How to Work with Service Tags
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How to Work with Service Tags
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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# config/services.yaml
services:
App\Twig\AppExtension:
tags: ['twig.extension']
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<!-- 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
https://symfony.com/schema/dic/services/services-1.0.xsd">
<services>
<service id="App\Twig\AppExtension">
<tag name="twig.extension"/>
</service>
</services>
</container>
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// config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;
use App\Twig\AppExtension;
return function(ContainerConfigurator $containerConfigurator) {
$services = $containerConfigurator->services();
$services->set(AppExtension::class)
->tag('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 the name
parameter).
For most users, this is all you need to know. If you want to go further and learn how to create your own custom tags, keep reading.
Autoconfiguring Tags
If you enable autoconfigure, then some tags are
automatically applied for you. That's true for the twig.extension
tag: the
container sees that your class extends AbstractExtension
(or more accurately,
that it implements ExtensionInterface
) and adds the tag for you.
If you want to apply tags automatically for your own services, use the
_instanceof
option:
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# config/services.yaml
services:
# this config only applies to the services created by this file
_instanceof:
# services whose classes are instances of CustomInterface will be tagged automatically
App\Security\CustomInterface:
tags: ['app.custom_tag']
# ...
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<!-- 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 https://symfony.com/schema/dic/services/services-1.0.xsd">
<services>
<!-- this config only applies to the services created by this file -->
<instanceof id="App\Security\CustomInterface" autowire="true">
<!-- services whose classes are instances of CustomInterface will be tagged automatically -->
<tag name="app.custom_tag"/>
</instanceof>
</services>
</container>
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// config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;
use App\Security\CustomInterface;
return function(ContainerConfigurator $containerConfigurator) {
$services = $containerConfigurator->services();
// this config only applies to the services created by this file
$services
->instanceof(CustomInterface::class)
// services whose classes are instances of CustomInterface will be tagged automatically
->tag('app.custom_tag');
};
It is also possible to use the #[AutoconfigureTag]
attribute directly on the
base class or interface:
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// src/Security/CustomInterface.php
namespace App\Security;
use Symfony\Component\DependencyInjection\Attribute\AutoconfigureTag;
#[AutoconfigureTag('app.custom_tag')]
interface CustomInterface
{
// ...
}
Tip
If you need more capabilities to autoconfigure instances of your base class like their laziness, their bindings or their calls for example, you may rely on the Autoconfigure attribute.
For more advanced needs, you can define the automatic tags using the registerForAutoconfiguration() method.
In a Symfony application, call this method in your kernel class:
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// src/Kernel.php
class Kernel extends BaseKernel
{
// ...
protected function build(ContainerBuilder $containerBuilder): void
{
$containerBuilder->registerForAutoconfiguration(CustomInterface::class)
->addTag('app.custom_tag')
;
}
}
In a Symfony bundle, call this method in the load()
method of the
bundle extension class:
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// src/DependencyInjection/MyBundleExtension.php
class MyBundleExtension extends Extension
{
// ...
public function load(array $configs, ContainerBuilder $containerBuilder): void
{
$containerBuilder->registerForAutoconfiguration(CustomInterface::class)
->addTag('app.custom_tag')
;
}
}
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 the Symfony Mailer component you might want
to implement a "transport chain", which is a collection of classes implementing
\MailerTransport
. Using the chain, you'll want Mailer to try several
ways of transporting the message until one succeeds.
To begin with, define the TransportChain
class:
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// src/Mail/TransportChain.php
namespace App\Mail;
class TransportChain
{
private $transports;
public function __construct()
{
$this->transports = [];
}
public function addTransport(\MailerTransport $transport): void
{
$this->transports[] = $transport;
}
}
Then, define the chain as a service:
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# config/services.yaml
services:
App\Mail\TransportChain: ~
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<!-- 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
https://symfony.com/schema/dic/services/services-1.0.xsd">
<services>
<service id="App\Mail\TransportChain"/>
</services>
</container>
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// config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;
use App\Mail\TransportChain;
return function(ContainerConfigurator $containerConfigurator) {
$services = $containerConfigurator->services();
$services->set(TransportChain::class);
};
Define Services with a Custom Tag
Now you might want several of the \MailerTransport
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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# config/services.yaml
services:
MailerSmtpTransport:
arguments: ['%mailer_host%']
tags: ['app.mail_transport']
MailerSendmailTransport:
tags: ['app.mail_transport']
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<!-- 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
https://symfony.com/schema/dic/services/services-1.0.xsd">
<services>
<service id="MailerSmtpTransport">
<argument>%mailer_host%</argument>
<tag name="app.mail_transport"/>
</service>
<service id="MailerSendmailTransport">
<tag name="app.mail_transport"/>
</service>
</services>
</container>
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// config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;
return function(ContainerConfigurator $containerConfigurator) {
$services = $containerConfigurator->services();
$services->set(\MailerSmtpTransport::class)
// the param() method was introduced in Symfony 5.2.
