Wordpress nav-walker for Bootstrap 4.0.0-alpha.6

header.php

	<nav class="navbar navbar-toggleable-md navbar-inverse bg-inverse">
		<button class="navbar-toggler navbar-toggler-right" type="button" data-toggle="collapse" data-target="#navbarCollapse" aria-controls="navbarCollapse" aria-expanded="false" aria-label="Toggle navigation">
			<span class="navbar-toggler-icon"></span>
		</button>
		<a class="navbar-brand" href="#"><?php bloginfo('name'); ?></a>
		<?php
			wp_nav_menu( array(
				'theme_location'	=> 'header-menu',
				'container'			=> 'div',
				'container_class'	=> 'collapse navbar-collapse',
				'container_id'      => 'navbarCollapse',
				'menu_class'		=> 'nav navbar-nav',
				'fallback_cb'		=> '__return_false',
				'items_wrap'		=> '<ul id="%1$s" class="%2$s">%3$s</ul>',
				'depth'				=> 2,
				'walker'			=> new bootstrap_4_walker_nav_menu()
			) );
		?>
	</nav>

wp-bootstrap-navwalker.php

<?php
/*
Bootstrap 4.0.0-alpha6 nav walker extension class
=================================================
Add this to your `functions.php`
*/
class bootstrap_4_walker_nav_menu extends Walker_Nav_menu {
	
	function start_lvl( &$output, $depth = 0, $args = array() ){ // ul
		$indent = str_repeat("\t",$depth); // indents the outputted HTML
		$submenu = ($depth > 0) ? ' sub-menu' : '';
		$output .= "\n$indent<ul class=\"dropdown-menu$submenu depth_$depth\">\n";
	}
	
	function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ){ // li a span
		$indent = ( $depth ) ? str_repeat("\t",$depth) : '';
		
		$li_attributes = '';
		$class_names = $value = '';
		
		$classes = empty( $item->classes ) ? array() : (array) $item->classes;
		
		$classes[] = ($args->walker->has_children) ? 'dropdown' : '';
		$classes[] = ($item->current || $item->current_item_anchestor) ? 'active' : '';
		$classes[] = 'nav-item';
		$classes[] = 'nav-item-' . $item->ID;
		if( $depth && $args->walker->has_children ){
			$classes[] = 'dropdown-menu';
		}
		
		$class_names =  join(' ', apply_filters('nav_menu_css_class', array_filter( $classes ), $item, $args ) );
		$class_names = ' class="' . esc_attr($class_names) . '"';
		
		$id = apply_filters('nav_menu_item_id', 'menu-item-'.$item->ID, $item, $args);
		$id = strlen( $id ) ? ' id="' . esc_attr( $id ) . '"' : '';
		
		$output .= $indent . '<li ' . $id . $value . $class_names . $li_attributes . '>';
		
		$attributes = ! empty( $item->attr_title ) ? ' title="' . esc_attr($item->attr_title) . '"' : '';
		$attributes .= ! empty( $item->target ) ? ' target="' . esc_attr($item->target) . '"' : '';
		$attributes .= ! empty( $item->xfn ) ? ' rel="' . esc_attr($item->xfn) . '"' : '';
		$attributes .= ! empty( $item->url ) ? ' href="' . esc_attr($item->url) . '"' : '';
		
		$attributes .= ( $args->walker->has_children ) ? ' class="nav-link dropdown-toggle" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false"' : ' class="nav-link"';
		
		$item_output = $args->before;
		$item_output .= ( $depth > 0 ) ? '<a class="dropdown-item"' . $attributes . '>' : '<a' . $attributes . '>';
		$item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
		$item_output .= '</a>';
		$item_output .= $args->after;
		
		$output .= apply_filters ( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
		
	}
	
}