silverstripe-docsviewer/code/extensions/DocumentationViewerVersionWarning.php

<?php
/**
 * @package docsviewer
 */
class DocumentationViewerVersionWarning extends Extension
{
    /**
     * Check to see if the currently accessed version is out of date or perhaps a
     * future version rather than the stable edition.
     *
     * @return false|ArrayData
     */
    public function VersionWarning()
    {
        $page = $this->owner->getPage();

        if (!$page) {
            return false;
        }

        $entity = $page->getEntity();

        if (!$entity) {
            return false;
        }

        $versions = $this->owner->getManifest()->getAllVersionsOfEntity($entity);

        if ($entity->getIsStable()) {
            return false;
        }

        $stable = $this->owner->getManifest()->getStableVersion($entity);
        $compare = $entity->compare($stable);

        if ($entity->getVersion() == 'master' || $compare > 0) {
            return $this->owner->customise(
                new ArrayData(
                    array(
                        'FutureRelease' => true,
                        'StableVersion' => DBField::create_field('HTMLText', $stable->getVersion())
                    )
                )
            );
        } else {
            return $this->owner->customise(
                new ArrayData(
                    array(
                        'OutdatedRelease' => true,
                        'StableVersion'   => DBField::create_field('HTMLText', $stable->getVersion())
                    )
                )
            );
        }

        return false;
    }
}