TestSettingsValidationBlock.php

<?php

namespace Drupal\block_test\Plugin\Block;

use Drupal\Core\Block\BlockBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Provides a test settings validation block.
 *
 * @Block(
 *  id = "test_settings_validation",
 *  admin_label = @Translation("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',
        ];
    }

}