silverstripe-userforms/code/Model/EditableFormField/EditableMemberListField.php

<?php

namespace SilverStripe\UserForms\Model\EditableFormField;

use SilverStripe\Forms\DropdownField;
use SilverStripe\Forms\FieldList;
use SilverStripe\Security\Group;
use SilverStripe\Security\Member;
use SilverStripe\UserForms\Model\EditableFormField;

/**
 * Creates an editable field that displays members in a given group
 *
 * @package userforms
 * @property int $GroupID
 * @method Group Group()
 */
class EditableMemberListField extends EditableFormField
{
    private static $singular_name = 'Member List Field';

    private static $plural_name = 'Member List Fields';

    private static $has_one = [
        'Group' => Group::class
    ];

    private static $table_name = 'EditableMemberListField';

    /**
     * @return FieldList
     */
    public function getCMSFields()
    {
        $this->beforeUpdateCMSFields(function (FieldList $fields) {
            $fields->removeByName('Default');
            $fields->removeByName('Validation');

            /** @skipUpgrade */
            $fields->addFieldToTab(
                'Root.Main',
                DropdownField::create(
                    'GroupID',
                    _t('SilverStripe\\UserForms\\Model\\EditableFormField.GROUP', 'Group'),
                    Group::get()->map()
                )->setEmptyString(' ')
            );
        });

        return parent::getCMSFields();
    }

    public function getFormField()
    {
        if (empty($this->GroupID)) {
            return false;
        }

        $members = Member::map_in_groups($this->GroupID);

        $field = DropdownField::create($this->Name, $this->Title ?: false, $members)
            ->setTemplate(EditableDropdown::class)
            ->setFieldHolderTemplate(EditableFormField::class . '_holder');

        $this->doUpdateFormField($field);

        return $field;
    }

    public function getValueFromData($data)
    {
        if (isset($data[$this->Name])) {
            $memberID = $data[$this->Name];
            $member = Member::get()->byID($memberID);
            return $member ? $member->getName() : '';
        }

        return false;
    }
}