
    // ...
}

The service config for these classes would look something like this:

• YAML

  # src/Acme/HelloBundle/Resources/config/services.yml
  parameters:
      # ...
      newsletter_manager.class: Acme\HelloBundle\Mail\NewsletterManager
      greeting_card_manager.class: Acme\HelloBundle\Mail\GreetingCardManager
  services:
      my_mailer:
          # ...
      my_email_formatter:
          # ...
      newsletter_manager:
          class:     %newsletter_manager.class%
          calls:
              - [ setMailer, [ @my_mailer ] ]
              - [ setEmailFormatter, [ @my_email_formatter] ]
  
      greeting_card_manager:
          class:     %greeting_card_manager.class%
          calls:
              - [ setMailer, [ @my_mailer ] ]
              - [ setEmailFormatter, [ @my_email_formatter] ]
  

• XML

  <!-- src/Acme/HelloBundle/Resources/config/services.xml -->
  <parameters>
      <!-- ... -->
      <parameter key="newsletter_manager.class">Acme\HelloBundle\Mail\NewsletterManager</parameter>
      <parameter key="greeting_card_manager.class">Acme\HelloBundle\Mail\GreetingCardManager</parameter>
  </parameters>
  
  <services>
      <service id="my_mailer" ... >
        <!-- ... -->
      </service>
      <service id="my_email_formatter" ... >
        <!-- ... -->
      </service>
      <service id="newsletter_manager" class="%newsletter_manager.class%">
          <call method="setMailer">
               <argument type="service" id="my_mailer" />
          </call>
          <call method="setEmailFormatter">
               <argument type="service" id="my_email_formatter" />
          </call>
      </service>
      <service id="greeting_card_manager" class="%greeting_card_manager.class%">
          <call method="setMailer">
               <argument type="service" id="my_mailer" />
          </call>
          <call method="setEmailFormatter">
               <argument type="service" id="my_email_formatter" />
          </call>
      </service>
  </services>
  

• PHP

  // src/Acme/HelloBundle/Resources/config/services.php
  use Symfony\Component\DependencyInjection\Definition;
  use Symfony\Component\DependencyInjection\Reference;
  
  // ...
  $container->setParameter('newsletter_manager.class', 'Acme\HelloBundle\Mail\NewsletterManager');
  $container->setParameter('greeting_card_manager.class', 'Acme\HelloBundle\Mail\GreetingCardManager');
  
  $container->setDefinition('my_mailer', ... );
  $container->setDefinition('my_email_formatter', ... );
  $container->setDefinition('newsletter_manager', new Definition(
      '%newsletter_manager.class%'
  ))->addMethodCall('setMailer', array(
      new Reference('my_mailer')
  ))->addMethodCall('setEmailFormatter', array(
      new Reference('my_email_formatter')
  ));
  $container->setDefinition('greeting_card_manager', new Definition(
      '%greeting_card_manager.class%'
  ))->addMethodCall('setMailer', array(
      new Reference('my_mailer')
  ))->addMethodCall('setEmailFormatter', array(
      new Reference('my_email_formatter')
  ));
  

There is a lot of repetition in both the classes and the configuration. This means that if you changed, for example, the Mailer of EmailFormatter classes to be injected via the constructor, you would need to update the config in two places. Likewise if you needed to make changes to the setter methods you would need to do this in both classes. The typical way to deal with the common methods of these related classes would be to extract them to a super class:

namespace Acme\HelloBundle\Mail;

use Acme\HelloBundle\Mailer;
use Acme\HelloBundle\EmailFormatter;

abstract class MailManager
{
    protected $mailer;
    protected $emailFormatter;

    public function setMailer(Mailer $mailer)
    {
        $this->mailer = $mailer;
    }

    public function setEmailFormatter(EmailFormatter $emailFormatter)
    {
        $this->emailFormatter = $emailFormatter;
    }
    // ...
}

The NewsletterManager and GreetingCardManager can then extend this super class:

namespace Acme\HelloBundle\Mail;

class NewsletterManager extends MailManager
{
    // ...
}

and:

namespace Acme\HelloBundle\Mail;

class GreetingCardManager extends MailManager
{
    // ...
}

In a similar fashion, the Symfony2 service container also supports extending services in the configuration so you can also reduce the repetition by specifying a parent for a service.

