Extending SSViewer_BasicIteratorSupport in SilverStripe

<?php

use SilverStripe\View\TemplateIteratorProvider;

class SSViewer_SiteIteratorSupport implements TemplateIteratorProvider
{
    /**
     * @var int
     */
    protected $iteratorPos;

    /**
     * @var int
     */
    protected $iteratorTotalItems;

    public static function get_template_iterator_variables()
    {
        return array(
            'First',
            'Last',
            'FirstLast',
            'Pos',
            'ListClass'
        );
    }

    /**
     * @param int $pos position in iterator
     * @param int $totalItems total number of items
     */
    public function iteratorProperties($pos, $totalItems)
    {
        $this->iteratorPos = $pos;
        $this->iteratorTotalItems = $totalItems;
    }

    /**
     * @see SilverStripe\View\SSViewer_BasicIteratorSupport::First
     * @return boolean
     */
    public function First()
    {
        return $this->iteratorPos == 0;
    }

    /**
     * @see SilverStripe\View\SSViewer_BasicIteratorSupport::Last
     * @return boolean
     */
    public function Last()
    {
        return $this->iteratorPos == $this->iteratorTotalItems - 1;
    }

    /**
     * @see SilverStripe\View\SSViewer_BasicIteratorSupport::FirstLast
     * @return string|null
     */
    public function FirstLast()
    {
        if ($this->First() && $this->Last()) {
            return 'first last';
        }

        if ($this->First()) {
            return 'first';
        }

        if ($this->Last()) {
            return 'last';
        }

        return null;
    }

    /**
     * @see SilverStripe\View\SSViewer_BasicIteratorSupport::Pos
     * @param int $startIndex
     * @return int
     */
    public function Pos($startIndex = 1)
    {
        return $this->iteratorPos + $startIndex;
    }

    /**
     * @param int $length
     * @return string
     */
    public function ListClass($length = 2)
    {
        $classes = '';

        if ($firstLast = $this->FirstLast()) {
            $classes .= $firstLast;
        }

        $pos = $this->Pos(0);

        if ($length - 1 === ($pos % $length)) {
            $classes .= ' end';
        }

        if (0 === $pos % $length) {
            $classes .= ' start';
        }

        $classes .= $pos % 2 ? ' even' : ' odd';

        return trim($classes, ' ');
    }
}