[BUGFIX] Migrate kitodo#1723 to main branch (kitodo#1734)

Co-authored-by: webpack-builder[bot] <webpack-builder[bot]@users.noreply.github.com>
Co-authored-by: Sebastian Meyer <sebastian.meyer@opencultureconsulting.com>

Author: 3 people

Classes/Controller/AbstractController.php

@@ -183,7 +183,7 @@ protected function buildMultiView(AbstractDocument $doc): void
             foreach ($this->requestData['multipleSource'] as $location) {
                 $document = AbstractDocument::getInstance($location, $this->settings);
                 if ($document !== null) {
-                    $this->documentArray['extra_' . $i] = $document;
+                    $this->documentArray['multipleSource_' . $i] = $document;
                 }
                 $i++;
             }

Classes/ViewHelpers/MultipleSourceViewHelper.php

@@ -0,0 +1,36 @@
+<?php
+/**
+ * (c) Kitodo. Key to digital objects e.V. <contact@kitodo.org>
+ *
+ * This file is part of the Kitodo and TYPO3 projects.
+ *
+ * @license GNU General Public License version 3 or later.
+ * For the full copyright and license information, please read the
+ * LICENSE.txt file that was distributed with this source code.
+ */
+
+namespace Kitodo\Dlf\ViewHelpers;
+
+use TYPO3Fluid\Fluid\Core\ViewHelper\AbstractViewHelper;
+
+class MultipleSourceViewHelper extends AbstractViewHelper
+{
+    public function initializeArguments()
+    {
+        parent::initializeArguments();
+        $this->registerArgument('docArrayKey', 'string', 'The array key of the doc array', true);
+    }
+
+    /**
+     * Checks whether docArray starts with the prefix multipleSource_ and returns the numeric postfix.
+     *
+     * @return int
+     */
+    public function render()
+    {
+        if (preg_match('/^multipleSource_(\d+)$/', $this->arguments['docArrayKey'], $matches)) {
+            return (int) $matches[1];
+        }
+        return -1;
+    }
+}