class AggregatorFeedBlock

class AggregatorFeedBlock extends BlockBase implements ContainerFactoryPluginInterface {
  
  /**
   * The entity storage for feeds.
   *
   * @var \Drupal\aggregator\FeedStorageInterface
   */
  protected $feedStorage;
  
  /**
   * The entity storage for items.
   *
   * @var \Drupal\aggregator\ItemStorageInterface
   */
  protected $itemStorage;
  
  /**
   * Constructs an AggregatorFeedBlock object.
   *
   * @param array $configuration
   *   A configuration array containing information about the plugin instance.
   * @param string $plugin_id
   *   The plugin_id for the plugin instance.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param \Drupal\aggregator\FeedStorageInterface $feed_storage
   *   The entity storage for feeds.
   * @param \Drupal\aggregator\ItemStorageInterface $item_storage
   *   The entity storage for feed items.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, FeedStorageInterface $feed_storage, ItemStorageInterface $item_storage) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
    $this->feedStorage = $feed_storage;
    $this->itemStorage = $item_storage;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static($configuration, $plugin_id, $plugin_definition, $container->get('entity_type.manager')
      ->getStorage('aggregator_feed'), $container->get('entity_type.manager')
      ->getStorage('aggregator_item'));
  }
  
  /**
   * {@inheritdoc}
   */
  public function defaultConfiguration() {
    // By default, the block will contain 10 feed items.
    return [
      'block_count' => 10,
      'feed' => NULL,
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  protected function blockAccess(AccountInterface $account) {
    // Only grant access to users with the 'access news feeds' permission.
    return AccessResult::allowedIfHasPermission($account, 'access news feeds');
  }
  
  /**
   * {@inheritdoc}
   */
  public function blockForm($form, FormStateInterface $form_state) {
    $feeds = $this->feedStorage
      ->loadMultiple();
    $options = [];
    foreach ($feeds as $feed) {
      $options[$feed->id()] = $feed->label();
    }
    $form['feed'] = [
      '#type' => 'select',
      '#title' => $this->t('Select the feed that should be displayed'),
      '#default_value' => $this->configuration['feed'],
      '#options' => $options,
    ];
    $range = range(2, 20);
    $form['block_count'] = [
      '#type' => 'select',
      '#title' => $this->t('Number of news items in block'),
      '#default_value' => $this->configuration['block_count'],
      '#options' => array_combine($range, $range),
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function blockSubmit($form, FormStateInterface $form_state) {
    $this->configuration['block_count'] = $form_state->getValue('block_count');
    $this->configuration['feed'] = $form_state->getValue('feed');
  }
  
  /**
   * {@inheritdoc}
   */
  public function build() {
    // Load the selected feed.
    if (!$feed = $this->feedStorage
      ->load($this->configuration['feed'])) {
      return [];
    }
    $result = $this->itemStorage
      ->getQuery()
      ->accessCheck(TRUE)
      ->condition('fid', $feed->id())
      ->range(0, $this->configuration['block_count'])
      ->sort('timestamp', 'DESC')
      ->sort('iid', 'DESC')
      ->execute();
    if ($result) {
      // Only display the block if there are items to show.
      $items = $this->itemStorage
        ->loadMultiple($result);
      $build['list'] = [
        '#theme' => 'item_list',
        '#items' => [],
      ];
      foreach ($items as $item) {
        $build['list']['#items'][$item->id()] = [
          '#type' => 'link',
          '#url' => $item->toUrl(),
          '#title' => $item->label(),
        ];
      }
      $build['more_link'] = [
        '#type' => 'more_link',
        '#url' => $feed->toUrl(),
        '#attributes' => [
          'title' => $this->t("View this feed's recent news."),
        ],
      ];
      return $build;
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function getCacheTags() {
    $cache_tags = parent::getCacheTags();
    if ($feed = $this->feedStorage
      ->load($this->configuration['feed'])) {
      $cache_tags = Cache::mergeTags($cache_tags, $feed->getCacheTags());
    }
    return $cache_tags;
  }

}