How to Work with Service Tags

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Read the updated version of this page for Symfony 7.1 (the current stable version).

Service tags are a way to tell Symfony or other third-party bundles that your service should be registered in some special way. Take the following example:

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# app/config/services.yml
services:
    app.twig_extension:
        class: AppBundle\Twig\AppExtension
        public: false
        tags:
            - { name: twig.extension }

Services tagged with the twig.extension tag are collected during the initialization of TwigBundle and added to Twig as extensions.

Other tags are used to integrate your services into other systems. For a list of all the tags available in the core Symfony Framework, check out Built-in Symfony Service Tags. Each of these has a different effect on your service and many tags require additional arguments (beyond just the name parameter).

Creating custom Tags

Tags on their own don't actually alter the functionality of your services in any way. But if you choose to, you can ask a container builder for a list of all services that were tagged with some specific tag. This is useful in compiler passes where you can find these services and use or modify them in some specific way.

For example, if you are using Swift Mailer you might imagine that you want to implement a "transport chain", which is a collection of classes implementing \Swift_Transport. Using the chain, you'll want Swift Mailer to try several ways of transporting the message until one succeeds.

To begin with, define the TransportChain class:

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// src/AppBundle/Mail/TransportChain.php
namespace AppBundle\Mail;

class TransportChain
{
    private $transports;

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

    public function addTransport(\Swift_Transport $transport)
    {
        $this->transports[] = $transport;
    }
}

Then, define the chain as a service:

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services:
    app.mailer_transport_chain:
        class: AppBundle\Mail\TransportChain

Define Services with a Custom Tag

Now you might want several of the \Swift_Transport classes to be instantiated and added to the chain automatically using the addTransport() method. For example, you may add the following transports as services:

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services:
    app.smtp_transport:
        class: \Swift_SmtpTransport
        arguments: ['%mailer_host%']
        tags:
            - { name: app.mail_transport }

    app.sendmail_transport:
        class: \Swift_SendmailTransport
        tags:
            - { name: app.mail_transport }

Notice that each service was given a tag named app.mail_transport. This is the custom tag that you'll use in your compiler pass. The compiler pass is what makes this tag "mean" something.

Create a Compiler Pass

You can now use a compiler pass to ask the container for any services with the app.mail_transport tag:

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// src/AppBundle/DependencyInjection/Compiler/MailTransportPass.php
namespace AppBundle\DependencyInjection\Compiler;

use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
use Symfony\Component\DependencyInjection\Reference;

class MailTransportPass implements CompilerPassInterface
{
    public function process(ContainerBuilder $container)
    {
        // always first check if the primary service is defined
        if (!$container->has('app.mailer_transport_chain')) {
            return;
        }

        $definition = $container->findDefinition('app.mailer_transport_chain');

        // find all service IDs with the app.mail_transport tag
        $taggedServices = $container->findTaggedServiceIds('app.mail_transport');

        foreach ($taggedServices as $id => $tags) {
            // add the transport service to the ChainTransport service
            $definition->addMethodCall('addTransport', array(new Reference($id)));
        }
    }
}

Register the Pass with the Container

In order to run the compiler pass when the container is compiled, you have to add the compiler pass to the container in the build() method of your bundle:

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// src/AppBundle/AppBundle.php

// ...
use Symfony\Component\DependencyInjection\ContainerBuilder;
use AppBundle\DependencyInjection\Compiler\MailTransportPass;

class AppBundle extends Bundle
{
    public function build(ContainerBuilder $container)
    {
        $container->addCompilerPass(new MailTransportPass());
    }
}

Tip

When implementing the CompilerPassInterface in a service extension, you do not need to register it. See the components documentation for more information.

Adding Additional Attributes on Tags

Sometimes you need additional information about each service that's tagged with your tag. For example, you might want to add an alias to each member of the transport chain.

To begin with, change the TransportChain class:

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class TransportChain
{
    private $transports;

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

    public function addTransport(\Swift_Transport $transport, $alias)
    {
        $this->transports[$alias] = $transport;
    }

    public function getTransport($alias)
    {
        if (array_key_exists($alias, $this->transports)) {
            return $this->transports[$alias];
        }
    }
}

As you can see, when addTransport() is called, it takes not only a Swift_Transport object, but also a string alias for that transport. So, how can you allow each tagged transport service to also supply an alias?

To answer this, change the service declaration:

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services:
    app.smtp_transport:
        class: \Swift_SmtpTransport
        arguments: ['%mailer_host%']
        tags:
            - { name: app.mail_transport, alias: foo }

    app.sendmail_transport:
        class: \Swift_SendmailTransport
        tags:
            - { name: app.mail_transport, alias: bar }

Notice that you've added a generic alias key to the tag. To actually use this, update the compiler:

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use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
use Symfony\Component\DependencyInjection\Reference;

class TransportCompilerPass implements CompilerPassInterface
{
    public function process(ContainerBuilder $container)
    {
        if (!$container->hasDefinition('app.mailer_transport_chain')) {
            return;
        }

        $definition = $container->getDefinition('app.mailer_transport_chain');
        $taggedServices = $container->findTaggedServiceIds('app.mail_transport');

        foreach ($taggedServices as $id => $tags) {
            foreach ($tags as $attributes) {
                $definition->addMethodCall('addTransport', array(
                    new Reference($id),
                    $attributes["alias"]
                ));
            }
        }
    }
}

The double loop may be confusing. This is because a service can have more than one tag. You tag a service twice or more with the app.mail_transport tag. The second foreach loop iterates over the app.mail_transport tags set for the current service and gives you the attributes.

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