How do I inject services in a service?

Question

How do I inject services in a service?

I am using the following code, but it throws an error.

class RoleNegotiator implements ThemeNegotiatorInterface {

  public static function create(ContainerInterface $interface) {
    return new static(
      $interface->get('router.admin_context')
    );
  }
  public function __construct(RouteMatchInterface $route) {
    $this->route = $route;
  }
}

Answer 1

You define the necessary services as arguments of the service you implement.

Since you are showing a class that implements ThemeNegotiatorInterface, see the theme.negotiator.default service and the class implementing it, Drupal\Core\Theme\DefaultNegotiator.

Service definition

  theme.negotiator.default:
    class: Drupal\Core\Theme\DefaultNegotiator
    arguments: ['@config.factory']
    tags:
      - { name: theme_negotiator, priority: -100 }

Class constructor

public function __construct(ConfigFactoryInterface $config_factory) {
  $this->configFactory = $config_factory;
}

There isn't any static create() method as in your code.

Answer 2

You need to implement ContainerInjectionInterface as well to inject the dependency in your class (if its not a service). But if you are creating a new service you can inject it in module.services.yml itself as said by @kiamlaluno in his answer.

class RoleNegotiator implements ThemeNegotiatorInterface, ContainerInjectionInterface {

}