Are blocks rendered yet

RenderedBlocks.php

<?php
final class RenderedBlocks {

    /**
     * instance.
     *
     * @var RenderedBlocks $instance class instance.
     */
    private static $instance = null;

    /**
     * rendered_blocks.
     *
     * @var array $rendered_blocks store all the blocks that have been rendered
     */
    private $rendered_blocks = null;


    public function __construct()
    {
        $this->init();
    }


    /**
     * instance.
     *
     * Return an instance of this class if it exists, else create it and return it..
     *
     */
    public static function instance(): RenderedBlocks
    {
        if (!isset(self::$instance) && !(self::$instance instanceof \RenderedBlocks)) {
            self::$instance = new Self();
        }

        return self::$instance;
    }


    /**
     * init
     *
     * Register to the pre_render_block filter, to store all the blocks that have been rendered
     *
     * @uses pre_render_block https://developer.wordpress.org/reference/hooks/pre_render_block/
     * @uses add_filter https://developer.wordpress.org/reference/functions/add_filter/
     *
     */
    public function init()
    {
        add_filter('pre_render_block', array($this, 'cache_rendered_blocks'), 2, 2);
    }


    /**
     * cache_rendered_blocks
     *
     * Callback for the pre_render_block filter.
     * Push all rendered blocks to the rendered_blocks array.
     *
     */
    public function cache_rendered_blocks($parsed_block, $source_block)
    {
        $this->rendered_blocks[] = $source_block;
    }


    /**
     * is_block_rendered
     *
     * Check if a block of a certain type has been rendered on the page yet
     *
     */
    public function is_block_rendered($block_name)
    {
        foreach ($this->rendered_blocks as $block) {
            if ($block['blockName'] == $block_name) {
                return true;
            }
        }

        return false;
    }


    /**
     * print_rendered_blocks
     *
     * Utility: Print.r all the rendered blocks.
     *
     */
    public function print_rendered_blocks()
    {
        echo '<pre>';
        print_r($this->rendered_blocks);
        echo '<pre>';
    }

}


/**
 *
 * Initialize the class on WordPress init
 *
 */
add_action('init', function () {
    $inst = RenderedBlocks::instance();
})
?>


<?php
/**
 *
 * Somewhere in your code you can check if a certain block has been rendered yet.
 *
 */
( RenderedBlocks::instance() )->is_block_rendered('core/heading');
?>