class BlockRegion

class BlockRegion extends StaticMap implements ContainerFactoryPluginInterface {
    
    /**
     * List of regions, keyed by theme.
     *
     * @var array[]
     */
    protected $regions;
    
    /**
     * Constructs a BlockRegion plugin instance.
     *
     * @param array $configuration
     *   The plugin configuration.
     * @param string $plugin_id
     *   The plugin ID.
     * @param mixed $plugin_definition
     *   The plugin definition.
     * @param array $regions
     *   Array of region maps, keyed by theme.
     */
    public function __construct(array $configuration, $plugin_id, $plugin_definition, array $regions) {
        parent::__construct($configuration, $plugin_id, $plugin_definition);
        $this->regions = $regions;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
        $regions = [];
        foreach ($container->get('theme_handler')
            ->listInfo() as $key => $theme) {
            $regions[$key] = $theme->info['regions'];
        }
        return new static($configuration, $plugin_id, $plugin_definition, $regions);
    }
    
    /**
     * {@inheritdoc}
     */
    public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
        // Set the destination region, based on the source region and theme as well
        // as the current destination default theme.
        [
            $source_theme,
            $destination_theme,
            $region,
        ] = $value;
        // Theme is the same on both source and destination, so ensure that the
        // region exists in the destination theme.
        if (strtolower($source_theme) == strtolower($destination_theme)) {
            if (isset($this->regions[$destination_theme][$region])) {
                return $region;
            }
        }
        // Fall back to static mapping.
        return parent::transform($value, $migrate_executable, $row, $destination_property);
    }

}