WP custom menu walker

wp_custom_menu_walker.php

// http://wordpress.stackexchange.com/questions/145984/change-an-li-class-name-in-a-wordpress-custom-menu-walker

<?php
class Main_Walker_Nav_Menu extends Walker_Nav_Menu {

	function start_lvl( &$output, $depth = 0, $args = [] ) {
		$indent = str_repeat( "\t", $depth );
		switch ( $depth ) {
			case 0:
				$class = 'menu__list-sub';
			break;
			default:
				$class = '';
			break;
		}
		if ( isset( $class ) ) {
			$output .= "\n$indent<ul class=\"$class\">\n";
		} else {
			$output .= "\n$indent<ul>\n";
		}
	}

	function start_el( &$output, $item, $depth = 0, $args = [], $id = 0 ) {
		$indent     = str_repeat( "\t", $depth );
		$attributes = '';

		! empty ( $item->attr_title ) and
		$item->attr_title !== $item->title and $attributes .= ' title="' . esc_attr( $item->attr_title ) . '"';

		! empty ( $item->url ) and $attributes .= ' href="' . esc_attr( $item->url ) . '"';

        	$current = in_array( "current_page_item", $item->classes ) ? ' current' : '';
		
		$classes = empty( $item->classes ) ? array() : (array) $item->classes;
        	$class_names = esc_attr( implode( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) ) );

		$attributes  = trim( $attributes );
		$title       = apply_filters( 'the_title', $item->title, $item->ID );
		$item_output = "$args->before<a $attributes class='$current $class_names'>$args->link_before$title</a>" . "$args->link_after$args->after";

		switch ( $depth ) {
			case 0:
				$class = 'menu__item';
			break;
			default:
			break;
		}

		if ( isset( $class ) ) {
			$output .= $indent . '<li class="' . $class . '">';
		} else {
			$output .= $indent . '<li>';
		}

		$output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
	}
}