If you are working with Bootstrap and like most Sage developers are brave enough to work with the Bootstrap 4 beta with all it’s goodies you will often want to add a Bootstrap menu. So how is a Bootstrap Navigation in Sage 9 done? Here is how.

Sage 9 and Bootstrap

When you install Sage 9 do remember to pick Bootstrap as your CSS framework. This way the latest Bootstrap 4 will be added as the framework of choice. And that way the scss files needed to make Bootstrap 4 run will be added to your theme.

Partials Header

To load the navigation with the proper html tags edit the default templates/partials/header.blade.php and make it have this code for content:

<header class="banner">
 <div class="container">
 <nav class="navbar navbar-toggleable-md navbar-light bg-faded">
<button class="navbar-toggler navbar-toggler-right" type="button" data-toggle="collapse"
 data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false"
 aria-label="Toggle navigation">
 <span class="navbar-toggler-icon"></span>
 </button>
<a class="navbar-brand" href="{{ home_url('/') }}">{{ get_bloginfo('name', 'display') }}</a>
<div class="collapse navbar-collapse" id="navbarSupportedContent">
 @if (has_nav_menu('primary_navigation'))
 {!! wp_nav_menu(['theme_location' => 'primary_navigation', 'walker' => new wp_bootstrap_navwalker(), 'menu_class' => 'navbar-nav nav pull-right']) !!}
 @endif
 </div>
 <!-- <form class="form-inline collapse navbar-collapse pull-right">
 <button class="btn btn-outline-success" type="button">Main button</button>
 </form> -->
 </nav>
 </div>
</header>

This will make it work with the right html tags as well as CSS classes and IDs.

Custom Walker

You will of course also be needing a custom walker. And that walker should be loaded in the theme. In scr/  (app/ in beta 3) we add wp_bootstrap_navwalker.php:

<?php

/**
 * Class Name: wp_bootstrap_navwalker
 * GitHub URI: https://github.com/twittem/wp-bootstrap-navwalker
 * Description: A custom WordPress nav walker class to implement the Bootstrap 4 navigation style in a custom theme using the WordPress built in menu manager.
 * Version: 2.0.4
 * Author: Edward McIntyre - @twittem
 * License: GPL-2.0+
 * License URI: http://www.gnu.org/licenses/gpl-2.0.txt
 */

class wp_bootstrap_navwalker extends Walker_Nav_Menu {

/**
 * @see Walker::start_lvl()
 * @since 3.0.0
 *
 * @param string $output Passed by reference. Used to append additional content.
 * @param int $depth Depth of page. Used for padding.
 */
 public function start_lvl( &$output, $depth = 0, $args = array() ) {
 $indent = str_repeat( "\t", $depth );
 $output .= "\n$indent<div role=\"menu\" class=\" dropdown-menu\">\n";
 }

/**
 * Ends the list of after the elements are added.
 *
 * @see Walker::end_lvl()
 *
 * @since 3.0.0
 *
 * @param string $output Passed by reference. Used to append additional content.
 * @param int $depth Depth of menu item. Used for padding.
 * @param array $args An array of arguments. @see wp_nav_menu()
 */
 public function end_lvl( &$output, $depth = 0, $args = array() ) {
 $indent = str_repeat("\t", $depth);
 $output .= "$indent</div>\n";
 }

/**
 * Start the element output.
 *
 * @see Walker::start_el()
 *
 * @since 3.0.0
 *
 * @param string $output Passed by reference. Used to append additional content.
 * @param object $item Menu item data object.
 * @param int $depth Depth of menu item. Used for padding.
 * @param array $args An array of arguments. @see wp_nav_menu()
 * @param int $id Current item ID.
 */
 public function end_el( &$output, $item, $depth = 0, $args = array() ) {
 if($depth === 1){
 if(strcasecmp( $item->attr_title, 'divider' ) == 0 || strcasecmp( $item->title, 'divider') == 0) {
 $output .= '</div>';
 }else if ($depth === 1 && (strcasecmp( $item->attr_title, 'header') == 0 && $depth === 1)) {
 $output .= '</h6>';
 }
 }else{
 $output .= '</li>';
 }
 }

/**
 * @see Walker::start_el()
 * @since 3.0.0
 *
 * @param string $output Passed by reference. Used to append additional content.
 * @param object $item Menu item data object.
 * @param int $depth Depth of menu item. Used for padding.
 * @param int $current_page Menu item ID.
 * @param object $args
 */
 public function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
 $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

/**
 * Dividers, Headers or Disabled
 * =============================
 * Determine whether the item is a Divider, Header, Disabled or regular
 * menu item. To prevent errors we use the strcasecmp() function to so a
 * comparison that is not case sensitive. The strcasecmp() function returns
 * a 0 if the strings are equal.
 */

//( strcasecmp($item->attr_title, 'disabled' ) == 0 ) 
 
 if($depth === 1 && (strcasecmp( $item->attr_title, 'divider' ) == 0 || strcasecmp( $item->title, 'divider') == 0)) {
 $output .= $indent . '<div class="dropdown-divider">';
 }else if ((strcasecmp( $item->attr_title, 'header') == 0 && $depth === 1) && $depth === 1){
 $output .= $indent . '<h6 class="dropdown-header">' . esc_attr( $item->title );
 }else{
 $class_names = $value = '';

$classes = empty( $item->classes ) ? array() : (array) $item->classes;
 
