Service Subscribers

Symfony's Service Locators provide a powerful way of passing a subset of

services from the service container to a service. Sometimes, you may want a

more concrete way of defining the services contained within a service locator.

By implementing ``ServiceSubscriberInterface`` you can specify the contents of

the service locator from the object itself. This is useful when a set of

services have the same parent class and similar dependencies.

Symfony's:doc:`Service Locators</service_container/service_locators>` provide

a powerful way of passing a subset of services from the service container to a

service. Sometimes, you may want a more concrete way of defining the services

contained within a service locator. By implementing

:class:`Symfony\\Component\\DependencyInjection\\ServiceSubscriberInterface`

you can specify the contents of the service locator from the object itself.

This is useful when a set of services have the same parent class and similar

dependencies.

Suppose you've got a mailing system with multiple ``Updater`` classes for

sending out mails on different occasions. Since every updater has the same

purpose, we start with a base updater::

Now that we have a base class, we can add updaters for different entities. Note

that we have to inject the dependencies of ``AbstractUpdater`` to each

updater.::

updater::

If you're using the `default services.yaml configuration`_, no additional

configuration is required for these services thanks to autowiring, but

maintaining the list of dependencies through constructor injection will quickly

become cumbersome, especially if you add a dependency in ``AbstractUpdater``.

If you're using the:ref:`default services.yml configuration<service-container-services-load-example>`,

no additional configuration is required for these services thanks to autowiring,

but maintaining the list of dependencies through constructor injection will

quickly become cumbersome, especially if you add a dependency in

``AbstractUpdater``.

By creating a service subscriber of the base class, we can create a more

practical solution for our dependencies.

First, turn ``AbstractUpdater`` into an implementation of

``ServiceSubscriberInterface`` by adding the static method

``getSubscribedServices`` which maintains a list of subscribed services and

replacing our dependencies with a service locator.::

replacing our dependencies with a service locator::

With this newly created service subscriber, the updaters can be adapted to

coincide with the service locator::

Optionally, you'll need to add the ``container.service_subscriber`` tag to

configurethe servicesto be recognized as service subscribers.

the servicesdefinitions of the updaters to enable the service subscribers.

..configuration-block::

..code-block::yaml

..code-block::xml

..code-block::php

Subscribed services

Service subscribers rely on the ``getSubscribedServices()`` method to return a

list of services types required to build a service locator for instances of

that service subscriber::

Service types can optionally be keyed by the service names used internally by

the service subscriber::

Optional dependencies

For optional dependencies, prepend the service type with a ``?`` to prevent

errors if the service type has not been implemented in the service container::

..note::

Make sure an optional service exists by calling ``has()`` on the service

locator before calling the service itself.

Usage

Subscribed services

Add the ``container.service_subscriber`` tag to a service definition to turn it

into a service subscriber and add a ``Psr\Container\ContainerInterface``

argument to pass the generated service locator to the service instance.

..configuration-block::

..code-block::yaml

Returns an array of service types required by such instances, optionally keyed by the service names used internally.

..code-block::xml

For mandatory dependencies:

..code-block::php

* array('logger' => 'Psr\Log\LoggerInterface') means the objects use the "logger" name

internally to fetch a service which must implement Psr\Log\LoggerInterface.

* array('Psr\Log\LoggerInterface') is a shortcut for

* array('Psr\Log\LoggerInterface' => 'Psr\Log\LoggerInterface')

An instance of this service would look like this::

* array('logger' => '?Psr\Log\LoggerInterface') denotes an optional dependency

* array('?Psr\Log\LoggerInterface') is a shortcut for

* array('Psr\Log\LoggerInterface' => '?Psr\Log\LoggerInterface')