farm-2.x-dev/modules/core/data_stream/modules/notification/data_stream_notification.module

<?php

/**
 * @file
 * Contains data_stream_notification.module.
 */

/**
 * Implements hook_mail().
 */
function data_stream_notification_mail($key, &$message, $params) {

  // Bail if not a notification email.
  if ($key !== 'notification_email') {
    return;
  }

  /** @var \Drupal\data_stream_notification\Entity\DataStreamNotification $data_stream_notification */
  $data_stream_notification = $params['data_stream_notification'];

  /** @var \Drupal\data_stream\Entity\DataStreamInterface $data_stream */
  $data_stream = $params['data_stream'];
  $url = $data_stream->toUrl()->setAbsolute()->toString();

  // Build the email subject.
  $message['subject'] = t(
    '@notification notification for data stream: @data_stream',
    [
      '@notification' => $data_stream_notification->label(),
      '@data_stream' => $data_stream->label(),
    ]
  );

  // Build the email body.
  $message['body'][] = t(
    'Data stream: <a href="@link">@label</a> Actual value: @value',
    [
      '@link' => $url,
      '@label' => $data_stream->label(),
      '@value' => $params['value'],
    ],
  );

  // Build list of condition summaries.
  $conditions = '<ul>';
  $conditions .= '<li>' . implode('</li><li>', $params['condition_summaries']) . '</li>';
  $conditions .= '</ul>';
  $message['body'][] = $conditions;
}