Region.php

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Magento\Customer\Model\Renderer;

use Magento\Framework\Data\Form\Element\AbstractElement;

/**
 * Region field renderer
 */
class Region implements \Magento\Framework\Data\Form\Element\Renderer\RendererInterface
{
    /**
     * Country region collections
     *
     * Structure:
     * array(
     *      [$countryId] => \Magento\Framework\Data\Collection\AbstractDb
     * )
     *
     * @var array
     */
    protected static $_regionCollections;

    /**
     * Adminhtml data
     *
     * @var \Magento\Framework\Escaper
     */
    protected $_escaper = null;

    /**
     * @var \Magento\Directory\Model\CountryFactory
     */
    protected $_countryFactory;

    /**
     * @var \Magento\Directory\Helper\Data
     */
    private $_directoryHelper;

    /**
     * @param \Magento\Directory\Model\CountryFactory $countryFactory
     * @param \Magento\Directory\Helper\Data $directoryHelper
     * @param \Magento\Framework\Escaper $escaper
     */
    public function __construct(
        \Magento\Directory\Model\CountryFactory $countryFactory,
        \Magento\Directory\Helper\Data $directoryHelper,
        \Magento\Framework\Escaper $escaper
    ) {
        $this->_countryFactory = $countryFactory;
        $this->_directoryHelper = $directoryHelper;
        $this->_escaper = $escaper;
    }

    /**
     * Render element
     *
     * @param AbstractElement $element
     * @return string
     * @SuppressWarnings(PHPMD.CyclomaticComplexity)
     * @SuppressWarnings(PHPMD.NPathComplexity)
     */
    public function render(AbstractElement $element)
    {
        $countryId = false;
        $isRegionRequired = false;
        if ($country = $element->getForm()->getElement('country_id')) {
            $countryId = $country->getValue();
            $isRegionRequired = $this->_directoryHelper->isRegionRequired($countryId);
        }

        $html = '<div class="field field-region ' . ($isRegionRequired ? 'required' : '') . '">' . "\n";

        $regionCollection = false;
        if ($countryId) {
            if (!isset(self::$_regionCollections[$countryId])) {
                self::$_regionCollections[$countryId] = $this->_countryFactory->create()->setId(
                    $countryId
                )->getLoadedRegionCollection()->toOptionArray();
            }
            $regionCollection = self::$_regionCollections[$countryId];
        }

        $regionId = (int)$element->getForm()->getElement('region_id')->getValue();

        $htmlAttributes = $element->getHtmlAttributes();
        foreach ($htmlAttributes as $key => $attribute) {
            if ('type' === $attribute) {
                unset($htmlAttributes[$key]);
                break;
            }
        }

        // Output two elements - for 'region' and for 'region_id'.
        // Two elements are needed later upon form post - to properly set data to address model,
        // otherwise old value can be left in region_id attribute and saved to DB.
        // Depending on country selected either 'region' (input text) or 'region_id' (selectbox) is visible to user
        $regionHtmlName = $element->getName();
        $regionIdHtmlName = str_replace('region', 'region_id', $regionHtmlName);
        $regionHtmlId = $element->getHtmlId();
        $regionIdHtmlId = str_replace('region', 'region_id', $regionHtmlId);

        if ($isRegionRequired) {
            $element->addClass('required-entry');
        }

        if ($regionCollection && count($regionCollection) > 0) {
            $elementClass = $element->getClass();
            $html .= '<label class="label" for="' .
                $regionIdHtmlId .
                '"><span>' .
                $element->getLabel() .
                '</span>' .
                '</label>';
            $html .= '<div class="control">';

            $html .= '<select id="' . $regionIdHtmlId . '" name="' . $regionIdHtmlName . '" ' . $element->serialize(
                $htmlAttributes
            ) . '>' . "\n";
            foreach ($regionCollection as $region) {
                $selected = $regionId == $region['value'] ? ' selected="selected"' : '';
                $regionVal = 0 == $region['value'] ? '' : (int)$region['value'];
                $html .= '<option value="' . $regionVal . '"' . $selected . '>' . $this->_escaper->escapeHtml(
                    __($region['label'])
                ) . '</option>';
            }
            $html .= '</select>' . "\n";

            $html .= '<input type="hidden" name="' . $regionHtmlName . '" id="' . $regionHtmlId . '" value=""/>';

            $html .= '</div>';
            $element->setClass($elementClass);
        } else {
            $html .= '<label class="label" for="' .
                $regionHtmlId .
                '"><span>' .
                $element->getLabel() .
                '</span></label>';
            $html .= '<div class="control">';
            $html .= '<input id="' .
                $regionHtmlId .
                '" name="' .
                $regionHtmlName .
                '" value="' .
                $element->getEscapedValue() .
                '" ' .
                $element->serialize(
                    $htmlAttributes
                ) . "/>" . "\n";
            $html .= '<input type="hidden" name="' . $regionIdHtmlName . '" id="' . $regionIdHtmlId . '" value=""/>';
            $html .= '</div>' . "\n";
        }
        $html .= '</div>' . "\n";
        return $html;
    }
}