Magento 2: Server side validation not showing filled up data while error [SOLVED]

Question

I have below code

app\code\Custom\Module\Controller\MyModule\PostForm.php

namespace Custom\Module\Controller\MyModule;

use Magento\Framework\App\Request\DataPersistorInterface;
use Magento\Framework\App\ObjectManager;
use Magento\Framework\App\Filesystem\DirectoryList;

class PostForm extends \Magento\Framework\App\Action\Action {

    /**
     * @var DataPersistorInterface
     */
    private $dataPersistor;

    /** @var  \Magento\Framework\View\Result\Page */
    protected $resultPageFactory;

    /**
     * @var \Magento\Framework\Translate\Inline\StateInterface
     */
    protected $inlineTranslation;

    /**
     * @var \Magento\Framework\Data\Form\FormKey\Validator
     */
    protected $_formKeyValidator;

    /**     * @param \Magento\Framework\App\Action\Context $context      */
    public function __construct(\Magento\Framework\App\Action\Context $context, \Magento\Framework\View\Result\PageFactory $resultPageFactory, \Magento\Framework\Data\Form\FormKey\Validator $formKeyValidator, \Magento\Framework\Translate\Inline\StateInterface $inlineTranslation) {
        $this->resultPageFactory = $resultPageFactory;
        $this->_formKeyValidator = $formKeyValidator;
        $this->inlineTranslation = $inlineTranslation;
        parent::__construct($context);
    }

    /**
     * Post user post
     *
     * @return void
     * @throws \Exception
     */
    public function execute() {

        $resultPage = $this->resultPageFactory->create();
        $resultRedirect = $this->resultRedirectFactory->create();

        $post = $this->getRequest()->getPostValue();

        if (!$post) {
            $this->_redirect('*/*/');
            return;
        }

        if (!$this->_formKeyValidator->validate($this->getRequest())) {
            return $resultRedirect->setPath('*/*/');
        }

        $this->inlineTranslation->suspend();

        $postObject = new \Magento\Framework\DataObject();
        $postObject->setData($post);

        $error = false;
        $errors = [];
        if (!\Zend_Validate::is(trim($post['title']), 'NotEmpty')) {
            $errors[] = __('Please enter title.');
            $error = true;
        }
        if (!\Zend_Validate::is(trim($post['description']), 'NotEmpty')) {
            $errors[] = __('Please enter description.');
            $error = true;
        }


        if ($error) {
            foreach ($errors as $error) {
                $this->messageManager->addErrorMessage($error);
            }
            $this->getDataPersistor()->set('mymodule_post', $post);
            return $resultRedirect->setPath('module/mymodule/new/');
        }
    }

       /**
     * Get Data Persistor
     *
     * @return DataPersistorInterface
     */
    public function getDataPersistor() {
        if ($this->dataPersistor === null) {
            $this->dataPersistor = ObjectManager::getInstance()
                    ->get(DataPersistorInterface::class);
        }

        return $this->dataPersistor;
    }
}

What is purpose of dataPersistor & when i got error my data are not filled up in Form. I have filled up Title, but Description is blank. So when error message display, it should display my filled up Title.

Answer 1

Finally solved by referring Magento Core Contact Module

vendor\magento\module-contact\Controller\Index\Post.php

public function execute(){
    $post = $this->getRequest()->getPostValue();
}

Will call vendor\magento\module-contact\Helper\Data.php

/**
 * Get value from POST by key
 *
 * @param string $key
 * @return string
 */
public function getPostValue($key)
{
    if (null === $this->postData) {
        $this->postData = (array) $this->getDataPersistor()->get('contact_us');
        $this->getDataPersistor()->clear('contact_us');
    }

    if (isset($this->postData[$key])) {
        return (string) $this->postData[$key];
    }

    return '';
}

Make sure in controller while error set name is same

vendor\magento\module-contact\Controller\Index\Post.php

 $this->getDataPersistor()->set('contact_us', $post);

So in vendor\magento\module-contact\view\frontend\templates\form.phtml

<input name="name" id="name" title="<?php /* @escapeNotVerified */ echo __('Name') ?>" value="<?php echo $block->escapeHtml($this->helper('Magento\Contact\Helper\Data')->getPostValue('name') ?: $this->helper('Magento\Contact\Helper\Data')->getUserName()) ?>" class="input-text" type="text" data-validate="{required:true}"/>

This way it always your post data. Hope it helps to others as well. :)