Mail subject and body remain empty using mailManager

Question

I have been fiddling with sending a mail in my custom module in Drupal 8. The mail gets sent but the subject and body are empty. It has been an issue for days and I can't get it to work. The code and var_dump are below. I just can't find the issue. Maybe someone can!

Code in hook_mail:

<?php
/**
 * Implements hook_mail().
 */
function document_mail($key, $message, $params)
{
    $options = array(
        'langcode' => $message['langcode'],
    );
    switch ($key) {
       case 'send_file':
            $message['from'] = \Drupal::config('system.site')->get('mail');
            $message['subject'] = t('@title', array('@title' => $params['mail_title']), $options);
            $message['body'][] = $params['message'];
        break;
    }
}

Code to send mail:

    $mailManager = \Drupal::service('plugin.manager.mail');
    $module = 'document';
    $key = 'send_file';
    $to = 'info@example.com';
    $params['mail_title'] = 'a title';
    $params['message'] = 'a message';
    $langcode = \Drupal::currentUser()->getPreferredLangcode();
    $send = true;
    $result = $mailManager->mail($module, $key, $to, $langcode, $params, null, $send);
    var_dump($result);
    if ($result['result'] !== true) {
        drupal_set_message(t('There was a problem sending your message and it was not sent.'), 'error');
    } else {
        drupal_set_message(t('Your message has been sent.'));
    }

var_dump of result from $result of mailManager

array(12) { 
["id"]=> string(18) "document_send_file" 
["module"]=> string(8) "document" 
["key"]=> string(9) "send_file" 
["to"]=> string(24) "info@example.com," 
["from"]=> string(22) "info@example.com" 
["reply-to"]=> NULL 
["langcode"]=> string(2) "en" 
["params"]=> array(2) 
{ 
    ["mail_title"]=> string(7) "a title" 
    ["message"]=> string(9) "a message" 
} 
["send"]=> bool(true) 
["subject"]=> object(Drupal\Core\StringTranslation\TranslatableMarkup)#410 (5) 
{ 
    ["string":protected]=> string(6) "@title" 
        ["translatedMarkup":protected]=> NULL 
        ["options":protected]=> array(1) 
        { 
                ["langcode"]=> string(2) "en" 
        } 
        ["stringTranslation":protected]=> NULL 
        ["arguments":protected]=> array(1) 
        { 
            ["@title"]=> string(7) "a title" 
        } 
    } 
    ["body"]=> array(1) 
    { 
        [0]=> string(9) "a message" 
    } 
    ["headers"]=> array(7) 
    { 
        ["MIME-Version"]=> string(3) "1.0" 
        ["Content-Type"]=> string(51) "text/plain; charset=UTF-8; format=flowed; delsp=yes" 
        ["Content-Transfer-Encoding"]=> string(4) "8Bit" 
        ["X-Mailer"]=> string(6) "Drupal" 
        ["Return-Path"]=> string(22) "info@example.com" 
        ["Sender"]=> string(22) "info@example.com" 
        ["From"]=> string(44) "a string" 
    }
    ["result"]=> bool(true)
} 

Answer 1

I believe you forgot to pass $message by reference (&), so please double check the implementation of your hook_mail as per details below:

hook_mail($key, &$message, $params)

Otherwise, any changes to $message won't take any effect.

Answer 2

Remove the empty array tag from body as below.

$message['body'] = $params['message'];