class CachePluginBase

abstract class CachePluginBase extends PluginBase {
    
    /**
     * Contains all data that should be written/read from cache.
     */
    public $storage = [];
    
    /**
     * Which cache bin to store query results in.
     *
     * @var string
     */
    protected $resultsBin = 'data';
    
    /**
     * Stores the cache ID used for the results cache.
     *
     * The cache ID is stored in generateResultsKey() got executed.
     *
     * @var string
     *
     * @see \Drupal\views\Plugin\views\cache\CachePluginBase::generateResultsKey()
     */
    protected $resultsKey;
    
    /**
     * Returns the resultsKey property.
     *
     * @return string
     *   The resultsKey property.
     */
    public function getResultsKey() {
        return $this->resultsKey;
    }
    
    /**
     * Return a string to display as the clickable title for the
     * access control.
     */
    public function summaryTitle() {
        return $this->t('Unknown');
    }
    
    /**
     * Determine the expiration time of the cache type, or NULL if no expire.
     *
     * Plugins must override this to implement expiration.
     *
     * @param $type
     *   The cache type, either 'query', 'result'.
     */
    protected function cacheExpire($type) {
    }
    
    /**
     * Determine expiration time in the cache table of the cache type
     * or CACHE_PERMANENT if item shouldn't be removed automatically from cache.
     *
     * Plugins must override this to implement expiration in the cache table.
     *
     * @param $type
     *   The cache type, either 'query', 'result'.
     */
    protected function cacheSetMaxAge($type) {
        return Cache::PERMANENT;
    }
    
    /**
     * Save data to the cache.
     *
     * A plugin should override this to provide specialized caching behavior.
     *
     * @param $type
     *   The cache type, either 'query', 'result'.
     */
    public function cacheSet($type) {
        switch ($type) {
            case 'query':
                // Not supported currently, but this is certainly where we'd put it.
                break;
            case 'results':
                $data = [
                    'result' => $this->prepareViewResult($this->view->result),
                    'total_rows' => isset($this->view->total_rows) ? $this->view->total_rows : 0,
                    'current_page' => $this->view
                        ->getCurrentPage(),
                ];
                $expire = $this->cacheSetMaxAge($type) === Cache::PERMANENT ? Cache::PERMANENT : (int) $this->view
                    ->getRequest()->server
                    ->get('REQUEST_TIME') + $this->cacheSetMaxAge($type);
                \Drupal::cache($this->resultsBin)
                    ->set($this->generateResultsKey(), $data, $expire, $this->getCacheTags());
                break;
        }
    }
    
    /**
     * Retrieve data from the cache.
     *
     * A plugin should override this to provide specialized caching behavior.
     *
     * @param $type
     *   The cache type, either 'query', 'result'.
     *
     * @return bool
     *   TRUE if data has been taken from the cache, otherwise FALSE.
     */
    public function cacheGet($type) {
        $cutoff = $this->cacheExpire($type);
        switch ($type) {
            case 'query':
                // Not supported currently, but this is certainly where we'd put it.
                return FALSE;
            case 'results':
                // Values to set: $view->result, $view->total_rows, $view->execute_time,
                // $view->current_page.
                if ($cache = \Drupal::cache($this->resultsBin)
                    ->get($this->generateResultsKey())) {
                    if (!$cutoff || $cache->created > $cutoff) {
                        $this->view->result = $cache->data['result'];
                        // Load entities for each result.
                        $this->view->query
                            ->loadEntities($this->view->result);
                        $this->view->total_rows = $cache->data['total_rows'];
                        $this->view
                            ->setCurrentPage($cache->data['current_page'], TRUE);
                        $this->view->execute_time = 0;
                        return TRUE;
                    }
                }
                return FALSE;
        }
    }
    
    /**
     * Clear out cached data for a view.
     */
    public function cacheFlush() {
        Cache::invalidateTags($this->view->storage
            ->getCacheTagsToInvalidate());
    }
    
    /**
     * Post process any rendered data.
     *
     * This can be valuable to be able to cache a view and still have some level
     * of dynamic output. In an ideal world, the actual output will include HTML
     * comment based tokens, and then the post process can replace those tokens.
     *
     * Example usage. If it is known that the view is a node view and that the
     * primary field will be a nid, you can do something like this:
     * @code
     *   <!--post-FIELD-NID-->
     * @endcode
     *
     * And then in the post render, create an array with the text that should
     * go there:
     *
     * @code
     *   strtr($output, array('<!--post-FIELD-1-->', 'output for FIELD of nid 1');
     * @endcode
     *
     * All of the cached result data will be available in $view->result, as well,
     * so all ids used in the query should be discoverable.
     */
    public function postRender(&$output) {
    }
    
