Service Subscribers & Locators

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3.3

Service subscribers and locators were introduced in Symfony 3.3.

Sometimes, a service needs access to several other services without being sure that all of them will actually be used. In those cases, you may want the instantiation of the services to be lazy. However, that's not possible using the explicit dependency injection since services are not all meant to be lazy (see Lazy Services).

This can typically be the case in your controllers, where you may inject several services in the constructor, but the action executed only uses some of them. Another example are applications that implement the Command pattern using a CommandBus to map command handlers by Command class names and use them to handle their respective command when it is asked for:

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// src/AppBundle/CommandBus.php
namespace AppBundle;

// ...
class CommandBus
{
    /**
     * @var CommandHandler[]
     */
    private $handlerMap;

    public function __construct(array $handlerMap)
    {
        $this->handlerMap = $handlerMap;
    }

    public function handle(Command $command)
    {
        $commandClass = get_class($command);

        if (!isset($this->handlerMap[$commandClass])) {
            return;
        }

        return $this->handlerMap[$commandClass]->handle($command);
    }
}

// ...
$commandBus->handle(new FooCommand());

Considering that only one command is handled at a time, instantiating all the other command handlers is unnecessary. A possible solution to lazy-load the handlers could be to inject the main dependency injection container.

However, injecting the entire container is discouraged because it gives too broad access to existing services and it hides the actual dependencies of the services. Doing so also requires services to be made public, which isn't the case by default in Symfony applications.

Service Subscribers are intended to solve this problem by giving access to a set of predefined services while instantiating them only when actually needed through a Service Locator, a separate lazy-loaded container.

Defining a Service Subscriber

First, turn CommandBus into an implementation of ServiceSubscriberInterface. Use its getSubscribedServices() method to include as many services as needed in the service subscriber and change the type hint of the container to a PSR-11 ContainerInterface:

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// src/AppBundle/CommandBus.php
namespace AppBundle;

use AppBundle\CommandHandler\BarHandler;
use AppBundle\CommandHandler\FooHandler;
use Psr\Container\ContainerInterface;
use Symfony\Component\DependencyInjection\ServiceSubscriberInterface;

class CommandBus implements ServiceSubscriberInterface
{
    private $locator;

    public function __construct(ContainerInterface $locator)
    {
        $this->locator = $locator;
    }

    public static function getSubscribedServices()
    {
        return [
            'AppBundle\FooCommand' => FooHandler::class,
            'AppBundle\BarCommand' => BarHandler::class,
        ];
    }

    public function handle(Command $command)
    {
        $commandClass = get_class($command);

        if ($this->locator->has($commandClass)) {
            $handler = $this->locator->get($commandClass);

            return $handler->handle($command);
        }
    }
}

Tip

If the container does not contain the subscribed services, double-check that you have autoconfigure enabled. You can also manually add the container.service_subscriber tag.

The injected service is an instance of ServiceLocator which implements the PSR-11 ContainerInterface, but it is also a callable:

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// ...
$handler = ($this->locator)($commandClass);

return $handler->handle($command);

Including Services

In order to add a new dependency to the service subscriber, use the getSubscribedServices() method to add service types to include in the service locator:

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use Psr\Log\LoggerInterface;

public static function getSubscribedServices()
{
    return [
        // ...
        LoggerInterface::class,
    ];
}

Service types can also be keyed by a service name for internal use:

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use Psr\Log\LoggerInterface;

public static function getSubscribedServices()
{
    return [
        // ...
        'logger' => LoggerInterface::class,
    ];
}

When extending a class that also implements ServiceSubscriberInterface, it's your responsibility to call the parent when overriding the method. This typically happens when extending AbstractController:

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use Psr\Log\LoggerInterface;
use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;

class MyController extends AbstractController
{
    public static function getSubscribedServices()
    {
        return array_merge(parent::getSubscribedServices(), [
            // ...
            'logger' => LoggerInterface::class,
        ]);
    }
}

Optional Services

For optional dependencies, prepend the service type with a ? to prevent errors if there's no matching service found in the service container:

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use Psr\Log\LoggerInterface;

public static function getSubscribedServices()
{
    return [
        // ...
        '?'.LoggerInterface::class,
    ];
}

Note

Make sure an optional service exists by calling has() on the service locator before calling the service itself.

Aliased Services

By default, autowiring is used to match a service type to a service from the service container. If you don't use autowiring or need to add a non-traditional service as a dependency, use the container.service_subscriber tag to map a service type to a service.

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# app/config/services.yml
services:
    AppBundle\CommandBus:
        tags:
            - { name: 'container.service_subscriber', key: 'logger', id: 'monolog.logger.event' }

Tip

The key attribute can be omitted if the service name internally is the same as in the service container.

Defining a Service Locator

To manually define a service locator, create a new service definition and add the container.service_locator tag to it. Use its arguments option to include as many services as needed in it.

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// app/config/services.yml
services:
    app.command_handler_locator:
        class: Symfony\Component\DependencyInjection\ServiceLocator
        tags: ['container.service_locator']
        arguments:
            -
                AppBundle\FooCommand: '@app.command_handler.foo'
                AppBundle\BarCommand: '@app.command_handler.bar'

Note

The services defined in the service locator argument must include keys, which later become their unique identifiers inside the locator.

Now you can use the service locator by injecting it in any other service:

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// app/config/services.yml
services:
    AppBundle\CommandBus:
        arguments: ['@app.command_handler_locator']

In compiler passes it's recommended to use the register() method to create the service locators. This will save you some boilerplate and will share identical locators among all the services referencing them:

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use Symfony\Component\DependencyInjection\Compiler\ServiceLocatorTagPass;
use Symfony\Component\DependencyInjection\ContainerBuilder;

public function process(ContainerBuilder $container)
{
    // ...

    $locateableServices = [
        // ...
        'logger' => new Reference('logger'),
    ];

    $myService->addArgument(ServiceLocatorTagPass::register($container, $locateableServices));
}
This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.
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