• YAML

  # src/Acme/HelloBundle/Resources/config/services.yml
  parameters:
      # ...
      newsletter_manager.class: Acme\HelloBundle\Mail\NewsletterManager
      greeting_card_manager.class: Acme\HelloBundle\Mail\GreetingCardManager
      mail_manager.class: Acme\HelloBundle\Mail\MailManager
  services:
      my_mailer:
          # ...
      my_email_formatter:
          # ...
      mail_manager:
          class:     %mail_manager.class%
          abstract:  true
          calls:
              - [ setMailer, [ @my_mailer ] ]
              - [ setEmailFormatter, [ @my_email_formatter] ]
  
      newsletter_manager:
          class:     %newsletter_manager.class%
          parent: mail_manager
  
      greeting_card_manager:
          class:     %greeting_card_manager.class%
          parent: mail_manager
  

• XML

  <!-- src/Acme/HelloBundle/Resources/config/services.xml -->
  <parameters>
      <!-- ... -->
      <parameter key="newsletter_manager.class">Acme\HelloBundle\Mail\NewsletterManager</parameter>
      <parameter key="greeting_card_manager.class">Acme\HelloBundle\Mail\GreetingCardManager</parameter>
      <parameter key="mail_manager.class">Acme\HelloBundle\Mail\MailManager</parameter>
  </parameters>
  
  <services>
      <service id="my_mailer" ... >
        <!-- ... -->
      </service>
      <service id="my_email_formatter" ... >
        <!-- ... -->
      </service>
      <service id="mail_manager" class="%mail_manager.class%" abstract="true">
          <call method="setMailer">
               <argument type="service" id="my_mailer" />
          </call>
          <call method="setEmailFormatter">
               <argument type="service" id="my_email_formatter" />
          </call>
      </service>
      <service id="newsletter_manager" class="%newsletter_manager.class%" parent="mail_manager"/>
      <service id="greeting_card_manager" class="%greeting_card_manager.class%" parent="mail_manager"/>
  </services>
  

• PHP

  // src/Acme/HelloBundle/Resources/config/services.php
  use Symfony\Component\DependencyInjection\Definition;
  use Symfony\Component\DependencyInjection\Reference;
  
  // ...
  $container->setParameter('newsletter_manager.class', 'Acme\HelloBundle\Mail\NewsletterManager');
  $container->setParameter('greeting_card_manager.class', 'Acme\HelloBundle\Mail\GreetingCardManager');
  $container->setParameter('mail_manager.class', 'Acme\HelloBundle\Mail\MailManager');
  
  $container->setDefinition('my_mailer', ... );
  $container->setDefinition('my_email_formatter', ... );
  $container->setDefinition('mail_manager', new Definition(
      '%mail_manager.class%'
  ))->SetAbstract(
      true
  )->addMethodCall('setMailer', array(
      new Reference('my_mailer')
  ))->addMethodCall('setEmailFormatter', array(
      new Reference('my_email_formatter')
  ));
  $container->setDefinition('newsletter_manager', new DefinitionDecorator(
      'mail_manager'
  ))->setClass(
      '%newsletter_manager.class%'
  );
  $container->setDefinition('greeting_card_manager', new DefinitionDecorator(
      'mail_manager'
  ))->setClass(
      '%greeting_card_manager.class%'
  );
  

In this context, having a parent service implies that the arguments and method calls of the parent service should be used for the child services. Specifically, the setter methods defined for the parent service will be called when the child services are instantiated.

Note

If you remove the parent config key, the services will still be instantiated and they will still of course extend the MailManager class. The difference is that omitting the parent config key will mean that the calls defined on the mail_manager service will not be executed when the child services are instantiated.

