TestSettingsValidationBlock.php

<?php

declare (strict_types=1);
namespace Drupal\block_test\Plugin\Block;

use Drupal\Core\Block\Attribute\Block;
use Drupal\Core\Block\BlockBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\StringTranslation\TranslatableMarkup;

/**
 * Provides a test settings validation block.
 */
class TestSettingsValidationBlock extends BlockBase {
  
  /**
   * {@inheritdoc}
   */
  public function blockForm($form, FormStateInterface $form_state) {
    return [
      'digits' => [
        '#type' => 'textfield',
      ],
    ] + $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function blockValidate($form, FormStateInterface $form_state) {
    if (!ctype_digit($form_state->getValue('digits'))) {
      $form_state->setErrorByName('digits', $this->t('Only digits are allowed'));
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function build() {
    return [
      '#markup' => 'foo',
    ];
  }

}