    /**
     * Calculates and sets a cache ID used for the result cache.
     *
     * @return string
     *   The generated cache ID.
     */
    public function generateResultsKey() {
        if (!isset($this->resultsKey)) {
            $build_info = $this->view->build_info;
            foreach ([
                'query',
                'count_query',
            ] as $index) {
                // If the default query back-end is used generate SQL query strings from
                // the query objects.
                if ($build_info[$index] instanceof Select) {
                    $query = clone $build_info[$index];
                    $query->preExecute();
                    $build_info[$index] = [
                        'query' => (string) $query,
                        'arguments' => $query->getArguments(),
                    ];
                }
            }
            $key_data = [
                'build_info' => $build_info,
            ];
            // @todo https://www.drupal.org/node/2433591 might solve it to not require
            //    the pager information here.
            $key_data['pager'] = [
                'page' => $this->view
                    ->getCurrentPage(),
                'items_per_page' => $this->view
                    ->getItemsPerPage(),
                'offset' => $this->view
                    ->getOffset(),
            ];
            $key_data += \Drupal::service('cache_contexts_manager')->convertTokensToKeys($this->displayHandler
                ->getCacheMetadata()
                ->getCacheContexts())
                ->getKeys();
            $this->resultsKey = $this->view->storage
                ->id() . ':' . $this->displayHandler->display['id'] . ':results:' . hash('sha256', serialize($key_data));
        }
        return $this->resultsKey;
    }
    
    /**
     * Gets an array of cache tags for the current view.
     *
     * @return string[]
     *   An array of cache tags based on the current view.
     */
    public function getCacheTags() {
        $tags = $this->view->storage
            ->getCacheTags();
        // The list cache tags for the entity types listed in this view.
        $entity_information = $this->view
            ->getQuery()
            ->getEntityTableInfo();
        if (!empty($entity_information)) {
            // Add the list cache tags for each entity type used by this view.
            foreach ($entity_information as $table => $metadata) {
                $tags = Cache::mergeTags($tags, \Drupal::entityTypeManager()->getDefinition($metadata['entity_type'])
                    ->getListCacheTags());
            }
        }
        $tags = Cache::mergeTags($tags, $this->view
            ->getQuery()
            ->getCacheTags());
        return $tags;
    }
    
    /**
     * Gets the max age for the current view.
     *
     * @return int
     */
    public function getCacheMaxAge() {
        $max_age = $this->getDefaultCacheMaxAge();
        $max_age = Cache::mergeMaxAges($max_age, $this->view
            ->getQuery()
            ->getCacheMaxAge());
        return $max_age;
    }
    
    /**
     * Returns the default cache max age.
     */
    protected function getDefaultCacheMaxAge() {
        // The default cache backend is not caching anything.
        return 0;
    }
    
    /**
     * Prepares the view result before putting it into cache.
     *
     * @param \Drupal\views\ResultRow[] $result
     *   The result containing loaded entities.
     *
     * @return \Drupal\views\ResultRow[]
     *   The result without loaded entities.
     */
    protected function prepareViewResult(array $result) {
        $return = [];
        // Clone each row object and remove any loaded entities, to keep the
        // original result rows intact.
        foreach ($result as $key => $row) {
            $clone = clone $row;
            $clone->resetEntityData();
            $return[$key] = $clone;
        }
        return $return;
    }
    
    /**
     * Alters the cache metadata of a display upon saving a view.
     *
     * @param \Drupal\Core\Cache\CacheableMetadata $cache_metadata
     *   The cache metadata.
     */
    public function alterCacheMetadata(CacheableMetadata $cache_metadata) {
    }
    
    /**
     * Returns the row cache tags.
     *
     * @param \Drupal\views\ResultRow $row
     *   A result row.
     *
     * @return string[]
     *   The row cache tags.
     */
    public function getRowCacheTags(ResultRow $row) {
        $tags = !empty($row->_entity) ? $row->_entity
            ->getCacheTags() : [];
        if (!empty($row->_relationship_entities)) {
            foreach ($row->_relationship_entities as $entity) {
                $tags = Cache::mergeTags($tags, $entity->getCacheTags());
            }
        }
        return $tags;
    }
    
    /**
     * Returns the row cache keys.
     *
     * @param \Drupal\views\ResultRow $row
     *   A result row.
     *
     * @return string[]
     *   The row cache keys.
     */
    public function getRowCacheKeys(ResultRow $row) {
        return [
            'views',
            'fields',
            $this->view
                ->id(),
            $this->view->current_display,
            $this->getRowId($row),
        ];
    }
    
    /**
     * Returns a unique identifier for the specified row.
     *
     * @param \Drupal\views\ResultRow $row
     *   A result row.
     *
     * @return string
     *   The row identifier.
     */
    public function getRowId(ResultRow $row) {
        // Here we compute a unique identifier for the row by computing the hash of
        // its data. We exclude the current index, since the same row could have a
        // different result index depending on the user permissions. We exclude also
        // entity data, since serializing entity objects is very expensive. Instead
        // we include entity cache tags, which are enough to identify all the
        // entities associated with the row.
        $row_data = array_diff_key((array) $row, array_flip([
            'index',
            '_entity',
            '_relationship_entities',
        ])) + $this->getRowCacheTags($row);
        // This ensures that we get a unique identifier taking field handler access
        // into account: users having access to different sets of fields will get
        // different row identifiers.
        $field_ids = array_keys($this->view->field);
        $row_data += array_flip($field_ids);
        // Finally we compute a hash of row data and return it as row identifier.
        return hash('sha256', serialize($row_data));
    }

}