class DerivativeDiscoveryDecorator

class DerivativeDiscoveryDecorator implements DiscoveryInterface {
    use DiscoveryTrait;
    
    /**
     * Plugin derivers.
     *
     * @var \Drupal\Component\Plugin\Derivative\DeriverInterface[]
     *   Keys are base plugin IDs.
     */
    protected $derivers = [];
    
    /**
     * The decorated plugin discovery.
     *
     * @var \Drupal\Component\Plugin\Discovery\DiscoveryInterface
     */
    protected $decorated;
    
    /**
     * Creates a new instance.
     *
     * @param \Drupal\Component\Plugin\Discovery\DiscoveryInterface $decorated
     *   The parent object implementing DiscoveryInterface that is being
     *   decorated.
     */
    public function __construct(DiscoveryInterface $decorated) {
        $this->decorated = $decorated;
    }
    
    /**
     * {@inheritdoc}
     *
     * @throws \Drupal\Component\Plugin\Exception\InvalidDeriverException
     *   Thrown if the 'deriver' class specified in the plugin definition
     *   does not implement \Drupal\Component\Plugin\Derivative\DeriverInterface.
     */
    public function getDefinition($plugin_id, $exception_on_invalid = TRUE) {
        // This check is only for derivative plugins that have explicitly provided
        // an ID. This is not common, and can be expected to fail. Therefore, opt
        // out of the thrown exception, which will be handled when checking the
        // $base_plugin_id.
        $plugin_definition = $this->decorated
            ->getDefinition($plugin_id, FALSE);
        [
            $base_plugin_id,
            $derivative_id,
        ] = $this->decodePluginId($plugin_id);
        $base_plugin_definition = $this->decorated
            ->getDefinition($base_plugin_id, $exception_on_invalid);
        if ($base_plugin_definition) {
            $deriver = $this->getDeriver($base_plugin_id, $base_plugin_definition);
            if ($deriver) {
                $derivative_plugin_definition = $deriver->getDerivativeDefinition($derivative_id, $base_plugin_definition);
                // If a plugin defined itself as a derivative, merge in possible
                // defaults from the derivative.
                if ($derivative_id && isset($plugin_definition)) {
                    $plugin_definition = $this->mergeDerivativeDefinition($plugin_definition, $derivative_plugin_definition);
                }
                else {
                    $plugin_definition = $derivative_plugin_definition;
                }
            }
        }
        return $plugin_definition;
    }
    
    /**
     * {@inheritdoc}
     *
     * @throws \Drupal\Component\Plugin\Exception\InvalidDeriverException
     *   Thrown if the 'deriver' class specified in the plugin definition
     *   does not implement \Drupal\Component\Plugin\Derivative\DeriverInterface.
     */
    public function getDefinitions() {
        $plugin_definitions = $this->decorated
            ->getDefinitions();
        return $this->getDerivatives($plugin_definitions);
    }
    
    /**
     * Adds derivatives to a list of plugin definitions.
     *
     * This should be called by the class extending this in
     * DiscoveryInterface::getDefinitions().
     */
    protected function getDerivatives(array $base_plugin_definitions) {
        $plugin_definitions = [];
        foreach ($base_plugin_definitions as $base_plugin_id => $plugin_definition) {
            $deriver = $this->getDeriver($base_plugin_id, $plugin_definition);
            if ($deriver) {
                $derivative_definitions = $deriver->getDerivativeDefinitions($plugin_definition);
                foreach ($derivative_definitions as $derivative_id => $derivative_definition) {
                    $plugin_id = $this->encodePluginId($base_plugin_id, $derivative_id);
                    // Use this definition as defaults if a plugin already defined
                    // itself as this derivative.
                    if ($derivative_id && isset($base_plugin_definitions[$plugin_id])) {
                        $derivative_definition = $this->mergeDerivativeDefinition($base_plugin_definitions[$plugin_id], $derivative_definition);
                    }
                    $plugin_definitions[$plugin_id] = $derivative_definition;
                }
            }
            elseif (!isset($plugin_definitions[$base_plugin_id])) {
                $plugin_definitions[$base_plugin_id] = $plugin_definition;
            }
        }
        return $plugin_definitions;
    }
    
