magento2-modules/app/code/Mageplaza/Seo/Observer/Markup.php

<?php

namespace Mageplaza\Seo\Observer;

use Magento\Framework\Event\ObserverInterface;
use Magento\Framework\Event\Observer;

class Markup implements ObserverInterface
{
    /**
     * @param \Magento\Framework\Event\Observer $observer
     */
    public function execute(Observer $observer)
    {
        /* @var \Magento\Framework\View\LayoutInterface $layout */
        $layout = $observer->getEvent()->getLayout();
        $action = $observer->getEvent()->getFullActionName();

        /**
         * Add Canonical tag
         */
        $headBlock = $this->getBlock('head.additional', $layout);

        if ($headBlock != false) {
            $afterBodyStartContainer = $this->renderContainer('after.body.start', $layout);
            $afterBodyStartContainer = str_replace([' ', "\n"], ['', ''], $afterBodyStartContainer);

            /**
             * Add rich snippets organization
             */
            $subString = '<scripttype="application/ld+json">{"@context":"http://schema.org","@type":"Organization"';
            if (!$afterBodyStartContainer || strpos($afterBodyStartContainer, $subString) === false) {
                $layout->addBlock('\Mageplaza\Seo\Block\Richsnippets\Organization', 'mageplaza_seo_organization', 'head.additional', '');
            }

            /**
             * Add rich snippets sitename, sitelinks
             */
            $subString = '<scripttype="application/ld+json">{"@context":"http://schema.org","@type":"WebSite"';
            if (!$afterBodyStartContainer || strpos($afterBodyStartContainer, $subString) === false) {
                $layout->addBlock('\Mageplaza\Seo\Block\Sitelinks', 'mageplaza_seo_sitelinks', 'head.additional', '');
            }

            /**
             * Add markup data to specify action
             */
            switch ($action) {
                case 'catalog_category_view':
                    $headBlock->addChild(
                        'mageplaza_seo_open_graph',
                        '\Mageplaza\Seo\Block\Page\Head\Category',
                        ['template' => 'opengraph/category.phtml']
                    );
                    break;
                case 'catalog_category_view_type_default':
                    $headBlock->addChild(
                        'mageplaza_seo_open_graph',
                        '\Mageplaza\Seo\Block\Page\Head\Category',
                        ['template' => 'opengraph/category.phtml']
                    );
                    break;
                case 'cms_page_view':
                    $headBlock->addChild(
                        'mageplaza_seo_open_graph',
                        '\Mageplaza\Seo\Block\Page\Head\Page',
                        ['template' => 'opengraph/cms.phtml']
                    );
                    break;
                case 'catalog_product_view':
                    $headBlock->addChild(
                        'mageplaza_seo_open_graph',
                        '\Mageplaza\Seo\Block\Page\Head\Product',
                        ['template' => 'opengraph/product.phtml']
                    );
                    /**
                     * Add rich snippet product
                     */

                    $layout->addBlock(
                        '\Mageplaza\Seo\Block\Richsnippets\Product',
                        'mageplaza_seo_richsnippets_product',
                        'head.additional',
                        ''
                    );
                    break;
            }
        }
    }

    /**
     * Get block by name
     *
     * @param string $name
     * @param \Magento\Framework\View\LayoutInterface $layout
     * @return \Magento\Framework\View\Element\AbstractBlock|bool
     */
    public function getBlock($name, $layout)
    {
        $blocks = $layout->getAllBlocks();

        return isset($blocks[$name]) ? $blocks[$name] : false;
    }

    /**
     * Get block by name
     *
     * @param string $name
     * @param \Magento\Framework\View\LayoutInterface $layout
     * @return string
     */
    public function renderContainer($name, $layout)
    {
        $html     = '';
        $children = $layout->getChildNames($name);
        foreach ($children as $child) {
            $html .= $layout->renderElement($child);
        }

        return $html;
    }

    protected $logger;

    public function __construct(\Psr\Log\LoggerInterface $logger)
    {
        $this->logger = $logger;
    }
}