class SiteInformationForm

system.routing.yml in drupal/core/modules/system/system.routing.yml

drupal/core/modules/system/system.routing.yml

class SiteInformationForm extends SystemConfigFormBase {

  /**
   * The path alias manager.
   *
   * @var \Drupal\Core\Path\AliasManagerInterface
   */
  protected $aliasManager;

  /**
   * Constructs a SiteInformationForm object.
   *
   * @param \Drupal\Core\Config\ConfigFactory $config_factory
   *   The factory for configuration objects.
   * @param \Drupal\Core\Config\Context\ContextInterface $context
   *   The configuration context used for this configuration object.
   * @param \Drupal\Core\Path\AliasManagerInterface $alias_manager
   *   The path alias manager.
   */
  public function __construct(ConfigFactory $config_factory, ContextInterface $context, AliasManagerInterface $alias_manager) {
    parent::__construct($config_factory, $context);
    $this->aliasManager = $alias_manager;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static($container
      ->get('config.factory'), $container
      ->get('config.context.free'), $container
      ->get('path.alias_manager'));
  }

  /**
   * {@inheritdoc}
   */
  public function getFormID() {
    return 'system_site_information_settings';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, array &$form_state) {
    $site_config = $this->configFactory
      ->get('system.site');
    $site_mail = $site_config
      ->get('mail');
    if (empty($site_mail)) {
      $site_mail = ini_get('sendmail_from');
    }
    $form['site_information'] = array(
      '#type' => 'details',
      '#title' => t('Site details'),
    );
    $form['site_information']['site_name'] = array(
      '#type' => 'textfield',
      '#title' => t('Site name'),
      '#default_value' => $site_config
        ->get('name'),
      '#required' => TRUE,
    );
    $form['site_information']['site_slogan'] = array(
      '#type' => 'textfield',
      '#title' => t('Slogan'),
      '#default_value' => $site_config
        ->get('slogan'),
      '#description' => t("How this is used depends on your site's theme."),
    );
    $form['site_information']['site_mail'] = array(
      '#type' => 'email',
      '#title' => t('E-mail address'),
      '#default_value' => $site_mail,
      '#description' => t("The <em>From</em> address in automated e-mails sent during registration and new password requests, and other notifications. (Use an address ending in your site's domain to help prevent this e-mail being flagged as spam.)"),
      '#required' => TRUE,
    );
    $form['front_page'] = array(
      '#type' => 'details',
      '#title' => t('Front page'),
    );
    $front_page = $site_config
      ->get('page.front') != 'user' ? $this->aliasManager
      ->getPathAlias($site_config
      ->get('page.front')) : '';
    $form['front_page']['site_frontpage'] = array(
      '#type' => 'textfield',
      '#title' => t('Default front page'),
      '#default_value' => $front_page,
      '#size' => 40,
      '#description' => t('Optionally, specify a relative URL to display as the front page. Leave blank to display the default front page.'),
      '#field_prefix' => url(NULL, array(
        'absolute' => TRUE,
      )),
    );
    $form['error_page'] = array(
      '#type' => 'details',
      '#title' => t('Error pages'),
    );
    $form['error_page']['site_403'] = array(
      '#type' => 'textfield',
      '#title' => t('Default 403 (access denied) page'),
      '#default_value' => $site_config
        ->get('page.403'),
      '#size' => 40,
      '#description' => t('This page is displayed when the requested document is denied to the current user. Leave blank to display a generic "access denied" page.'),
      '#field_prefix' => url(NULL, array(
        'absolute' => TRUE,
      )),
    );
    $form['error_page']['site_404'] = array(
      '#type' => 'textfield',
      '#title' => t('Default 404 (not found) page'),
      '#default_value' => $site_config
        ->get('page.404'),
      '#size' => 40,
      '#description' => t('This page is displayed when no other content matches the requested document. Leave blank to display a generic "page not found" page.'),
      '#field_prefix' => url(NULL, array(
        'absolute' => TRUE,
      )),
    );
    return parent::buildForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, array &$form_state) {

    // Check for empty front page path.
    if (empty($form_state['values']['site_frontpage'])) {

      // Set to default "user".
      form_set_value($form['front_page']['site_frontpage'], 'user', $form_state);
    }
    else {

      // Get the normal path of the front page.
      form_set_value($form['front_page']['site_frontpage'], $this->aliasManager
        ->getSystemPath($form_state['values']['site_frontpage']), $form_state);
    }

    // Validate front page path.
    if (!drupal_valid_path($form_state['values']['site_frontpage'])) {
      form_set_error('site_frontpage', t("The path '%path' is either invalid or you do not have access to it.", array(
        '%path' => $form_state['values']['site_frontpage'],
      )));
    }

    // Get the normal paths of both error pages.
    if (!empty($form_state['values']['site_403'])) {
      form_set_value($form['error_page']['site_403'], $this->aliasManager
        ->getSystemPath($form_state['values']['site_403']), $form_state);
    }
    if (!empty($form_state['values']['site_404'])) {
      form_set_value($form['error_page']['site_404'], $this->aliasManager
        ->getSystemPath($form_state['values']['site_404']), $form_state);
    }

    // Validate 403 error path.
    if (!empty($form_state['values']['site_403']) && !drupal_valid_path($form_state['values']['site_403'])) {
      form_set_error('site_403', t("The path '%path' is either invalid or you do not have access to it.", array(
        '%path' => $form_state['values']['site_403'],
      )));
    }

    // Validate 404 error path.
    if (!empty($form_state['values']['site_404']) && !drupal_valid_path($form_state['values']['site_404'])) {
      form_set_error('site_404', t("The path '%path' is either invalid or you do not have access to it.", array(
        '%path' => $form_state['values']['site_404'],
      )));
    }
    parent::validateForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, array &$form_state) {
    $this->configFactory
      ->get('system.site')
      ->set('name', $form_state['values']['site_name'])
      ->set('mail', $form_state['values']['site_mail'])
      ->set('slogan', $form_state['values']['site_slogan'])
      ->set('page.front', $form_state['values']['site_frontpage'])
      ->set('page.403', $form_state['values']['site_403'])
      ->set('page.404', $form_state['values']['site_404'])
      ->save();
    parent::submitForm($form, $form_state);
  }

}