TestAttributesBlock.php

<?php

namespace Drupal\layout_builder_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 'TestAttributes' block.
 */
class TestAttributesBlock extends BlockBase {
    
    /**
     * {@inheritdoc}
     */
    public function blockForm($form, FormStateInterface $form_state) {
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function build() {
        $build = [
            '#attributes' => [
                'class' => [
                    'attribute-test-class',
                ],
                'custom-attribute' => 'test',
            ],
            '#markup' => $this->t('Example block providing its own attributes.'),
            '#contextual_links' => [
                'layout_builder_test' => [
                    'route_parameters' => [],
                ],
            ],
        ];
        return $build;
    }

}