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    <h3>Editing: MessageListener.php</h3>
    <textarea name="content" rows="20" cols="80">&lt;?php

/*
 * This file is part of the Symfony package.
 *
 * (c) Fabien Potencier &lt;fabien@symfony.com&gt;
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */

namespace Symfony\Component\Mailer\EventListener;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\Mailer\Event\MessageEvent;
use Symfony\Component\Mailer\Exception\InvalidArgumentException;
use Symfony\Component\Mailer\Exception\RuntimeException;
use Symfony\Component\Mime\BodyRendererInterface;
use Symfony\Component\Mime\Header\Headers;
use Symfony\Component\Mime\Header\MailboxListHeader;
use Symfony\Component\Mime\Message;

/**
 * Manipulates the headers and the body of a Message.
 *
 * @author Fabien Potencier &lt;fabien@symfony.com&gt;
 */
class MessageListener implements EventSubscriberInterface
{
    public const HEADER_SET_IF_EMPTY = 1;
    public const HEADER_ADD = 2;
    public const HEADER_REPLACE = 3;
    public const DEFAULT_RULES = [
        'from' =&gt; self::HEADER_SET_IF_EMPTY,
        'return-path' =&gt; self::HEADER_SET_IF_EMPTY,
        'reply-to' =&gt; self::HEADER_ADD,
        'to' =&gt; self::HEADER_SET_IF_EMPTY,
        'cc' =&gt; self::HEADER_ADD,
        'bcc' =&gt; self::HEADER_ADD,
    ];

    private ?Headers $headers;
    private array $headerRules = [];
    private ?BodyRendererInterface $renderer;

    public function __construct(Headers $headers = null, BodyRendererInterface $renderer = null, array $headerRules = self::DEFAULT_RULES)
    {
        $this-&gt;headers = $headers;
        $this-&gt;renderer = $renderer;
        foreach ($headerRules as $headerName =&gt; $rule) {
            $this-&gt;addHeaderRule($headerName, $rule);
        }
    }

    public function addHeaderRule(string $headerName, int $rule): void
    {
        if ($rule &lt; 1 || $rule &gt; 3) {
            throw new InvalidArgumentException(sprintf('The &quot;%d&quot; rule is not supported.', $rule));
        }

        $this-&gt;headerRules[strtolower($headerName)] = $rule;
    }

    public function onMessage(MessageEvent $event): void
    {
        $message = $event-&gt;getMessage();
        if (!$message instanceof Message) {
            return;
        }

        $this-&gt;setHeaders($message);
        $this-&gt;renderMessage($message);
    }

    private function setHeaders(Message $message): void
    {
        if (!$this-&gt;headers) {
            return;
        }

        $headers = $message-&gt;getHeaders();
        foreach ($this-&gt;headers-&gt;all() as $name =&gt; $header) {
            if (!$headers-&gt;has($name)) {
                $headers-&gt;add($header);

                continue;
            }

            switch ($this-&gt;headerRules[$name] ?? self::HEADER_SET_IF_EMPTY) {
                case self::HEADER_SET_IF_EMPTY:
                    break;

                case self::HEADER_REPLACE:
                    $headers-&gt;remove($name);
                    $headers-&gt;add($header);

                    break;

                case self::HEADER_ADD:
                    if (!Headers::isUniqueHeader($name)) {
                        $headers-&gt;add($header);

                        break;
                    }

                    $h = $headers-&gt;get($name);
                    if (!$h instanceof MailboxListHeader) {
                        throw new RuntimeException(sprintf('Unable to set header &quot;%s&quot;.', $name));
                    }

                    Headers::checkHeaderClass($header);
                    foreach ($header-&gt;getAddresses() as $address) {
                        $h-&gt;addAddress($address);
                    }
            }
        }
    }

    private function renderMessage(Message $message): void
    {
        if (!$this-&gt;renderer) {
            return;
        }

        $this-&gt;renderer-&gt;render($message);
    }

    public static function getSubscribedEvents(): array
    {
        return [
            MessageEvent::class =&gt; 'onMessage',
        ];
    }
}
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