->args([param('mailer_host')])
->tag('app.mail_transport')
;
$services->set(\MailerSendmailTransport::class)
->tag('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/DependencyInjection/Compiler/MailTransportPass.php
namespace App\DependencyInjection\Compiler;
use App\Mail\TransportChain;
use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Reference;
class MailTransportPass implements CompilerPassInterface
{
public function process(ContainerBuilder $containerBuilder): void
{
// always first check if the primary service is defined
if (!$containerBuilder->has(TransportChain::class)) {
return;
}
$definition = $containerBuilder->findDefinition(TransportChain::class);
// find all service IDs with the app.mail_transport tag
$taggedServices = $containerBuilder->findTaggedServiceIds('app.mail_transport');
foreach ($taggedServices as $id => $tags) {
// add the transport service to the TransportChain service
$definition->addMethodCall('addTransport', [new Reference($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 a bundle extension or from your kernel:
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// src/Kernel.php
namespace App;
use App\DependencyInjection\Compiler\MailTransportPass;
use Symfony\Component\HttpKernel\Kernel as BaseKernel;
// ...
class Kernel extends BaseKernel
{
// ...
protected function build(ContainerBuilder $containerBuilder): void
{
$containerBuilder->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 = [];
}
public function addTransport(\MailerTransport $transport, $alias): void
{
$this->transports[$alias] = $transport;
}
public function getTransport($alias): ?\MailerTransport
{
if (array_key_exists($alias, $this->transports)) {
return $this->transports[$alias];
}
return null;
}
}
As you can see, when addTransport()
is called, it takes not only a MailerTransport
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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# config/services.yaml
services:
MailerSmtpTransport:
arguments: ['%mailer_host%']
tags:
- { name: 'app.mail_transport', alias: 'smtp' }
MailerSendmailTransport:
tags:
- { name: 'app.mail_transport', alias: 'sendmail' }
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<!-- 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
https://symfony.com/schema/dic/services/services-1.0.xsd">
<services>
<service id="MailerSmtpTransport">
<argument>%mailer_host%</argument>
<tag name="app.mail_transport" alias="smtp"/>
</service>
<service id="MailerSendmailTransport">
<tag name="app.mail_transport" alias="sendmail"/>
</service>
</services>
</container>
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// config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;
return function(ContainerConfigurator $containerConfigurator) {
$services = $containerConfigurator->services();
$services->set(\MailerSmtpTransport::class)
// the param() method was introduced in Symfony 5.2.
->args([param('mailer_host')])
->tag('app.mail_transport', ['alias' => 'smtp'])
;
$services->set(\MailerSendmailTransport::class)
->tag('app.mail_transport', ['alias' => 'sendmail'])
;
};
Tip
In YAML format, you may provide the tag as a simple string as long as you don't need to specify additional attributes. The following definitions are equivalent.
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# config/services.yaml
services:
# Compact syntax
MailerSendmailTransport:
class: \MailerSendmailTransport
tags: ['app.mail_transport']
# Verbose syntax
MailerSendmailTransport:
class: \MailerSendmailTransport
tags:
- { name: 'app.mail_transport' }
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\Compiler\CompilerPassInterface;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Reference;
class TransportCompilerPass implements CompilerPassInterface
{
public function process(ContainerBuilder $containerBuilder): void
{
// ...
foreach ($taggedServices as $id => $tags) {
// a service could have the same tag twice
foreach ($tags as $attributes) {
$definition->addMethodCall('addTransport', [
new Reference($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.
Reference Tagged Services
Symfony provides a shortcut to inject all services tagged with a specific tag, which is a common need in some applications, so you don't have to write a compiler pass just for that.
