class-menu-item-meta-fields.php

<?php
/**
 * Created by PhpStorm.
 * User: rahul
 * Date: 5/8/16
 * Time: 3:16 PM
 * 
 * @ref https://gist.github.com/kucrut/3804376
 */

require_once ABSPATH . 'wp-admin/includes/nav-menu.php';

class Nav_Menu_Item_Meta_Fields {
	/**
	 * @var array $options Fields templates for meta box of menu items
	 */
	static $options = array(
		
		'item_tpl' => '<p class="nav-country-meta-field-{name} description description-wide">
						   <label for="edit-menu-item-{name}-{id}">{label}<br>
						        {tpl_type}
						   </label>
					   </p>',

		'select_tpl' => '<select name="menu-item-{name}[{id}]" class="widefat">
							<option value="">--Select--</option>
							{options}
						 </select>',
		
		'input_tpl'  => '<input
							type="{field_type}"
							id="edit-menu-item-{name}-{id}"
							class="widefat edit-menu-item-{name}"
							name="menu-item-{name}[{id}]"
							value="{value}">',
		);

	/**
	 * Setup all meta fields from $options['fields'] for nav menu items.
	 */
	static function setup() {
		
		if ( ! is_admin() ) {
			return;
		}
		
		$meta_fields = apply_filters( 'nav_menu_item_meta_fields', array() );
		
		if ( empty($meta_fields) )
			return;
		
		self::$options['fields'] = $meta_fields;
		
		add_filter( 'wp_edit_nav_menu_walker', function () {
			return 'Custom_Fields_Walker_Nav_Menu_Edit';
		} );

		add_action( 'save_post', array( __CLASS__, '_save_post' ), 10, 2 );
	}

	/**
	 * @param string $name Meta field name
	 *
	 * @return string Nav Menu Item metabox-key ( post-meta key )
	 */
	static function get_menu_item_postmeta_key($name) {
		return '_menu_item_' . $name;
	}

	/**
	 * @param object $item Menu item data object.
	 *
	 * @return string
	 */
	static function get_field( $item ) {

		$meta_fields = '';

		foreach( self::$options['fields'] as $field ) {
			$field['value'] = get_post_meta($item->ID, self::get_menu_item_postmeta_key($field['name']), true);
			$field['id'] = $item->ID;
			$item_tpl_key = "item_tpl";
			$item_field_tpl_key = "input_tpl";

			if ( $field[ "field_type" ] == "select" ) {
				$item_field_tpl_key = "select_tpl";
			}

			$field_tpl = str_replace(
				'{tpl_type}',
				self::$options[$item_field_tpl_key],
				self::$options[$item_tpl_key]
			);

			if ( isset( $field[ 'options' ] ) && ! empty( $field[ 'options' ] ) && is_array( $field[ 'options' ] ) ) {
				$options = "";
				foreach ( $field['options'] as $val => $name ) {
					$selected = "";
					if( $field["value"] == $val ) {
						$selected = "selected=selected";
					}
					$options .= "<option value=$val $selected>$name</option>";
				}
				$field['options'] = $options;
			}

			$meta_fields .= str_replace(
				array_map(function($key){ return '{' . $key . '}'; }, array_keys($field)),
				array_values(array_map('esc_attr', $field)),
				$field_tpl
			);
		}
		return html_entity_decode( $meta_fields );
	}

	/**
	 * Save the newly submitted fields
	 *
	 * @hook {action} save_post
	 *
	 * @param int    $post_id   Menu item id
	 * @param object $post      Menu item data object.
	 *
	 * @return mixed
	 */
	static function _save_post( $post_id, $post ) {
		if ( $post->post_type !== 'nav_menu_item' ) {
			return $post_id;
		}

		foreach( self::$options['fields'] as $field_schema ) {
			$form_field_name = 'menu-item-' . $field_schema['name'];
			// @todo FALSE should always be used as the default $value, otherwise we wouldn't be able to clear checkboxes
			if (isset($_POST[$form_field_name][$post_id])) {
				$key = self::get_menu_item_postmeta_key($field_schema['name']);
				$value = stripslashes($_POST[$form_field_name][$post_id]);
				update_post_meta($post_id, $key, $value);
			}
		}
	}

}

add_action( 'init', array( 'Nav_Menu_Item_Meta_Fields', 'setup' ) );

class Custom_Fields_Walker_Nav_Menu_Edit extends Walker_Nav_Menu_Edit {

	/**
	 * Start the element output.
	 *
	 * @ref wp-admin/includes/class-walker-nav-menu-edit.php
	 *
	 * @see Walker_Nav_Menu::start_el()
	 *
	 *
	 * @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   Not used.
	 * @param int    $id     Not used.
	 */
	function start_el( &$output, $item, $depth, $args, $id = 0 ) {
		$item_output = '';
		
		parent::start_el( $item_output, $item, $depth, $args, $id );
		
		if ( $meta_fields = Nav_Menu_Item_Meta_Fields::get_field( $item ) ) {
			// Inject $new_fields before: <div class="menu-item-actions description-wide submitbox">
			$item_output = preg_replace( '/(?=<div[^>]+class="[^"]*submitbox)/', $meta_fields, $item_output );
		}

		$output .= $item_output;
	}
}

Add fields using this filter

add_filter( 'nav_menu_item_meta_fields', 'custom_nav_menu_item_meta_fields' );

function custom_nav_menu_item_meta_fields( $fields ) {

    $fields[] = array(
        'name' => 'country',
        'label' => __('Country', 'nav_meta_fields'),
        'container_class' => 'country-type',
        'options' => array(
            'in'  => 'India',
            'usa' => 'USA',
        ),
        'field_type' => 'select',
    );

    $fields[] = array(
        'name' => 'sub-title',
        'label' => __('Sub Title', 'nav_meta_fields'),
        'container_class' => 'sub-title',
        'field_type' => 'text',
    );

    return $fields;
}