Method configure

/**
 * Add children buttons as dropdown.
 *
 * @param   callable  $handler  The callback to configure dropdown items.
 *
 * @return  static
 *
 * @since  4.0.0
 */
public function configure(callable $handler) : self
{
    $child = $this->getChildToolbar();
    $handler($child);
    return $this;
}