 $atts = array();
 $atts['title'] = ! empty( $item->title ) ? $item->title : '';
 $atts['target'] = ! empty( $item->target ) ? $item->target : '';
 $atts['rel'] = ! empty( $item->xfn ) ? $item->xfn : '';
 $atts['href'] = ! empty( $item->url ) ? $item->url : '';

$id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args );
 
 if ( in_array( 'current-menu-item', $classes ) )
 $classes[] = ' active';

if($depth === 0){
 $classes[] = 'nav-item';
 $classes[] = 'nav-item-' . $item->ID;
 $atts['class'] = 'nav-link';
 if ( $args->has_children ){
 $classes[] = ' dropdown';
 $atts['href'] = '#';
 $atts['data-toggle'] = 'dropdown';
 $atts['class'] = 'dropdown-toggle nav-link';
 $atts['role'] = 'button';
 $atts['aria-haspopup'] = 'true';
 }
 $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );
 $class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';

$id = $id ? ' id="' . esc_attr( $id ) . '"' : '';
 $output .= $indent . '<li' . $id . $value . $class_names .'>';
 }else{
 $classes[] = 'dropdown-item';
 $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );
 $atts['class'] = $class_names;
 $atts['id'] = $id;
 }
 
 $atts = apply_filters( 'nav_menu_link_attributes', $atts, $item, $args );

$attributes = '';
 foreach ( $atts as $attr => $value ) {
 if ( ! empty( $value ) ) {
 $value = ( 'href' === $attr ) ? esc_url( $value ) : esc_attr( $value );
 $attributes .= ' ' . $attr . '="' . $value . '"';
 }
 }

$item_output = $args->before;
 $item_output .= '<a'. $attributes .'>';
 
 /*
 * Icons
 * ===========
 * Since the the menu item is NOT a Divider or Header we check the see
 * if there is a value in the attr_title property. If the attr_title
 * property is NOT null we apply it as the class name for the icon
 */
 if ( ! empty( $item->attr_title ) ){
 $item_output .= '<span class="' . esc_attr( $item->attr_title ) . '"></span>&nbsp;';
 }

$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 );
 }
 }

/**
 * Traverse elements to create list from elements.
 *
 * Display one element if the element doesn't have any children otherwise,
 * display the element and its children. Will only traverse up to the max
 * depth and no ignore elements under that depth.
 *
 * This method shouldn't be called directly, use the walk() method instead.
 *
 * @see Walker::start_el()
 * @since 2.5.0
 *
 * @param object $element Data object
 * @param array $children_elements List of elements to continue traversing.
 * @param int $max_depth Max depth to traverse.
 * @param int $depth Depth of current element.
 * @param array $args
 * @param string $output Passed by reference. Used to append additional content.
 * @return null Null on failure with no changes to parameters.
 */
 public function display_element( $element, &$children_elements, $max_depth, $depth, $args, &$output ) {
 if ( ! $element )
 return;

$id_field = $this->db_fields['id'];

// Display this element.
 if ( is_object( $args[0] ) )
 $args[0]->has_children = ! empty( $children_elements[ $element->$id_field ] );

parent::display_element( $element, $children_elements, $max_depth, $depth, $args, $output );
 }

/**
 * Menu Fallback
 * =============
 * If this function is assigned to the wp_nav_menu's fallback_cb variable
 * and a manu has not been assigned to the theme location in the WordPress
 * menu manager the function with display nothing to a non-logged in user,
 * and will add a link to the WordPress menu manager if logged in as an admin.
 *
 * @param array $args passed from the wp_nav_menu function.
 *
 */
 public static function fallback( $args ) {
 if ( current_user_can( 'manage_options' ) ) {

extract( $args );

$fb_output = null;

if ( $container ) {
 $fb_output = '<' . $container;

if ( $container_id )
 $fb_output .= ' id="' . $container_id . '"';

if ( $container_class )
 $fb_output .= ' class="' . $container_class . '"';

$fb_output .= '>';
 }

$fb_output .= '<ul';

if ( $menu_id )
 $fb_output .= ' id="' . $menu_id . '"';

if ( $menu_class )
 $fb_output .= ' class="' . $menu_class . '"';

$fb_output .= '>';
 $fb_output .= '<li><a href="' . admin_url( 'nav-menus.php' ) . '">Add a menu</a></li>';
 $fb_output .= '</ul>';

if ( $container )
 $fb_output .= '</' . $container . '>';

echo $fb_output;
 }
 }
}

This one is from a fork of the original Twittem repo. It has been created by @sebakerckhof . A bit older now, but despite some BS changes it works well.

Including Custom Walker

To load it inside the theme you should have this in resources/functions.php (in latest Sage 9 beta3)

array_map(function ($file) use ($sage_error) {
 $file = "src/{$file}.php";
 if (!locate_template($file, true, true)) {
 $sage_error(sprintf(__('Error locating <code>%s</code> for inclusion.', 'sage'), $file), 'File not found');
 }
}, ['helpers', 'setup', 'filters', 'admin', 'wp_bootstrap_navwalker']);

The array_map(function..) is already there. You just need to add the file to the array there to have it included. This is different from working in Sage 9 so it may be a little getting used to.

Once you have done that the Bootstrap Navigation in Sage 9 will be a fact. Of course plenty of styling to be done and perhaps you want other classes for different displays within the page. But I am sure you will be able to work that out )) .