Appends the listing sub categories mega menu to the category menu item.

sub-category-mega-menu.php

<?php
/**
 * Appends the listing sub categories mega menu to the category menu item.
 *
 * @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 wp_nav_menu() arguments.
 */
function va_sub_category_mega_menu( $item_output, $item, $depth, $args ) {

	// Must be a page and in the secondary menu location.
	if ( VA_LISTING_CATEGORY !== $item->object || 'secondary' != $args->theme_location ) {
		return $item_output;
	}

	$defaults = array(
		'child_of'          => $item->object_id,
		'menu_cols'         => 3, // AppThemes custom.
		'cat_parent_count'  => true,
		'cat_child_count'   => true,
		'taxonomy'          => VA_LISTING_CATEGORY,
		'app_pad_counts'    => true, // AppThemes custom. Used in 'app-term-counts' feature.
		'column_wrapper'    => '%s',
		'main_list_wrapper' => '%s',
		'main_item_wrapper' => '<div class="parent-cat-wrap column column-block"><div class="parent-cat cat-item-%1$d"><a href="%2$s">%4$s%5$s</a></div><!-- .parent-cat -->%6$s' . "\r\n</div><!-- .parent-cat-wrap -->\r\n",
		'sub_list_wrapper'  => '<div class="sub-cat-list">%s' . "</div>\r\n",
		'sub_item_wrapper'  => '<div class="cat-item cat-item-%1$d"><a href="%2$s">%4$s%5$s</a>%6$s' . "</div>\r\n",
		'count_wrapper'     => ' <span class="cat-item-count label">%d</span>',
		'nocats_wrapper'    => '<div class="sub-cat-list"><div class="cat-item">%s' . "</div>\r\n</div>\r\n",
	);

	$categories_list = appthemes_categories_list( $defaults, array(
		'app_pad_counts' => $defaults['app_pad_counts'],
		'child_of' => ! empty( $defaults['child_of'] ) ? $defaults['child_of'] : 0,
	) );

	if ( ! empty(  $categories_list) ) {
		$item_output .= sprintf(
			apply_filters( 'va_category_mega_menu', '<ul class="listing-cats listing-cats-dropdown"><div class="cat-column row collapse small-up-1 medium-up-2 large-up-3">%s</div></ul>' ),
			$categories_list
		);
	}

	return $item_output;
}
add_filter( 'walker_nav_menu_start_el', 'va_sub_category_mega_menu', 10, 4 );