SystemMainBlock.php

<?php

namespace Drupal\system\Plugin\Block;

use Drupal\Core\Block\Attribute\Block;
use Drupal\Core\Block\BlockBase;
use Drupal\Core\Block\MainContentBlockPluginInterface;
use Drupal\Core\StringTranslation\TranslatableMarkup;

/**
 * Provides a 'Main page content' block.
 */
class SystemMainBlock extends BlockBase implements MainContentBlockPluginInterface {
    
    /**
     * The render array representing the main page content.
     *
     * @var array
     */
    protected $mainContent;
    
    /**
     * {@inheritdoc}
     */
    public function setMainContent(array $main_content) {
        $this->mainContent = $main_content;
    }
    
    /**
     * {@inheritdoc}
     */
    public function build() {
        return $this->mainContent;
    }

}