class RoleSettingsForm

class RoleSettingsForm extends FormBase {
    
    /**
     * The role storage used when changing the admin role.
     *
     * @var \Drupal\user\RoleStorageInterface
     */
    protected $roleStorage;
    
    /**
     * Constructs a \Drupal\user\Form\RoleSettingsForm object.
     *
     * @param \Drupal\user\RoleStorageInterface $role_storage
     *   The role storage.
     */
    public function __construct(RoleStorageInterface $role_storage) {
        $this->roleStorage = $role_storage;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('entity_type.manager')
            ->getStorage('user_role'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'role_settings';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        // Administrative role option.
        $form['admin_role'] = [
            '#type' => 'details',
            '#title' => $this->t('Administrator role'),
            '#open' => TRUE,
        ];
        // Do not allow users to set the anonymous or authenticated user roles as
        // the administrator role.
        $roles = user_role_names(TRUE);
        unset($roles[RoleInterface::AUTHENTICATED_ID]);
        $admin_roles = $this->roleStorage
            ->getQuery()
            ->condition('is_admin', TRUE)
            ->execute();
        $default_value = reset($admin_roles);
        $form['admin_role']['user_admin_role'] = [
            '#type' => 'select',
            '#title' => $this->t('Administrator role'),
            '#empty_value' => '',
            '#default_value' => $default_value,
            '#options' => $roles,
            '#description' => $this->t('This role will be automatically granted all permissions.'),
            // Don't allow to select a single admin role in case multiple roles got
            // marked as admin role already.
'#access' => count($admin_roles) <= 1,
        ];
        $form['actions'] = [
            '#type' => 'actions',
        ];
        $form['actions']['submit'] = [
            '#type' => 'submit',
            '#value' => $this->t('Save configuration'),
            '#button_type' => 'primary',
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        if ($form_state->hasValue('user_admin_role')) {
            $admin_roles = $this->roleStorage
                ->getQuery()
                ->condition('is_admin', TRUE)
                ->execute();
            foreach ($admin_roles as $rid) {
                $this->roleStorage
                    ->load($rid)
                    ->setIsAdmin(FALSE)
                    ->save();
            }
            $new_admin_role = $form_state->getValue('user_admin_role');
            if ($new_admin_role) {
                $this->roleStorage
                    ->load($new_admin_role)
                    ->setIsAdmin(TRUE)
                    ->save();
            }
        }
    }

}