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<?php

class custom_feed {
  
	public $feed = 'custom-xml';

	public function __construct() {
	
		add_action( 'init', array( $this, 'init' ) );
		
		add_filter( 'pre_get_posts', array( $this, 'pre_get_posts' ) );
		
	}
  
	public function init() {
	
		// feed name to access in URL eg. /feed/custom-xml/
		add_feed( $this->feed, array( $this, 'xml' ) );
	
	}
  
	public function pre_get_posts( $query ) {
	
		if ( $query->is_main_query() && $query->is_feed( $this->feed ) ) {
			
			// modify query here eg. show all posts
			$query->set( 'nopaging', 1 );
			
		}
		
		return $query;
	
	}
	
	public function xml() {
		
		// either output template & loop here or include a template
		
		if ( have_posts() ) : while( have_posts() ) : the_post();
		
			// standard loop functions can be used here
		
		endwhile; endif;
		
	}
  
}

$custom_feed = new custom_feed();