Allow fallbacks for hreflang in TYPO3 9

ext_localconf.php

<?php

$GLOBALS['TYPO3_CONF_VARS']['SYS']['Objects'][\TYPO3\CMS\Seo\HrefLang\HrefLangGenerator::class] = [
            'className' => \JosefGlatz\Theme\Xclass\HrefLangGeneratorXclass::class,
        ];

HrefLangGenerator.php

<?php
declare(strict_types=1);

namespace JosefGlatz\Theme\Xclass;

use JosefGlatz\Theme\Service\LanguageAvailability;
use TYPO3\CMS\Core\Site\Entity\Site;
use TYPO3\CMS\Core\Utility\GeneralUtility;
use TYPO3\CMS\Frontend\DataProcessing\LanguageMenuProcessor;
use TYPO3\CMS\Seo\HrefLang\HrefLangGenerator;

class HrefLangGeneratorXclass extends HrefLangGenerator {

    public function generate(): string
    {
        $hreflangs = [];
        if ((int)$this->typoScriptFrontendController->page['no_index'] === 1) {
            return '';
        }

        if ($this->request->getAttribute('site') instanceof Site) {
            $languageMenu = GeneralUtility::makeInstance(LanguageMenuProcessor::class);
            $languages = $languageMenu->process($this->cObj, [], [], []);
            foreach ($languages['languagemenu'] as $language) {
                if (LanguageAvailability::checkLanguageAvailability($language) && !empty($language['link'])) {
                    $href = $this->getAbsoluteUrl($language['link']);
                    $hreflangs[] =
                        '<link rel="alternate" hreflang="' . htmlspecialchars($language['hreflang']) . '" href="' . htmlspecialchars($href) . '"/>';
                }
            }

            if (count($hreflangs) > 1) {
                $href = $this->getAbsoluteUrl($languages['languagemenu'][0]['link']);
                $hreflangs[] =
                    '<link rel="alternate" hreflang="x-default" href="' . htmlspecialchars($href) . '"/>' . LF;

                $this->getTypoScriptFrontendController()->additionalHeaderData[] = implode(LF, $hreflangs);
            }
        }

        return implode(LF, $hreflangs);
    }

}

LanguageAvailability.php

<?php
declare(strict_types=1);

namespace JosefGlatz\Theme\Service;

use Psr\Http\Message\ServerRequestInterface;
use TYPO3\CMS\Core\Database\ConnectionPool;
use TYPO3\CMS\Core\Site\Entity\Site;
use TYPO3\CMS\Core\Site\Entity\SiteLanguage;
use TYPO3\CMS\Core\Utility\GeneralUtility;
use TYPO3\CMS\Frontend\Controller\TypoScriptFrontendController;

class LanguageAvailability
{


    /**
     * @param $language
     * @return bool
     */
    public static function checkLanguageAvailability(array $language): bool
    {
        if ($language['available']) {
            return true;
        }
        $currentPage = self::getTsfe()->page;
        $currentSite = self::getCurrentSite();
        if ($currentSite) {
            $languageId = $language['languageId'];
            /** @var SiteLanguage $languageConfiguration */
            $languageConfiguration = $currentSite->getAllLanguages()[$languageId];

            // check fallbacks
            $fallbacks = $languageConfiguration->getFallbackLanguageIds();
            foreach ($fallbacks as $fallbackLanguageId) {
                if ($fallbackLanguageId === 0) {
                    return true;
                }

                $row = self::getTranslationOfPage($currentPage['uid'], $fallbackLanguageId);
                if (is_array($row) && !empty($row)) {
                    return true;
                }
            }

        }

        return false;
    }

    protected static function getTranslationOfPage(int $pageId, int $languageId)
    {
        $queryBuilder = GeneralUtility::makeInstance(ConnectionPool::class)
            ->getQueryBuilderForTable('pages');
        $row = $queryBuilder
            ->select('uid', 'title', 'pid')
            ->from('pages')
            ->where(
                $queryBuilder->expr()->eq(
                    'l10n_parent',
                    $queryBuilder->createNamedParameter($pageId, \PDO::PARAM_INT)
                ),
                $queryBuilder->expr()->eq(
                    'sys_language_uid',
                    $queryBuilder->createNamedParameter($languageId, \PDO::PARAM_INT)
                )
            )
            ->execute()
            ->fetch();

        return $row;
    }

    protected static function getTsfe(): TypoScriptFrontendController
    {
        return $GLOBALS['TSFE'];
    }

    protected static function getCurrentSite(): ?Site
    {
        if ($GLOBALS['TYPO3_REQUEST'] instanceof ServerRequestInterface
            && $GLOBALS['TYPO3_REQUEST']->getAttribute('site') instanceof Site) {
            return $GLOBALS['TYPO3_REQUEST']->getAttribute('site');
        }
        return null;
    }
}