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 App\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: App\Mailer. One way to do this is with an expression:

1
2
3
4
5
6
7
8
9
10
11

# config/services.yaml
services:
    # ...

    App\Mail\MailerConfiguration: ~

    App\Mailer:
        # the '@=' prefix is required when using expressions for arguments in YAML files
        arguments: ['@=service("App\\Mail\\MailerConfiguration").getMailerMethod()']
        # when using double-quoted strings, the backslash needs to be escaped twice (see https://yaml.org/spec/1.2/spec.html#id2787109)
        # arguments: ["@=service('App\\\\Mail\\\\MailerConfiguration').getMailerMethod()"]

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

<!-- 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\MailerConfiguration"></service>

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

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15

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

use App\Mail\MailerConfiguration;
use App\Mailer;

return function(ContainerConfigurator $configurator) {
    // ...

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

    $services->set(Mailer::class)
        // because of the escaping applied by PHP, you must add 4 backslashes for each original backslash
        ->args([expr("service('App\\\\Mail\\\\MailerConfiguration').getMailerMethod()")]);
};

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

In this context, you have access to 3 functions:

service

Returns a given service (see the example above).

parameter

Returns a specific parameter value (syntax is like service).

env

Returns the value of an env variable.

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

1
2
3
4
5

# config/services.yaml
services:
    App\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
13

<!-- 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\Mailer">
            <argument type="expression">container.hasParameter('some_param') ? parameter('some_param') : 'default_value'</argument>
        </service>
    </services>
</container>

1
2
3
4
5
6
7
8
9
10
11

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

use App\Mailer;

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

    $services->set(Mailer::class)
        ->args([expr("container.hasParameter('some_param') ? parameter('some_param') : 'default_value'")]);
};

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