ThreadHeadersHandler.php

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<?php

namespace Elgg\Email;

class ThreadHeadersHandler {
    
    public function __invoke(\Elgg\Event $event) {
        $email = $event->getValue();
        if (!$email instanceof \Elgg\Email) {
            return;
        }
        
        $params = $email->getParams();
        
        $notification = elgg_extract('notification', $params);
        if (!$notification instanceof \Elgg\Notifications\Notification) {
            return;
        }
        
        $object = elgg_extract('object', $notification->params);
        if (!$object instanceof \ElggEntity) {
            return;
        }
        
        $event = elgg_extract('event', $notification->params);
        if (!$event instanceof \Elgg\Notifications\NotificationEvent) {
            return;
        }
        
        $message_id = $email->createEntityMessageID($object, $event->getAction() !== 'create');
        $email->addHeader('Message-ID', $message_id);
    
        // let's just thread comments by default
        $container = $object->getContainerEntity();
        if ($container instanceof \ElggEntity && $object instanceof \ElggComment) {
            $thread_message_id = "<{$email->createEntityMessageID($container)}>";
            
            $email->addHeader('In-Reply-To', $thread_message_id);
            $email->addHeader('References', $thread_message_id);
        }
        
        return $email;
    }
}