class LazyContextRepository

class LazyContextRepository implements ContextRepositoryInterface {
    
    /**
     * The set of available context providers service IDs.
     *
     * @var string[]
     *   Context provider service IDs.
     */
    protected $contextProviderServiceIDs = [];
    
    /**
     * The service container.
     *
     * @var \Symfony\Component\DependencyInjection\ContainerInterface
     */
    protected $container;
    
    /**
     * The statically cached contexts.
     *
     * @var \Drupal\Core\Plugin\Context\ContextInterface[]
     */
    protected $contexts = [];
    
    /**
     * Constructs a LazyContextRepository object.
     *
     * @param \Symfony\Component\DependencyInjection\ContainerInterface $container
     *   The current service container.
     * @param string[] $context_provider_service_ids
     *   The set of the available context provider service IDs.
     */
    public function __construct(ContainerInterface $container, array $context_provider_service_ids) {
        $this->container = $container;
        $this->contextProviderServiceIDs = $context_provider_service_ids;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getRuntimeContexts(array $context_ids) {
        $contexts = [];
        // Create a map of context providers (service IDs) to unqualified context
        // IDs.
        $context_ids_by_service = [];
        foreach ($context_ids as $id) {
            if (isset($this->contexts[$id])) {
                $contexts[$id] = $this->contexts[$id];
                continue;
            }
            // The IDs have been passed in @{service_id}:{unqualified_context_id}
            // format.
            // @todo Convert to an assert once https://www.drupal.org/node/2408013 is
            //   in.
            if ($id[0] === '@' && strpos($id, ':') !== FALSE) {
                [
                    $service_id,
                    $unqualified_context_id,
                ] = explode(':', $id, 2);
                // Remove the leading '@'.
                $service_id = substr($service_id, 1);
            }
            else {
                throw new \InvalidArgumentException('You must provide the context IDs in the @{service_id}:{unqualified_context_id} format.');
            }
            $context_ids_by_service[$service_id][] = $unqualified_context_id;
        }
        // Iterate over all missing context providers (services), gather the
        // runtime contexts and assign them as requested.
        foreach ($context_ids_by_service as $service_id => $unqualified_context_ids) {
            $contexts_by_service = $this->container
                ->get($service_id)
                ->getRuntimeContexts($unqualified_context_ids);
            $wanted_contexts = array_intersect_key($contexts_by_service, array_flip($unqualified_context_ids));
            foreach ($wanted_contexts as $unqualified_context_id => $context) {
                $context_id = '@' . $service_id . ':' . $unqualified_context_id;
                $this->contexts[$context_id] = $contexts[$context_id] = $context;
            }
        }
        return $contexts;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getAvailableContexts() {
        $contexts = [];
        foreach ($this->contextProviderServiceIDs as $service_id) {
            $contexts_by_service = $this->container
                ->get($service_id)
                ->getAvailableContexts();
            foreach ($contexts_by_service as $unqualified_context_id => $context) {
                $context_id = '@' . $service_id . ':' . $unqualified_context_id;
                $contexts[$context_id] = $context;
            }
        }
        return $contexts;
    }

}