contextual-block-example.php

<?php

namespace Drupal\module\Plugin\Block;

use Drupal\contextual_views\Plugin\Block\ContextualViewsBlock;
use Drupal\Core\Form\FormStateInterface;
use Drupal\node\NodeInterface;

/**
 * Provides a 'NewsByChannel' block.
 *
 * @Block(
 *  id = "views_block:news-news_by_channel",
 *  admin_label = @Translation("News by channel"),
 *  category = @Translation("News"),
 *  context = {
 *   "node" = @ContextDefinition("entity:node", required = FALSE)
 *  }
 * )
 */
class NewsByChannel extends ContextualViewsBlock {

  /**
   * {@inheritdoc}
   */
  public function defaultConfiguration() {
    return [
      'channel' => '',
    ] + parent::defaultConfiguration();
  }

  /**
   * {@inheritdoc}
   */
  public function blockForm($form, FormStateInterface $form_state) {
    $form = parent::blockForm($form, $form_state);

    // @todo: This should make use of a data selector.
    $form['channel'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Channel field'),
      '#description' => $this->t('The field on the node (context) containing the channel.'),
      '#default_value' => $this->configuration['channel'],
      '#weight' => '10',
      '#required' => TRUE,
    ];
    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function blockSubmit($form, FormStateInterface $form_state) {
    parent::blockSubmit($form, $form_state);
    $this->configuration['channel'] = $form_state->getValue('channel');
  }

  /**
   * {@inheritdoc}
   */
  public function build() {
    $node = $this->getContextValue('node');
    assert($node instanceof NodeInterface);

    $channel = NULL;
    $field_name = $this->configuration['channel'];

    if ($node->hasField($field_name)) {
      $channel = $node->get($field_name)->entity ? $node->get($field_name)->entity->id() : NULL;
    }
    $this->view->setArguments([$channel]);
    return parent::build();
  }

}