The parent class is abstract as it should not be directly instantiated. Setting it to abstract in the config file as has been done above will mean that it can only be used as a parent service and cannot be used directly as a service to inject and will be removed at compile time. In other words, it exists merely as a “template” that other services can use.

Overriding Parent Dependencies

There may be times where you want to override what class is passed in for a dependency of one child service only. Fortunately, by adding the method call config for the child service, the dependencies set by the parent class will be overridden. So if you needed to pass a different dependency just to the NewsletterManager class, the config would look like this:

• YAML

  # src/Acme/HelloBundle/Resources/config/services.yml
  parameters:
      # ...
      newsletter_manager.class: Acme\HelloBundle\Mail\NewsletterManager
      greeting_card_manager.class: Acme\HelloBundle\Mail\GreetingCardManager
      mail_manager.class: Acme\HelloBundle\Mail\MailManager
  services:
      my_mailer:
          # ...
      my_alternative_mailer:
          # ...
      my_email_formatter:
          # ...
      mail_manager:
          class:     %mail_manager.class%
          abstract:  true
          calls:
              - [ setMailer, [ @my_mailer ] ]
              - [ setEmailFormatter, [ @my_email_formatter] ]
  
      newsletter_manager:
          class:     %newsletter_manager.class%
          parent: mail_manager
          calls:
              - [ setMailer, [ @my_alternative_mailer ] ]
  
      greeting_card_manager:
          class:     %greeting_card_manager.class%
          parent: mail_manager
  

• XML

  <!-- src/Acme/HelloBundle/Resources/config/services.xml -->
  <parameters>
      <!-- ... -->
      <parameter key="newsletter_manager.class">Acme\HelloBundle\Mail\NewsletterManager</parameter>
      <parameter key="greeting_card_manager.class">Acme\HelloBundle\Mail\GreetingCardManager</parameter>
      <parameter key="mail_manager.class">Acme\HelloBundle\Mail\MailManager</parameter>
  </parameters>
  
  <services>
      <service id="my_mailer" ... >
        <!-- ... -->
      </service>
      <service id="my_alternative_mailer" ... >
        <!-- ... -->
      </service>
      <service id="my_email_formatter" ... >
        <!-- ... -->
      </service>
      <service id="mail_manager" class="%mail_manager.class%" abstract="true">
          <call method="setMailer">
               <argument type="service" id="my_mailer" />
          </call>
          <call method="setEmailFormatter">
               <argument type="service" id="my_email_formatter" />
          </call>
      </service>
      <service id="newsletter_manager" class="%newsletter_manager.class%" parent="mail_manager">
           <call method="setMailer">
               <argument type="service" id="my_alternative_mailer" />
          </call>
      </service>
      <service id="greeting_card_manager" class="%greeting_card_manager.class%" parent="mail_manager"/>
  </services>
  

• PHP

  // src/Acme/HelloBundle/Resources/config/services.php
  use Symfony\Component\DependencyInjection\Definition;
  use Symfony\Component\DependencyInjection\Reference;
  
  // ...
  $container->setParameter('newsletter_manager.class', 'Acme\HelloBundle\Mail\NewsletterManager');
  $container->setParameter('greeting_card_manager.class', 'Acme\HelloBundle\Mail\GreetingCardManager');
  $container->setParameter('mail_manager.class', 'Acme\HelloBundle\Mail\MailManager');
  
  $container->setDefinition('my_mailer', ... );
  $container->setDefinition('my_alternative_mailer', ... );
  $container->setDefinition('my_email_formatter', ... );
  $container->setDefinition('mail_manager', new Definition(
      '%mail_manager.class%'
  ))->SetAbstract(
      true
  )->addMethodCall('setMailer', array(
      new Reference('my_mailer')
  ))->addMethodCall('setEmailFormatter', array(
      new Reference('my_email_formatter')
  ));
  $container->setDefinition('newsletter_manager', new DefinitionDecorator(
      'mail_manager'
  ))->setClass(
      '%newsletter_manager.class%'
  )->addMethodCall('setMailer', array(
      new Reference('my_alternative_mailer')
  ));
  $container->setDefinition('greeting_card_manager', new DefinitionDecorator(
      'mail_manager'
  ))->setClass(
      '%greeting_card_manager.class%'
  );
  

The GreetingCardManager will receive the same dependencies as before, but the NewsletterManager will be passed the my_alternative_mailer instead of the my_mailer service.

