ExchangeRateForm.php

<?php

namespace Drupal\guia_commerce\Form;

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

/**
 * Class ExchangeRateForm.
 */
class ExchangeRateForm extends ConfigFormBase {

  /**
   * {@inheritdoc}
   */
  protected function getEditableConfigNames() {
    return [
      'guia_commerce.exchangerate',
    ];
  }

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

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    #$config = $this->config('guia_commerce.exchangerate');
    $config = $this->config('guia_commerce.exchangerate');
    $form['exchange_rate_us'] = [
      '#type' => 'textfield',
      '#element_validate' => array('_element_validate_number'),
      '#title' => $this->t('USD Dólar americano a BOB Boliviano'),
      '#default_value' => $config->get('exchange_rate_us'),
    ];
    $form['exchange_rate_bo'] = [
      '#type' => 'textfield',
      '#element_validate' => array('_element_validate_number'),
      '#title' => $this->t('BOB Boliviano a USD Dólar americano'),
      '#default_value' => $config->get('exchange_rate_bo'),
    ];

    return parent::buildForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    $value_us = $form_state->getValue('exchange_rate_us');
    $value_bo = $form_state->getValue('exchange_rate_bo');
    if (!is_numeric($value_us)) {
      $form_state->setErrorByName('exchange_rate_us', $this->t('El valor debe ser numérico'));
    }
    if (!is_numeric($value_bo)) {
      $form_state->setErrorByName('exchange_rate_bo', $this->t('El valor debe ser numérico'));
    }
    parent::validateForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    parent::submitForm($form, $form_state);

    $this->config('guia_commerce.exchangerate')
      ->set('exchange_rate_us', $form_state->getValue('exchange_rate_us'))
      ->set('exchange_rate_bo', $form_state->getValue('exchange_rate_bo'))
      ->save();
  }

}