Custom indexer configuration for TYPO3 Extension ke_search. This Script is made for TYPO3 7 LTS (and above, but untested atm) and respects - besides the "normal" sysfolder - the storagepids_recursive field too.

class.user_kesearchhooks.php

<?php

/**
 * Created by PhpStorm.
 * User: hziegenhain
 * Date: 27.12.2016
 * Time: 14:42
 */

use TYPO3\CMS\Core\Utility\ExtensionManagementUtility;
use TYPO3\CMS\Core\Utility\GeneralUtility;

class user_kesearchhooks
{
    /**
     * needed to get all recursive pids
     *
     * @var \TYPO3\CMS\Core\Database\QueryGenerator
     */
    protected $queryGen;

    public function registerIndexerConfiguration(&$params, $pObj) {

        // add item to "type" field
        $newArray = array(
            'My Custom Extension',
            'customindexer',
            ExtensionManagementUtility::extRelPath('myext') . 'ext_icon.gif'
        );
        $params['items'][] = $newArray;
    }

    /**
     * Custom indexer for ke_search
     *
     * @param array $indexerConfig Configuration from TYPO3 Backend
     * @param array $indexerObject Reference to indexer class.
     * @return string Output.
     * @author Henrik Ziegenhain <ziegenhain@team-digital.de>
     * @since Tue Dec 27 2016 14:42:00 GMT+0100
     */
    public function customIndexer(&$indexerConfig, &$indexerObject) {
        if ($indexerConfig['type'] === 'machinery') {
            $content = '';

            // get all the entries to index
            // don't index hidden or deleted elements, BUT
            // get the elements with frontend user group access restrictions
            // or time (start / stop) restrictions.
            // Copy those restrictions to the index.
            $fields = '*';
            $table = 'tx_machinery_domain_model_machinery';

            // get the pages from where to index the records
            $indexPids = $this->getPidList($indexerConfig['startingpoints_recursive'], $indexerConfig['sysfolder']);

            $where = 'pid IN (' . implode(',', $indexPids) . ') AND hidden = 0 AND deleted = 0';
            $groupBy = '';
            $orderBy = '';
            $limit = '';
            $res = $GLOBALS['TYPO3_DB']->exec_SELECTquery($fields, $table, $where, $groupBy, $orderBy, $limit);

            $resCount = $GLOBALS['TYPO3_DB']->sql_num_rows($res);

            // Loop through the records and write them to the index.
            if ($resCount) {
                while (($record = $GLOBALS['TYPO3_DB']->sql_fetch_assoc($res))) {

                    // compile the information which should go into the index
                    // the field names depend on the table you want to index!
                    $title = strip_tags($record['name']);
                    $abstract = '';
                    //$abstract = strip_tags($record['teaser']);
                    $content = strip_tags($record['description']);
                    $fullContent = $title . "\n" . $content;
                    $params = '&tx_myext_pi1[record]=' . $record['uid'] . '&tx_myext_pi1[action]=show&tx_myext_pi1[controller]=Record';
                    $tags = '#customindexer#';
                    $additionalFields = array(
                        'sortdate' => $record['tstamp'],
                        'orig_uid' => $record['uid'],
                        'orig_pid' => $record['pid'],
                    );

                    // add something to the title, just to identify the entries
                    // in the frontend
                    //$title = $title;

                    // ... and store the information in the index
                    $indexerObject->storeInIndex(
                        $indexerConfig['storagepid'], // storage PID
                        $title, // record title
                        'customindexer', // content type
                        $indexerConfig['targetpid'], // target PID: where is the single view?
                        $fullContent, // indexed content, includes the title (linebreak after title)
                        $tags, // tags for faceted search
                        $params, // typolink params for singleview
                        $abstract, // abstract; shown in result list if not empty
                        $record['sys_language_uid'], // language uid
                        $record['starttime'], // starttime
                        $record['endtime'], // endtime
                        $record['fe_group'], // fe_group
                        false, // debug only?
                        $additionalFields // additionalFields
                    );
                }
                $content = '<p><b>Indexer "' . $indexerConfig['title'] . '":</b><br />' . $resCount . ' Elements have been indexed.</p>';
            }
            return $content;
        }
    }

    public function getPidList($startingPointsRecursive = '', $singlePages = '') {
        $this->queryGen = GeneralUtility::makeInstance('TYPO3\\CMS\\Core\\Database\\QueryGenerator');
        $pidsRecursive = GeneralUtility::trimExplode(',', $startingPointsRecursive, true);
        $pidsNonRecursive = GeneralUtility::trimExplode(',', $singlePages, true);
        $pageList = '';
        foreach ($pidsRecursive as $pid) {
            $pageList .= $this->queryGen->getTreeList($pid, 99, 0, '1=1') . ',';
        }
        foreach ($pidsNonRecursive as $pid) {
            $pageList .= $pid . ',';
        }
        return GeneralUtility::trimExplode(',', $pageList, true);
    }
}

Configuration-TCA-Overrides-tx_kesearch_indexerconfig.php

<?php
// enable "sysfolder" and "startingpoints_recursive" field
$GLOBALS['TCA']['tx_kesearch_indexerconfig']['columns']['sysfolder']['displayCond'] .= ',customindexer';
$GLOBALS['TCA']['tx_kesearch_indexerconfig']['columns']['startingpoints_recursive']['displayCond'] .= ',customindexer';

Very cool.
Unfortunatley this doesn't work for TYPO3 Version 9. How could I adapt this accordingly?

Currently not working with ke_search in Version 9.
Please may have a look into official documentation here: https://docs.typo3.org/p/teaminmedias-pluswerk/ke_search/3.1/en-us/Indexing/IndexerTypes/Custom.html

or the examples extension here: https://github.com/teaminmedias-pluswerk/ke_search_hooks - it should be up to date I think

I did and it works. But there is a bug.
If your extension uses multiple pages ("sysfolder"), where records can be stored, only the first one will be taken into account:
teaminmedias-pluswerk/ke_search_hooks#12
It would be also nice to add "startingpoints_recursive", but I'd be happy if multiple single pages work (as it did in the earlier versions).

so please be so kind and report it to the maintainers of ke_search and ke_search_hooks ;-)

I think any help there is welcome.