class Item

class Item extends ContentEntityBase implements ItemInterface {
    
    /**
     * {@inheritdoc}
     */
    public function label() {
        return $this->get('title')->value;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
        
        /** @var \Drupal\Core\Field\BaseFieldDefinition[] $fields */
        $fields = parent::baseFieldDefinitions($entity_type);
        $fields['iid']->setLabel(t('Aggregator item ID'))
            ->setDescription(t('The ID of the feed item.'));
        $fields['langcode']->setLabel(t('Language code'))
            ->setDescription(t('The feed item language code.'));
        $fields['fid'] = BaseFieldDefinition::create('entity_reference')->setLabel(t('Source feed'))
            ->setRequired(TRUE)
            ->setDescription(t('The aggregator feed entity associated with this item.'))
            ->setSetting('target_type', 'aggregator_feed')
            ->setDisplayOptions('view', [
            'label' => 'hidden',
            'type' => 'entity_reference_label',
            'weight' => 0,
        ])
            ->setDisplayConfigurable('form', TRUE);
        $fields['title'] = BaseFieldDefinition::create('string')->setLabel(t('Title'))
            ->setDescription(t('The title of the feed item.'));
        $fields['link'] = BaseFieldDefinition::create('uri')->setLabel(t('Link'))
            ->setDescription(t('The link of the feed item.'))
            ->setDisplayOptions('view', [
            'region' => 'hidden',
        ])
            ->setDisplayConfigurable('view', TRUE);
        $fields['author'] = BaseFieldDefinition::create('string')->setLabel(t('Author'))
            ->setDescription(t('The author of the feed item.'))
            ->setDisplayOptions('view', [
            'label' => 'hidden',
            'weight' => 3,
        ])
            ->setDisplayConfigurable('view', TRUE);
        $fields['description'] = BaseFieldDefinition::create('string_long')->setLabel(t('Description'))
            ->setDescription(t('The body of the feed item.'));
        $fields['timestamp'] = BaseFieldDefinition::create('created')->setLabel(t('Posted on'))
            ->setDescription(t('Posted date of the feed item, as a Unix timestamp.'))
            ->setDisplayOptions('view', [
            'label' => 'hidden',
            'type' => 'timestamp_ago',
            'weight' => 1,
        ])
            ->setDisplayConfigurable('view', TRUE);
        // @todo Convert to a real UUID field in
        //   https://www.drupal.org/node/2149851.
        $fields['guid'] = BaseFieldDefinition::create('string_long')->setLabel(t('GUID'))
            ->setDescription(t('Unique identifier for the feed item.'));
        return $fields;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFeedId() {
        return $this->get('fid')->target_id;
    }
    
    /**
     * {@inheritdoc}
     */
    public function setFeedId($fid) {
        return $this->set('fid', $fid);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getTitle() {
        return $this->get('title')->value;
    }
    
    /**
     * {@inheritdoc}
     */
    public function setTitle($title) {
        return $this->set('title', $title);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getLink() {
        return $this->get('link')->value;
    }
    
    /**
     * {@inheritdoc}
     */
    public function setLink($link) {
        return $this->set('link', $link);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getAuthor() {
        return $this->get('author')->value;
    }
    
    /**
     * {@inheritdoc}
     */
    public function setAuthor($author) {
        return $this->set('author', $author);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getDescription() {
        return $this->get('description')->value;
    }
    
    /**
     * {@inheritdoc}
     */
    public function setDescription($description) {
        return $this->set('description', $description);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getPostedTime() {
        return $this->get('timestamp')->value;
    }
    
    /**
     * {@inheritdoc}
     */
    public function setPostedTime($timestamp) {
        return $this->set('timestamp', $timestamp);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getGuid() {
        return $this->get('guid')->value;
    }
    
    /**
     * {@inheritdoc}
     */
    public function setGuid($guid) {
        return $this->set('guid', $guid);
    }
    
    /**
     * {@inheritdoc}
     */
    public function postSave(EntityStorageInterface $storage, $update = TRUE) {
        parent::postSave($storage, $update);
        // Entity::postSave() calls Entity::invalidateTagsOnSave(), which only
        // handles the regular cases. The Item entity has one special case: a newly
        // created Item is *also* associated with a Feed, so we must invalidate the
        // associated Feed's cache tag.
        if (!$update) {
            Cache::invalidateTags($this->getCacheTagsToInvalidate());
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function getCacheTagsToInvalidate() {
        return Feed::load($this->getFeedId())
            ->getCacheTags();
    }
    
    /**
     * Entity URI callback.
     */
    public static function buildUri(ItemInterface $item) {
        return Url::fromUri($item->getLink());
    }

}