newsupernova/app/code/Magefan/Blog/Block/Adminhtml/Edit/PreviewButton.php

<?php
/**
 * Copyright © 2015-2017 Ihor Vansach (ihor@magefan.com). All rights reserved.
 * See LICENSE.txt for license details (http://opensource.org/licenses/osl-3.0.php).
 *
 * Glory to Ukraine! Glory to the heroes!
 */
namespace Magefan\Blog\Block\Adminhtml\Edit;

use Magento\Framework\View\Element\UiComponent\Control\ButtonProviderInterface;

/**
 * Class PreviewButton
 */
class PreviewButton extends GenericButton implements ButtonProviderInterface
{
    /**
     * @return array
     */
    public function getButtonData()
    {
        $data = [];
        if ($this->getPostId()) {
            $data = [
                'label' => __('Preview'),
                'class' => 'preview',
                'on_click' => 'window.open(\'' . $this->getPreviewUrl() . '\');',
                'sort_order' => 35,
            ];
        }
        return $data;
    }

    /**
     * @return string
     */
    public function getPreviewUrl()
    {
        return $this->getUrl('*/*/preview', ['id' => $this->getPostId()]);
    }
}