MenuBlock.php

<?php

/**
 * @file
 * Contains \Drupal\menu\Plugin\Block\MenuBlock.
 */

namespace Drupal\menu\Plugin\Block;

use Drupal\system\Plugin\Block\SystemMenuBlock;
use Drupal\Component\Annotation\Plugin;
use Drupal\Core\Annotation\Translation;

/**
 * Provides a generic Menu block.
 *
 * @Plugin(
 *   id = "menu_menu_block",
 *   admin_label = @Translation("Menu"),
 *   module = "menu",
 *   derivative = "Drupal\menu\Plugin\Derivative\MenuBlock"
 * )
 */
class MenuBlock extends SystemMenuBlock {

  /**
   * {@inheritdoc}
   */
  public function build() {
    list($plugin, $menu) = explode(':', $this->getPluginId());

    // @todo i dunno the exactness of this part, esp. if we want the context to be 'optional'
    $url = $this->getContext('url thinger');
    if ($url) {
      // Force the menu tree renderer to use the custom path.
      menu_tree_set_path($menu, $url->getContextValue());
    }

    $ret = menu_tree($menu);

    if ($url) {
      // Undo the damage.
      $paths = drupal_static('menu_tree_set_path');
      unset($paths[$menu]);
    }

    return $ret;
  }

}