AccountSettingsForm::submitForm

public AccountSettingsForm::submitForm(array &$form, FormStateInterface $form_state)

public function submitForm(array &$form, FormStateInterface $form_state) {
  parent::submitForm($form, $form_state);

  $this->config('user.settings')
    ->set('anonymous', $form_state->getValue('anonymous'))
    ->set('register', $form_state->getValue('user_register'))
    ->set('password_strength', $form_state->getValue('user_password_strength'))
    ->set('verify_mail', $form_state->getValue('user_email_verification'))
    ->set('cancel_method', $form_state->getValue('user_cancel_method'))
    ->set('notify.status_activated', $form_state->getValue('user_mail_status_activated_notify'))
    ->set('notify.status_blocked', $form_state->getValue('user_mail_status_blocked_notify'))
    ->set('notify.status_canceled', $form_state->getValue('user_mail_status_canceled_notify'))
    ->save();
  $this->config('user.mail')
    ->set('cancel_confirm.body', $form_state->getValue('user_mail_cancel_confirm_body'))
    ->set('cancel_confirm.subject', $form_state->getValue('user_mail_cancel_confirm_subject'))
    ->set('password_reset.body', $form_state->getValue('user_mail_password_reset_body'))
    ->set('password_reset.subject', $form_state->getValue('user_mail_password_reset_subject'))
    ->set('register_admin_created.body', $form_state->getValue('user_mail_register_admin_created_body'))
    ->set('register_admin_created.subject', $form_state->getValue('user_mail_register_admin_created_subject'))
    ->set('register_no_approval_required.body', $form_state->getValue('user_mail_register_no_approval_required_body'))
    ->set('register_no_approval_required.subject', $form_state->getValue('user_mail_register_no_approval_required_subject'))
    ->set('register_pending_approval.body', $form_state->getValue('user_mail_register_pending_approval_body'))
    ->set('register_pending_approval.subject', $form_state->getValue('user_mail_register_pending_approval_subject'))
    ->set('register_pending_approval_admin.body', $form_state->getValue('register_pending_approval_admin_body'))
    ->set('register_pending_approval_admin.subject', $form_state->getValue('register_pending_approval_admin_subject'))
    ->set('status_activated.body', $form_state->getValue('user_mail_status_activated_body'))
    ->set('status_activated.subject', $form_state->getValue('user_mail_status_activated_subject'))
    ->set('status_blocked.body', $form_state->getValue('user_mail_status_blocked_body'))
    ->set('status_blocked.subject', $form_state->getValue('user_mail_status_blocked_subject'))
    ->set('status_canceled.body', $form_state->getValue('user_mail_status_canceled_body'))
    ->set('status_canceled.subject', $form_state->getValue('user_mail_status_canceled_subject'))
    ->save();
  $this->config('system.site')
    ->set('mail_notification', $form_state->getValue('mail_notification_address'))
    ->save();

  // Change the admin role.
  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();
    }
  }
}