wetboew_webform_example-1.0.x-dev/src/Form/SettingsForm.php

<?php

namespace Drupal\wetboew_webform_example\Form;

use Drupal\Core\Form\ConfigFormBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Configure WetBoew Webform Example settings for this site.
 */
class SettingsForm extends ConfigFormBase {

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'wetboew_webform_example_settings';
  }

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

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['example'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Example setting'),
      '#default_value' => $this->config('wetboew_webform_example.settings')->get('example'),
    ];
    return parent::buildForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    if (is_numeric($form_state->getValue('example'))) {
      $form_state->setErrorByName('example', $this->t('The value is not supposed to be a number, please make it alpha numeric or alpha string.'));
    }
    parent::validateForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $this->config('wetboew_webform_example.settings')
      ->set('example', $form_state->getValue('example'))
      ->save();
    parent::submitForm($form, $form_state);
  }

}