Collections of Dependencies

It should be noted that the overridden setter method in the previous example is actually called twice - once per the parent definition and once per the child definition. In the previous example, that was fine, since the second setMailer call replaces mailer object set by the first call.

In some cases, however, this can be a problem. For example, if the overridden method call involves adding something to a collection, then two objects will be added to that collection. The following shows such a case, if the parent class looks like this:

namespace Acme\HelloBundle\Mail;

use Acme\HelloBundle\Mailer;
use Acme\HelloBundle\EmailFormatter;

abstract class MailManager
{
    protected $filters;

    public function setFilter($filter)
    {
        $this->filters[] = $filter;
    }
    // ...
}

If you had the following config:

• YAML

  # src/Acme/HelloBundle/Resources/config/services.yml
  parameters:
      # ...
      newsletter_manager.class: Acme\HelloBundle\Mail\NewsletterManager
      mail_manager.class: Acme\HelloBundle\Mail\MailManager
  services:
      my_filter:
          # ...
      another_filter:
          # ...
      mail_manager:
          class:     %mail_manager.class%
          abstract:  true
          calls:
              - [ setFilter, [ @my_filter ] ]
  
      newsletter_manager:
          class:     %newsletter_manager.class%
          parent: mail_manager
          calls:
              - [ setFilter, [ @another_filter ] ]
  

• XML

  <!-- src/Acme/HelloBundle/Resources/config/services.xml -->
  <parameters>
      <!-- ... -->
      <parameter key="newsletter_manager.class">Acme\HelloBundle\Mail\NewsletterManager</parameter>
      <parameter key="mail_manager.class">Acme\HelloBundle\Mail\MailManager</parameter>
  </parameters>
  
  <services>
      <service id="my_filter" ... >
        <!-- ... -->
      </service>
      <service id="another_filter" ... >
        <!-- ... -->
      </service>
      <service id="mail_manager" class="%mail_manager.class%" abstract="true">
          <call method="setFilter">
               <argument type="service" id="my_filter" />
          </call>
      </service>
      <service id="newsletter_manager" class="%newsletter_manager.class%" parent="mail_manager">
           <call method="setFilter">
               <argument type="service" id="another_filter" />
          </call>
      </service>
  </services>
  

• PHP

  // src/Acme/HelloBundle/Resources/config/services.php
  use Symfony\Component\DependencyInjection\Definition;
  use Symfony\Component\DependencyInjection\Reference;
  
  // ...
  $container->setParameter('newsletter_manager.class', 'Acme\HelloBundle\Mail\NewsletterManager');
  $container->setParameter('mail_manager.class', 'Acme\HelloBundle\Mail\MailManager');
  
  $container->setDefinition('my_filter', ... );
  $container->setDefinition('another_filter', ... );
  $container->setDefinition('mail_manager', new Definition(
      '%mail_manager.class%'
  ))->SetAbstract(
      true
  )->addMethodCall('setFilter', array(
      new Reference('my_filter')
  ));
  $container->setDefinition('newsletter_manager', new DefinitionDecorator(
      'mail_manager'
  ))->setClass(
      '%newsletter_manager.class%'
  )->addMethodCall('setFilter', array(
      new Reference('another_filter')
  ));
  

In this example, the setFilter of the newsletter_manager service will be called twice, resulting in the $filters array containing both my_filter and another_filter objects. This is great if you just want to add additional filters to the subclasses. If you want to replace the filters passed to the subclass, removing the parent setting from the config will prevent the base class from calling to setFilter.