Consider the following HandlerCollection
class where you want to inject
all services tagged with app.handler
into its constructor argument:
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// src/HandlerCollection.php
namespace App;
class HandlerCollection
{
public function __construct(iterable $handlers)
{
}
}
Symfony allows you to inject the services using YAML/XML/PHP configuration or directly via PHP attributes:
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// src/HandlerCollection.php
namespace App;
use Symfony\Component\DependencyInjection\Attribute\TaggedIterator;
class HandlerCollection
{
public function __construct(
// the attribute must be applied directly to the argument to autowire
#[TaggedIterator('app.handler')] iterable $handlers
) {
}
}
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# config/services.yaml
services:
App\Handler\One:
tags: ['app.handler']
App\Handler\Two:
tags: ['app.handler']
App\HandlerCollection:
# inject all services tagged with app.handler as first argument
arguments:
- !tagged_iterator app.handler
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<!-- 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
https://symfony.com/schema/dic/services/services-1.0.xsd">
<services>
<service id="App\Handler\One">
<tag name="app.handler"/>
</service>
<service id="App\Handler\Two">
<tag name="app.handler"/>
</service>
<service id="App\HandlerCollection">
<!-- inject all services tagged with app.handler as first argument -->
<argument type="tagged_iterator" tag="app.handler"/>
</service>
</services>
</container>
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// config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;
return function(ContainerConfigurator $containerConfigurator) {
$services = $containerConfigurator->services();
$services->set(App\Handler\One::class)
->tag('app.handler')
;
$services->set(App\Handler\Two::class)
->tag('app.handler')
;
$services->set(App\HandlerCollection::class)
// inject all services tagged with app.handler as first argument
->args([tagged_iterator('app.handler')])
;
};
See also
See also tagged locator services
Tagged Services with Priority
The tagged services can be prioritized using the priority
attribute. The
priority is a positive or negative integer that defaults to 0
. The higher
the number, the earlier the tagged service will be located in the collection:
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# config/services.yaml
services:
App\Handler\One:
tags:
- { name: 'app.handler', priority: 20 }
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<!-- 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
https://symfony.com/schema/dic/services/services-1.0.xsd">
<services>
<service id="App\Handler\One">
<tag name="app.handler" priority="20"/>
</service>
</services>
</container>
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// config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;
use App\Handler\One;
return function(ContainerConfigurator $containerConfigurator) {
$services = $containerConfigurator->services();
$services->set(One::class)
->tag('app.handler', ['priority' => 20])
;
};
Another option, which is particularly useful when using autoconfiguring
tags, is to implement the static getDefaultPriority()
method on the
service itself:
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// src/Handler/One.php
namespace App\Handler;
class One
{
public static function getDefaultPriority(): int
{
return 3;
}
}
If you want to have another method defining the priority
(e.g. getPriority()
rather than getDefaultPriority()
),
you can define it in the configuration of the collecting service:
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// src/HandlerCollection.php
namespace App;
use Symfony\Component\DependencyInjection\Attribute\TaggedIterator;
class HandlerCollection
{
public function __construct(
#[TaggedIterator('app.handler', defaultPriorityMethod: 'getPriority')]
iterable $handlers
) {
}
}
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# config/services.yaml
services:
App\HandlerCollection:
# inject all services tagged with app.handler as first argument
arguments:
- !tagged_iterator { tag: app.handler, default_priority_method: getPriority }
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<!-- 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
https://symfony.com/schema/dic/services/services-1.0.xsd">
<services>
<service id="App\HandlerCollection">
<argument type="tagged_iterator" tag="app.handler" default-priority-method="getPriority"/>
</service>
</services>
</container>
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// config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;
use Symfony\Component\DependencyInjection\Argument\TaggedIteratorArgument;
return function (ContainerConfigurator $containerConfigurator) {
$services = $containerConfigurator->services();
// ...
$services->set(App\HandlerCollection::class)
->args([
tagged_iterator('app.handler', null, null, 'getPriority'),
])
;
};
Tagged Services with Index
If you want to retrieve a specific service within the injected collection
you can use the index_by
and default_index_method
options of the
argument in combination with !tagged_iterator
.
