silverstripe-framework/src/Forms/TreeMultiselectField_Readonly.php

<?php

namespace SilverStripe\Forms;

class TreeMultiselectField_Readonly extends TreeMultiselectField
{

    protected $readonly = true;

    public function Field($properties = array())
    {
        $titleArray = $itemIDs = array();
        $titleList = $itemIDsList = "";
        if ($items = $this->getItems()) {
            foreach ($items as $item) {
                $titleArray[] = $item->Title;
            }
            foreach ($items as $item) {
                $itemIDs[] = $item->ID;
            }
            if ($titleArray) {
                $titleList = implode(", ", $titleArray);
            }
            if ($itemIDs) {
                $itemIDsList = implode(",", $itemIDs);
            }
        }

        $field = new ReadonlyField($this->name . '_ReadonlyValue', $this->title);
        $field->setValue($titleList);
        $field->setForm($this->form);

        $valueField = new HiddenField($this->name);
        $valueField->setValue($itemIDsList);
        $valueField->setForm($this->form);

        return $field->Field() . $valueField->Field();
    }
}