How to Inject Values Based on Complex Expressions

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

For example, suppose you have a service (not shown here), called AppBundle\Mail\MailerConfiguration, which has a getMailerMethod() method on it. This returns a string - like sendmail based on some configuration.

Suppose that you want to pass the result of this method as a constructor argument to another service: AppBundle\Mailer. One way to do this is with an expression:

YAML
XML
PHP

        1
2
3
4
5
6
7
8
9
        # app/config/services.yml
services:
    # ...

    AppBundle\Mail\MailerConfiguration: ~

    AppBundle\Mailer:
        # the '@=' prefix is required when using expressions for arguments in YAML files
        arguments: ["@=service('AppBundle\\\\Mail\\\\MailerConfiguration').getMailerMethod()"]
    

        1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
        <!-- 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
        https://symfony.com/schema/dic/services/services-1.0.xsd">

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

        <service id="AppBundle\Mail\MailerConfiguration"></service>

        <service id="AppBundle\Mailer">
            <argument type="expression">service('App\\Mail\\MailerConfiguration').getMailerMethod()</argument>
        </service>
    </services>
</container>
    

        1
2
3
4
5
6
7
8
9
        // app/config/services.php
use AppBundle\Mail\MailerConfiguration;
use AppBundle\Mailer;
use Symfony\Component\ExpressionLanguage\Expression;

$container->autowire(MailerConfiguration::class);

$container->autowire(Mailer::class)
    ->addArgument(new Expression('service("AppBundle\\\\Mail\\\\MailerConfiguration").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:

YAML
XML
PHP

        1
2
3
4
        services:
    AppBundle\Mailer:
        # the '@=' prefix is required when using expressions for arguments in YAML files
        arguments: ["@=container.hasParameter('some_param') ? parameter('some_param') : 'default_value'"]
    

        1
2
3
4
5
6
7
8
9
10
11
12
        <?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="AppBundle\Mailer">
            <argument type="expression">container.hasParameter('some_param') ? parameter('some_param') : 'default_value'</argument>
        </service>
    </services>
</container>
    

        1
2
3
4
5
6
7
        use AppBundle\Mailer;
use Symfony\Component\ExpressionLanguage\Expression;

$container->autowire(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.