How to Decorate Services

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
    

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        <!-- config/services.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsd="http://www.w3.org/2001/XMLSchema-instance"
    xsd:schemaLocation="http://symfony.com/schema/dic/services https://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>
        <service id="App\Mailer"/>

        <!-- this replaces the old App\Mailer definition with the new
             one, the old definition is lost -->
        <service id="App\Mailer" class="App\NewMailer"/>
    </services>
</container>
    

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        // config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;

use App\Mailer;
use App\NewMailer;

return function(ContainerConfigurator $configurator) {
    $services = $configurator->services();

    $services->set(Mailer::class);

    // this replaces the old App\Mailer definition with the new one, the
    // old definition is lost
    $services->set(Mailer::class, NewMailer::class);
};
    

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 App\DecoratingMailer.inner:

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

    App\DecoratingMailer:
        # overrides the App\Mailer service
        # but that service is still available as App\DecoratingMailer.inner
        decorates: App\Mailer
    

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        <!-- config/services.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsd="http://www.w3.org/2001/XMLSchema-instance"
    xsd:schemaLocation="http://symfony.com/schema/dic/services https://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>
        <service id="App\Mailer"/>

        <service id="App\DecoratingMailer"
            decorates="App\Mailer"
        />

    </services>
</container>
    

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        // config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;

use App\DecoratingMailer;
use App\Mailer;

return function(ContainerConfigurator $configurator) {
    $services = $configurator->services();

    $services->set(Mailer::class);

    $services->set(DecoratingMailer::class)
        // overrides the App\Mailer service
        // but that service is still available as App\DecoratingMailer.inner
        ->decorate(Mailer::class);
};
    

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 decorating_service_id + '.inner'):

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

    App\DecoratingMailer:
        decorates: App\Mailer
        # pass the old service as an argument
        arguments: ['@App\DecoratingMailer.inner']
    

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        <!-- config/services.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsd="http://www.w3.org/2001/XMLSchema-instance"
    xsd:schemaLocation="http://symfony.com/schema/dic/services https://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>
        <service id="App\Mailer"/>

        <service id="App\DecoratingMailer"
            decorates="App\Mailer"
        >
            <argument type="service" id="App\DecoratingMailer.inner"/>
        </service>

    </services>
</container>
    

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        // config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;

use App\DecoratingMailer;
use App\Mailer;

return function(ContainerConfigurator $configurator) {
    $services = $configurator->services();

    $services->set(Mailer::class);

    $services->set(DecoratingMailer::class)
        ->decorate(Mailer::class)
        // pass the old service as an argument
        ->args([ref(DecoratingMailer::class.'.inner')]);
};
    

Tip

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

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']
    

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        <!-- config/services.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsd="http://www.w3.org/2001/XMLSchema-instance"
    xsd:schemaLocation="http://symfony.com/schema/dic/services https://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>
        <!-- ... -->

        <service
            id="App\DecoratingMailer"
            decorates="App\Mailer"
            decoration-inner-name="App\DecoratingMailer.wooz"
            public="false"
        >
            <argument type="service" id="App\DecoratingMailer.wooz"/>
        </service>

    </services>
</container>
    

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        // config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;

use App\DecoratingMailer;
use App\Mailer;

return function(ContainerConfigurator $configurator) {
    $services = $configurator->services();

    $services->set(Mailer::class);

    $services->set(DecoratingMailer::class)
        ->decorate(Mailer::class, DecoratingMailer::class.'.wooz')
        ->args([ref(DecoratingMailer::class.'.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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        # config/services.yaml
Foo: ~

Bar:
    public: false
    decorates: Foo
    decoration_priority: 5
    arguments: ['@Bar.inner']

Baz:
    public: false
    decorates: Foo
    decoration_priority: 1
    arguments: ['@Baz.inner']
    

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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="Foo"/>

        <service id="Bar" decorates="Foo" decoration-priority="5" public="false">
            <argument type="service" id="Bar.inner"/>
        </service>

        <service id="Baz" decorates="Foo" decoration-priority="1" public="false">
            <argument type="service" id="Baz.inner"/>
        </service>
    </services>
</container>
    

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        // config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;

return function(ContainerConfigurator $configurator) {
    $services = $configurator->services();

    $services->set(Foo::class);

    $services->set(Bar::class)
        ->private()
        ->decorate(Foo::class, null, 5)
        ->args([ref(Bar::class.'.inner')]);

    $services->set(Baz::class)
        ->private()
        ->decorate(Foo::class, null, 1)
        ->args([ref(Baz::class.'.inner')]);
};
    

The generated code will be the following:

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

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