The best way to customize "nav-menu-template.php" to add <input> if the 'link_before' is "checkbox"

Question

I would like to add a 'link_before' control for a wp_nav_menu(), like that:

if ( strpos( $args->link_before, 'checkbox' ) == true ) {
    $args->link_before = '<input id="toggle' . $item->ID . '" type="checkbox" /> <label for="toggle' . $item->ID . '">';
}

To use the "checkbox before the link" trick in some menu.

How can I do that? Is it possible with "functions.php?

Answer 1

The best way to customize a navigation menu is to use a custom 'walker' function.. Here are the steps how to achieve the customization that you want!

1. Create a "wp-custom-nav-walker.php" file in your theme directory for a new custom 'walker' class.

2. Extend the Walker_Nav_Menu class and copy the class' function that you want to modify and override it in the new class, such like this:

   <?php
   class WP_Custom_Nav_Walker extends Walker_Nav_Menu {
   
   /**
    * 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    $id     Current item ID.
    */
   public function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
       $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';
   
       $classes = empty( $item->classes ) ? array() : (array) $item->classes;
       $classes[] = 'menu-item-' . $item->ID;
   
       /**
        * Filter the CSS class(es) applied to a menu item's list item element.
        *
        * @since 3.0.0
        * @since 4.1.0 The `$depth` parameter was added.
        *
        * @param array  $classes The CSS classes that are applied to the menu item's `<li>` element.
        * @param object $item    The current menu item.
        * @param array  $args    An array of {@see wp_nav_menu()} arguments.
        * @param int    $depth   Depth of menu item. Used for padding.
        */
       $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args, $depth ) );
       $class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';
   
       /**
        * Filter the ID applied to a menu item's list item element.
        *
        * @since 3.0.1
        * @since 4.1.0 The `$depth` parameter was added.
        *
        * @param string $menu_id The ID that is applied to the menu item's `<li>` element.
        * @param object $item    The current menu item.
        * @param array  $args    An array of {@see wp_nav_menu()} arguments.
        * @param int    $depth   Depth of menu item. Used for padding.
        */
       $id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args, $depth );
       $id = $id ? ' id="' . esc_attr( $id ) . '"' : '';
   
       $output .= $indent . '<li' . $id . $class_names .'>';
   
       $atts = array();
       $atts['title']  = ! empty( $item->attr_title ) ? $item->attr_title : '';
       $atts['target'] = ! empty( $item->target )     ? $item->target     : '';
       $atts['rel']    = ! empty( $item->xfn )        ? $item->xfn        : '';
       $atts['href']   = ! empty( $item->url )        ? $item->url        : '';
   
       /**
        * Filter the HTML attributes applied to a menu item's anchor element.
        *
        * @since 3.6.0
        * @since 4.1.0 The `$depth` parameter was added.
        *
        * @param array $atts {
        *     The HTML attributes applied to the menu item's `<a>` element, empty strings are ignored.
        *
        *     @type string $title  Title attribute.
        *     @type string $target Target attribute.
        *     @type string $rel    The rel attribute.
        *     @type string $href   The href attribute.
        * }
        * @param object $item  The current menu item.
        * @param array  $args  An array of {@see wp_nav_menu()} arguments.
        * @param int    $depth Depth of menu item. Used for padding.
        */
       $atts = apply_filters( 'nav_menu_link_attributes', $atts, $item, $args, $depth );
   
       $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 .'>';
       /** This filter is documented in wp-includes/post-template.php */
   
   
       // YOUR ADDED CONTROL STARTS HERE!!
       if ( $args->link_before === 'checkbox' ) {
           $item_output .= '<input id="toggle' . $item->ID . '" type="checkbox" /> <label for="toggle' . $item->ID . '">';
       }
       $item_output .= apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
       // YOUR ADDED CONTROL ENDS HERE!!
   
   
       $item_output .= '</a>';
       $item_output .= $args->after;
   
       /**
        * Filter a menu item's starting output.
        *
        * The menu item's starting output only includes `$args->before`, the opening `<a>`,
        * the menu item's title, the closing `</a>`, and `$args->after`. Currently, there is
        * no filter for modifying the opening and closing `<li>` for a menu item.
        *
        * @since 3.0.0
        *
        * @param string $item_output The menu item's starting HTML output.
        * @param object $item        Menu item data object.
        * @param int    $depth       Depth of menu item. Used for padding.
        * @param array  $args        An array of {@see wp_nav_menu()} arguments.
        */
       $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
   }
   
   }
   

3. Include the "wp-custom-nav-walker.php" file in your "functions.php" file:

   // Register Custom Navigation Walker
   require get_template_directory() . '/wp-custom-nav-walker.php';
   // OR
   require_once('wp-custom-nav-walker.php');
   

4. And use it like this:

   // Primary navigation menu.
   wp_nav_menu( array(
       'menu_class'     => 'nav-menu',
       'theme_location' => 'primary',
       'link_before'    => 'checkbox',
       'walker'         => new WP_Custom_Nav_Walker(),
   ) );
   

For more info you can check the wp_nav_menu() code reference.