9.15.4. Dependencies Injection in ConfigFormBase settings form.

In previous articles, we discussed what Services, Dependency Injection (DI) are and how to use them in your controller, block and form:

9.15. Services and Dependency Injection.
9.15.1. Dependency Injection in Controller
9.15.2. Dependency Injection in BlockBase
9.15.3. Dependencies Injection in BaseForm

Code examples can be viewed on github:
https://github.com/levmyshkin/drupalbook8

In this article, an example of how to add services via DI to a configuration form class inheriting ConfigFormBase. You can download Commerce Order Number module from drupal.org:
https://www.drupal.org/project/commerce_order_number 

/modules/contrib/commerce_order_number/src/Form/SettingsForm.php:

<?php

namespace Drupal\commerce_order_number\Form;

use Drupal\commerce_order_number\OrderNumber;
use Drupal\commerce_order_number\OrderNumberFormatterInterface;
use Drupal\commerce_order_number\OrderNumberGeneratorManager;
use Drupal\Core\Config\ConfigFactoryInterface;
use Drupal\Core\Form\ConfigFormBase;
use Drupal\Core\Form\FormStateInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Configure commerce_order_number settings for this site.
 */
class SettingsForm extends ConfigFormBase {

  /**
   * The order number generator manager.
   *
   * @var \Drupal\commerce_order_number\OrderNumberGeneratorManager
   */
  protected $orderNumberGeneratorManager;

  /**
   * The order number formatter.
   *
   * @var \Drupal\commerce_order_number\OrderNumberFormatterInterface
   */
  protected $orderNumberFormatter;

  /**
   * Constructs a new SettingsForm object.
   *
   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
   *   The factory for configuration objects.
   * @param \Drupal\commerce_order_number\OrderNumberGeneratorManager $order_number_generator_manager
   *   The order number generator manager.
   * @param \Drupal\commerce_order_number\OrderNumberFormatterInterface $order_number_formatter
   *   The order number formatter.
   */
  public function __construct(ConfigFactoryInterface $config_factory, OrderNumberGeneratorManager $order_number_generator_manager, OrderNumberFormatterInterface $order_number_formatter) {
    parent::__construct($config_factory);

    $this->orderNumberGeneratorManager = $order_number_generator_manager;
    $this->orderNumberFormatter = $order_number_formatter;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('config.factory'),
      $container->get('plugin.manager.commerce_order_number_generator'),
      $container->get('commerce_order_number.order_number_formatter')
    );
  }

  /**
   * @inheritDoc
   */
  protected function getEditableConfigNames() {
    return ['commerce_order_number.settings'];
  }

  /**
   * @inheritDoc
   */
  public function getFormId() {
    return 'commerce_order_number_settings';
  }

  /**
   * @inheritDoc
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $config = $this->config('commerce_order_number.settings');
    ...
    return parent::buildForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $this->config('commerce_order_number.settings')
      ->set('generator', $form_state->getValue('generator'))
      ->set('padding', $form_state->getValue('padding'))
      ->set('pattern', $form_state->getValue('pattern'))
      ->set('force', $form_state->getValue('force'))
      ->save();

    parent::submitForm($form, $form_state);
  }

  /**
   * @inheritDoc
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    parent::validateForm($form, $form_state);
    ...
    $example_order_number_formatted = $this->orderNumberFormatter->format($example_order_number, $padding, $pattern);
    ...
  }

}

Services for a configuration form are connected in the same way as for a regular form, using the __construct() and create() methods.