How to Decorate Services

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When overriding an existing definition, the original service is lost:

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# config/services.yaml
services:
    App\Mailer: ~

    # this replaces the old App\Mailer definition with the new one, the
    # old definition is lost
    App\Mailer:
        class: App\NewMailer

Most of the time, that's exactly what you want to do. But sometimes, you might want to decorate the old one instead (i.e. apply the Decorator pattern). In this case, the old service should be kept around to be able to reference it in the new one. This configuration replaces App\Mailer with a new one, but keeps a reference of the old one as .inner:

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// src/DecoratingMailer.php
namespace App;

// ...
use Symfony\Component\DependencyInjection\Attribute\AsDecorator;

#[AsDecorator(decorates: Mailer::class)]
class DecoratingMailer
{
    // ...
}

The decorates option tells the container that the App\DecoratingMailer service replaces the App\Mailer service. If you're using the default services.yaml configuration, the decorated service is automatically injected when the constructor of the decorating service has one argument type-hinted with the decorated service class.

If you are not using autowiring or the decorating service has more than one constructor argument type-hinted with the decorated service class, you must inject the decorated service explicitly (the ID of the decorated service is automatically changed to '.inner'):

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// src/DecoratingMailer.php
namespace App;

// ...
use Symfony\Component\DependencyInjection\Attribute\AsDecorator;
use Symfony\Component\DependencyInjection\Attribute\AutowireDecorated;

#[AsDecorator(decorates: Mailer::class)]
class DecoratingMailer
{
    public function __construct(
        #[AutowireDecorated]
        private object $inner,
    ) {
    }

    // ...
}

Note

The visibility of the decorated App\Mailer service (which is an alias for the new service) will still be the same as the original App\Mailer visibility.

Note

All custom service tags from the decorated service are removed in the new service. Only certain built-in service tags defined by Symfony are retained: container.service_locator, container.service_subscriber, kernel.event_subscriber, kernel.event_listener, kernel.locale_aware, and kernel.reset.

Note

The generated inner id is based on the id of the decorator service (App\DecoratingMailer here), not of the decorated service (App\Mailer here). You can control the inner service name via the decoration_inner_name option:

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# config/services.yaml
services:
    App\DecoratingMailer:
        # ...
        decoration_inner_name: App\DecoratingMailer.wooz
        arguments: ['@App\DecoratingMailer.wooz']

Decoration Priority

When applying multiple decorators to a service, you can control their order with the decoration_priority option. Its value is an integer that defaults to 0 and higher priorities mean that decorators will be applied earlier.

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// ...
use Symfony\Component\DependencyInjection\Attribute\AsDecorator;
use Symfony\Component\DependencyInjection\Attribute\AutowireDecorated;

#[AsDecorator(decorates: Foo::class, priority: 5)]
class Bar
{
    public function __construct(
        #[AutowireDecorated]
        private $inner,
    ) {
    }
    // ...
}

#[AsDecorator(decorates: Foo::class, priority: 1)]
class Baz
{
    public function __construct(
        #[AutowireDecorated]
        private $inner,
    ) {
    }

    // ...
}

The generated code will be the following:

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$this->services[Foo::class] = new Baz(new Bar(new Foo()));

Stacking Decorators

An alternative to using decoration priorities is to create a stack of ordered services, each one decorating the next:

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# config/services.yaml
services:
    decorated_foo_stack:
        stack:
            - class: Baz
              arguments: ['@.inner']
            - class: Bar
              arguments: ['@.inner']
            - class: Foo

    # using the short syntax:
    decorated_foo_stack:
        stack:
            - Baz: ['@.inner']
            - Bar: ['@.inner']
            - Foo: ~

    # can be simplified when autowiring is enabled:
    decorated_foo_stack:
        stack:
            - Baz: ~
            - Bar: ~
            - Foo: ~

The result will be the same as in the previous section:

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$this->services['decorated_foo_stack'] = new Baz(new Bar(new Foo()));

Like aliases, a stack can only use public and deprecated attributes.

Each frame of the stack can be either an inlined service, a reference or a child definition. The latter allows embedding stack definitions into each others, here's an advanced example of composition:

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# config/services.yaml
services:
    some_decorator:
        class: App\Decorator

    embedded_stack:
        stack:
            - alias: some_decorator
            - App\Decorated: ~

    decorated_foo_stack:
        stack:
            - parent: embedded_stack
            - Baz: ~
            - Bar: ~
            - Foo: ~

The result will be:

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$this->services['decorated_foo_stack'] = new App\Decorator(new App\Decorated(new Baz(new Bar(new Foo()))));

Note

To change existing stacks (i.e. from a compiler pass), you can access each frame by its generated id with the following structure: .stack_id.frame_key. From the example above, .decorated_foo_stack.1 would be a reference to the inlined Baz service and .decorated_foo_stack.0 to the embedded stack. To get more explicit ids, you can give a name to each frame:

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# ...
decorated_foo_stack:
    stack:
        first:
            parent: embedded_stack
        second:
            Baz: ~
        # ...

The Baz frame id will now be .decorated_foo_stack.second.

Control the Behavior When the Decorated Service Does Not Exist

When you decorate a service that doesn't exist, the decoration_on_invalid option allows you to choose the behavior to adopt.

Three different behaviors are available:

  • exception: A ServiceNotFoundException will be thrown telling that decorator's dependency is missing. (default)
  • ignore: The container will remove the decorator.
  • null: The container will keep the decorator service and will set the decorated one to null.
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// ...
use Symfony\Component\DependencyInjection\Attribute\AsDecorator;
use Symfony\Component\DependencyInjection\Attribute\AutowireDecorated;
use Symfony\Component\DependencyInjection\ContainerInterface;

#[AsDecorator(decorates: Mailer::class, onInvalid: ContainerInterface::IGNORE_ON_INVALID_REFERENCE)]
class Bar
{
    public function __construct(
        private #[AutowireDecorated] $inner,
    ) {
    }

    // ...
}

Caution

When using null, you may have to update the decorator constructor in order to make decorated dependency nullable:

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// src/Service/DecoratorService.php
namespace App\Service;

use Acme\OptionalBundle\Service\OptionalService;

class DecoratorService
{
    public function __construct(
        private ?OptionalService $decorated,
    ) {
    }

    public function tellInterestingStuff(): string
    {
        if (!$this->decorated) {
            return 'Just one interesting thing';
        }

        return $this->decorated->tellInterestingStuff().' + one more interesting thing';
    }
}

Note

Sometimes, you may want to add a compiler pass that creates service definitions on the fly. If you want to decorate such a service, be sure that your compiler pass is registered with PassConfig::TYPE_BEFORE_OPTIMIZATION type so that the decoration pass will be able to find the created services.

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