TestBlockInstantiation.php

<?php

namespace Drupal\block_test\Plugin\Block;

use Drupal\Core\Access\AccessResult;
use Drupal\Core\Block\BlockBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Session\AccountInterface;

/**
 * Provides a basic block for testing block instantiation and configuration.
 *
 * @Block(
 *   id = "test_block_instantiation",
 *   admin_label = @Translation("Display message")
 * )
 */
class TestBlockInstantiation extends BlockBase {
    
    /**
     * {@inheritdoc}
     */
    public function defaultConfiguration() {
        return [
            'display_message' => 'no message set',
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    protected function blockAccess(AccountInterface $account) {
        return AccessResult::allowedIfHasPermission($account, 'access content');
    }
    
    /**
     * {@inheritdoc}
     */
    public function blockForm($form, FormStateInterface $form_state) {
        $form['display_message'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Display message'),
            '#default_value' => $this->configuration['display_message'],
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function blockSubmit($form, FormStateInterface $form_state) {
        $this->configuration['display_message'] = $form_state->getValue('display_message');
    }
    
    /**
     * {@inheritdoc}
     */
    public function build() {
        return [
            '#children' => $this->configuration['display_message'],
        ];
    }

}