Inject configuration values into services in Drupal 8

in this article, I'll show you how to inject configuration values into your custom service using service factory.

Create mail_module.services.yml file:

services:
  app.mailer:
    class:       Drupal/mail_module/Mailer
    factory:      Drupal/mail_module/MailerFactory:create
    arguments:    ['@config.factory']

Create MailerFactory.php file:

class MailerFactory {
    static function create($config) {
        return new Mailer($config->get('mail.config')->get('transport'));
    }
}

Create MailerFactory.php file:

class Mailer {
    public function __construct($transport) {
        $this->mailTransport = $transport;
    }
}