Add submenu using the bootstrap wp_nav_menu

I have a problem to generate a submenu using wp_nav_menu . How can I change the ClassA submenu generated by the wp_nav_menu for ClassA the Bootstrap ?

see my problem:

This is my code that generates the menu:

<nav class="row menu">
        <?php wp_nav_menu( array( 
            'theme_location'      => 'category-menu',
            'container'             => '',
            'container_class'   => '',
            'container_id'      => 'main-navbar-collapse',
            'menu_class'        => 'nav nav-pills',
            'menu_id'           => '',
            'echo'              => true,
            'fallback_cb'       => 'wp_page_menu',
            'before'            => '',
            'after'             => '',
            'link_before'       => '',
            'link_after'        => '',
            'items_wrap'        => '<ul id="%1$s" class="%2$s">%3$s</ul>',
            'depth'             => 0,
            'walker'            => ''
           )); ?>
    </nav>      

How to modify it for ClassA the dropdown bootstrap ?

You will need to write a custom menu walker.

See this code for the walker. (Or do a search for “Bootstrap Menu Walker”.)

Then use:

    wp_nav_menu( array(
      'menu' => 'header',
      'menu_class' => 'nav nav-pills',
      'container' => 'nav',
      'container_class' => $classes,
      'fallback_cb' => false,
      'depth' => 2,
      'walker' => new Bootstrap_Walker_Menu_Nav(),
      'theme_location' => 'header',
      'echo' => false,
      ));