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;
    }

}