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How to Inject Values Based on Complex Expressions

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How to Inject Values Based on Complex ExpressionsΒΆ

The service container also supports an "expression" that allows you to inject very specific values into a service.

For example, suppose you have a third service (not shown here), called mailer_configuration, which has a getMailerMethod() method on it, which will return a string like sendmail based on some configuration. Remember that the first argument to the my_mailer service is the simple string sendmail:

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    # app/config/services.yml
services:
    app.mailer:
        class:        AppBundle\Mailer
        arguments:    [sendmail]
  • XML
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    <!-- app/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
        http://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>
        <service id="app.mailer" class="AppBundle\Mailer">
            <argument>sendmail</argument>
        </service>
    </services>
</container>
  • PHP
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    // app/config/services.php
use AppBundle\Mailer;

$container->register('app.mailer', Mailer::class)
    ->addArgument('sendmail');

But instead of hardcoding this, how could we get this value from the getMailerMethod() of the new mailer_configuration service? One way is to use an expression:

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    # app/config/services.yml
services:
    my_mailer:
        class:     AppBundle\Mailer
        arguments: ["@=service('mailer_configuration').getMailerMethod()"]
  • XML
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    <!-- app/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
        http://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>
        <service id="my_mailer" class="AppBundle\Mailer">
            <argument type="expression">service('mailer_configuration').getMailerMethod()</argument>
        </service>
    </services>
</container>
  • PHP
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    // app/config/services.php
use AppBundle\Mailer;
use Symfony\Component\ExpressionLanguage\Expression;

$container->register('my_mailer', Mailer::class)
    ->addArgument(new Expression('service("mailer_configuration").getMailerMethod()'));

To learn more about the expression language syntax, see The Expression Syntax.

In this context, you have access to 2 functions:

service
Returns a given service (see the example above).
parameter
Returns a specific parameter value (syntax is just like service).

You also have access to the Container via a container variable. Here's another example:

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    services:
    my_mailer:
        class:     AppBundle\Mailer
        arguments: ["@=container.hasParameter('some_param') ? parameter('some_param') : 'default_value'"]
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    <?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
        http://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>
        <service id="my_mailer" class="AppBundle\Mailer">
            <argument type="expression">container.hasParameter('some_param') ? parameter('some_param') : 'default_value'</argument>
        </service>
    </services>
</container>
  • PHP
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    use AppBundle\Mailer;
use Symfony\Component\ExpressionLanguage\Expression;

$container->register('my_mailer', Mailer::class)
    ->addArgument(new Expression(
        "container.hasParameter('some_param') ? parameter('some_param') : 'default_value'"
    ));

Expressions can be used in arguments, properties, as arguments with configurator and as arguments to calls (method calls).

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