    /**
     * Decodes derivative id and plugin id from a string.
     *
     * @param string $plugin_id
     *   Plugin identifier that may point to a derivative plugin.
     *
     * @return array
     *   An array with the base plugin id as the first index and the derivative id
     *   as the second. If there is no derivative id it will be null.
     */
    protected function decodePluginId($plugin_id) {
        // Try and split the passed plugin definition into a plugin and a
        // derivative id. We don't need to check for !== FALSE because a leading
        // colon would break the derivative system and doesn't makes sense.
        if (strpos($plugin_id, ':')) {
            return explode(':', $plugin_id, 2);
        }
        return [
            $plugin_id,
            NULL,
        ];
    }
    
    /**
     * Encodes plugin and derivative id's into a string.
     *
     * @param string $base_plugin_id
     *   The base plugin identifier.
     * @param string $derivative_id
     *   The derivative identifier.
     *
     * @return string
     *   A uniquely encoded combination of the $base_plugin_id and $derivative_id.
     */
    protected function encodePluginId($base_plugin_id, $derivative_id) {
        if ($derivative_id) {
            return "{$base_plugin_id}:{$derivative_id}";
        }
        // By returning the unmerged plugin_id, we are able to support derivative
        // plugins that support fetching the base definitions.
        return $base_plugin_id;
    }
    
    /**
     * Gets a deriver for a base plugin.
     *
     * @param string $base_plugin_id
     *   The base plugin id of the plugin.
     * @param mixed $base_definition
     *   The base plugin definition to build derivatives.
     *
     * @return \Drupal\Component\Plugin\Derivative\DeriverInterface|null
     *   A DerivativeInterface or NULL if none exists for the plugin.
     *
     * @throws \Drupal\Component\Plugin\Exception\InvalidDeriverException
     *   Thrown if the 'deriver' class specified in the plugin definition
     *   does not implement \Drupal\Component\Plugin\Derivative\DeriverInterface.
     */
    protected function getDeriver($base_plugin_id, $base_definition) {
        if (!isset($this->derivers[$base_plugin_id])) {
            $this->derivers[$base_plugin_id] = FALSE;
            $class = $this->getDeriverClass($base_definition);
            if ($class) {
                $this->derivers[$base_plugin_id] = new $class($base_plugin_id);
            }
        }
        return $this->derivers[$base_plugin_id] ?: NULL;
    }
    
    /**
     * Gets the deriver class name from the base plugin definition.
     *
     * @param array $base_definition
     *   The base plugin definition to build derivatives.
     *
     * @return string|null
     *   The name of a class implementing
     *   \Drupal\Component\Plugin\Derivative\DeriverInterface.
     *
     * @throws \Drupal\Component\Plugin\Exception\InvalidDeriverException
     *   Thrown if the 'deriver' class specified in the plugin definition
     *   does not implement
     *   \Drupal\Component\Plugin\Derivative\DerivativeInterface.
     */
    protected function getDeriverClass($base_definition) {
        $class = NULL;
        $id = NULL;
        if ($base_definition instanceof DerivablePluginDefinitionInterface) {
            $class = $base_definition->getDeriver();
            $id = $base_definition->id();
        }
        if ((is_array($base_definition) || ($base_definition = (array) $base_definition)) && isset($base_definition['deriver'])) {
            $class = $base_definition['deriver'];
            $id = $base_definition['id'];
        }
        if ($class) {
            if (!class_exists($class)) {
                throw new InvalidDeriverException(sprintf('Plugin (%s) deriver "%s" does not exist.', $id, $class));
            }
            if (!is_subclass_of($class, '\\Drupal\\Component\\Plugin\\Derivative\\DeriverInterface')) {
                throw new InvalidDeriverException(sprintf('Plugin (%s) deriver "%s" must implement \\Drupal\\Component\\Plugin\\Derivative\\DeriverInterface.', $id, $class));
            }
        }
        return $class;
    }
    
    /**
     * Merges a base and derivative definition, taking into account empty values.
     *
     * @param array $base_plugin_definition
     *   The base plugin definition.
     * @param array $derivative_definition
     *   The derivative plugin definition.
     *
     * @return array
     *   The merged definition.
     */
    protected function mergeDerivativeDefinition($base_plugin_definition, $derivative_definition) {
        // Use this definition as defaults if a plugin already defined itself as
        // this derivative, but filter out empty values first.
        $filtered_base = array_filter($base_plugin_definition);
        $derivative_definition = $filtered_base + ($derivative_definition ?: []);
        // Add back any empty keys that the derivative didn't have.
        return $derivative_definition + $base_plugin_definition;
    }
    
    /**
     * Passes through all unknown calls onto the decorated object.
     */
    public function __call($method, $args) {
        return call_user_func_array([
            $this->decorated,
            $method,
        ], $args);
    }

}