How to pass parameters to add_feed in an OOP singleton

I struggle creating separate page feeds (list of URLs) in each language when WPML is active.

The plan is to have different feed names for each language, such as feed_name_en or feed_name_fr.

I wrote the following function that iterates through each available language on the site and should output a feed for each language containing only the pages in the particular language.

I found other examples that show how to pass parameters within add_action using closures. (eg. add_action('init', function() use($param) { some_out_of_scope_function($param) } )

But in my function I am getting the following error:

Object of class Closure could not be converted to string in

I have the suspicion that it is because I am using OOP to encapsule the function in an array add_feed( $feedname, array( $this, ... ) ). But I have no idea how to work around this.

How shall I approach this?

$feedname = 'feed_name'; 

private function createLanguageFeeds(){

    foreach($this->wpml_languages as $lang){

        // add the lang to the feed name. example: feed_name_en
        $feedname = $this->feedname . '_' . $lang;

        // making sure that the feed is created
        if ( ! in_array( $feedname, $wp_rewrite->feeds ) ) {
            $wp_rewrite->feeds[] = $feedname;
            flush_rewrite_rules( FALSE );
        }

        add_feed( $feedname, array( $this, function() use ($lang) {$this->create_page_feed($lang);} ) );
    }
}

private function create_page_feed($lang){
    // echo pages
}

Answer

If you’re using PHP 5.4 or newer, you can just pass the closure with the $lang parameter to the add_feed() function and $this will retain the reference to the object:

add_feed( $feedname, function() use ($lang) {$this->create_page_feed($lang);} );

Note that your create_page_feed() method needs to be public so WordPress can access it:

public function create_page_feed($lang){
    // echo pages
}

(see What is the difference between public, private, and protected? for more details.)

Here’s a tested & working demo plugin that registers a feed using the Singleton pattern:

<?php
/**
 * Plugin Name:       Singleton RSS Feed
 * Plugin URI:        https://cabrerahector.com
 * Description:       A demo RSS feed built using the Singleton pattern.
 * Version:           1.0.0
 * Author:            Hector Cabrera
 * Author URI:        https://cabrerahector.com/
 * License:           GPL-2.0+
 * License URI:       http://www.gnu.org/licenses/gpl-2.0.txt
 * Text Domain:       singleton-rss-feed
 * Domain Path:       /languages
 */

class Singleton_RSS_Feed
{
    /**
     * The unique instance of the plugin.
     *
     * @var Singleton_RSS_Feed
     */
    private static $instance;

    /**
     * Gets an instance of our plugin.
     *
     * @return Singleton_RSS_Feed
     */
    public static function get_instance()
    {
        if (null === self::$instance) {
            self::$instance = new self();
        }

        return self::$instance;
    }

    /**
     * Constructor.
     */
    private function __construct()
    {
        // Actions
        add_action('init', array($this, 'createFeed'));
    }

    /**
     * Register the feed.
     */
    public function createFeed()
    {
        global $wp_rewrite;

        $feedname = 'some_rrs_feed';
        $lang = 'en';

        // Making sure that the feed is created
        if ( ! in_array( $feedname, $wp_rewrite->feeds ) ) {
            $wp_rewrite->feeds[] = $feedname;
            flush_rewrite_rules( FALSE );
        }

        add_feed( $feedname, function() use ($lang) {$this->renderFeed($lang);} );
    }

    /**
     * Renders the feed.
     *
     * @param string $lang
     */
    public function renderFeed($lang)
    {
        header( 'Content-Type: application/rss+xml' );
        ?>
        <xml version="1.0" encoding="UTF-8">
        <rss version="2.0">
            <channel><title>RSS Feed in <?php echo $lang; ?> language</title></channel>
        </rss>
        <?php
    }
}

$Singleton_RSS_Feed = Singleton_RSS_Feed::get_instance();

The feed will be available here: https://www.example.com/feed/some_rrs_feed.