Feature: #86614 - Add PSR-14 event to control hreflang tags to be rendered

See Issue #86614

Description

It is now possible to alter the hreflang tags just before they get rendered. You can do this by registering an event listener for the event TYPO3\CMS\Frontend\Event\ModifyHrefLangTagsEvent.

Also the class TYPO3\CMS\Seo\HrefLang\HrefLangGenerator has been refactored to be a listener (identifier 'typo3-seo/hreflangGenerator') to the newly introduced event. This way the system extension seo still provides hreflang tags but it is now possible to simply register after or instead of the implementation.

Example

An example implementation could look like this:

EXT:my_extension/Configuration/Services.yaml

services:
  Vendor\MyExtension\HrefLang\EventListener\OwnHrefLang:
    tags:
      - name: event.listener
        identifier: 'my-ext/ownHrefLang'
        after: 'typo3-seo/hreflangGenerator'
        event: TYPO3\CMS\Frontend\Event\ModifyHrefLangTagsEvent

With after and before, you can make sure your own listener is executed after or before the given identifiers.

EXT:my_extension/Classes/HrefLang/EventListener/OwnHrefLang.php

namespace Vendor\MyExtension\HrefLang\EventListener;

use TYPO3\CMS\Frontend\Event\ModifyHrefLangTagsEvent;

class OwnHrefLang
{
   public function __invoke(ModifyHrefLangTagsEvent $event): void
   {
      $hrefLangs = $event->getHrefLangs();
      $request = $event->getRequest();

      // Do anything you want with $hrefLangs
      $hrefLangs = [
         'en-US' => 'https://example.com',
         'nl-NL' => 'https://example.com/nl'
      ];

      // Override all hrefLang tags
      $event->setHrefLangs($hrefLangs);

      // Or add a single hrefLang tag
      $event->addHrefLang('de-DE', 'https://example.com/de');
    }
}