Using the previous example, this service configuration creates a collection
indexed by the key
attribute:
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// src/HandlerCollection.php
namespace App;
use Symfony\Component\DependencyInjection\Attribute\TaggedIterator;
class HandlerCollection
{
public function __construct(
#[TaggedIterator('app.handler', indexAttribute: 'key')]
iterable $handlers
) {
}
}
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# config/services.yaml
services:
App\Handler\One:
tags:
- { name: 'app.handler', key: 'handler_one' }
App\Handler\Two:
tags:
- { name: 'app.handler', key: 'handler_two' }
App\HandlerCollection:
arguments: [!tagged_iterator { tag: 'app.handler', index_by: 'key' }]
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<!-- 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
https://symfony.com/schema/dic/services/services-1.0.xsd">
<services>
<service id="App\Handler\One">
<tag name="app.handler" key="handler_one"/>
</service>
<service id="App\Handler\Two">
<tag name="app.handler" key="handler_two"/>
</service>
<service id="App\HandlerCollection">
<argument type="tagged_iterator" tag="app.handler" index-by="key"/>
</service>
</services>
</container>
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// config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;
use App\Handler\One;
use App\Handler\Two;
use Symfony\Component\DependencyInjection\Argument\TaggedIteratorArgument;
return function (ContainerConfigurator $containerConfigurator) {
$services = $containerConfigurator->services();
$services->set(One::class)
->tag('app.handler', ['key' => 'handler_one']);
$services->set(Two::class)
->tag('app.handler', ['key' => 'handler_two']);
$services->set(App\HandlerCollection::class)
->args([
// 2nd argument is the index attribute name
tagged_iterator('app.handler', 'key'),
])
;
};
After compilation the HandlerCollection
is able to iterate over your
application handlers. To retrieve a specific service from the iterator, call the
iterator_to_array()
function and then use the key
attribute to get the
array element. For example, to retrieve the handler_two
handler:
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// src/Handler/HandlerCollection.php
namespace App\Handler;
class HandlerCollection
{
public function __construct(iterable $handlers)
{
$handlers = $handlers instanceof \Traversable ? iterator_to_array($handlers) : $handlers;
$handlerTwo = $handlers['handler_two'];
}
}
Tip
Just like the priority, you can also implement a static
getDefaultIndexName()
method in the handlers and omit the
index attribute (key
):
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// src/Handler/One.php
namespace App\Handler;
class One
{
// ...
public static function getDefaultIndexName(): string
{
return 'handler_one';
}
}
You also can define the name of the static method to implement on each service
with the default_index_method
attribute on the tagged argument:
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// src/HandlerCollection.php
namespace App;
use Symfony\Component\DependencyInjection\Attribute\TaggedIterator;
class HandlerCollection
{
public function __construct(
#[TaggedIterator('app.handler', defaultIndexMethod: 'getIndex')]
iterable $handlers
) {
}
}
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# config/services.yaml
services:
# ...
App\HandlerCollection:
# use getIndex() instead of getDefaultIndexName()
arguments: [!tagged_iterator { tag: 'app.handler', default_index_method: 'getIndex' }]
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<!-- 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
https://symfony.com/schema/dic/services/services-1.0.xsd">
<services>
<!-- ... -->
<service id="App\HandlerCollection">
<!-- use getIndex() instead of getDefaultIndexName() -->
<argument type="tagged_iterator"
tag="app.handler"
default-index-method="someFunctionName"
/>
</service>
</services>
</container>
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// config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;
use App\HandlerCollection;
use Symfony\Component\DependencyInjection\Argument\TaggedIteratorArgument;
return function (ContainerConfigurator $containerConfigurator) {
$services = $containerConfigurator->services();
// ...
// use getIndex() instead of getDefaultIndexName()
$services->set(HandlerCollection::class)
->args([
tagged_iterator('app.handler', null, 'getIndex'),
])
;
};
The #[AsTaggedItem]
attribute
It is possible to define both the priority and the index of a tagged
item thanks to the #[AsTaggedItem]
attribute. This attribute must
be used directly on the class of the service you want to configure:
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// src/Handler/One.php
namespace App\Handler;
use Symfony\Component\DependencyInjection\Attribute\AsTaggedItem;
#[AsTaggedItem(index: 'handler_one', priority: 10)]
class One
{
// ...
}