TheCartPress allows, to plugin developers, to add new sorting methods using three filters. Those filters allow to add new sorting methods or to remove the existing ones.
Those filters are:

• tcp_sorting_fields: Allows to add or delete the types of sorts.
• tcp_sort_query: Allows to sort the query for TCP shortcodes and Custom post type widgets.
• tcp_sort_main_loop: Allows to add a sorting method to execute in the main loop.

Example

The example is a plugin to add a new classification by Stock:

<?php
/*
Plugin Name: TheCartPress Order by Stock
Plugin URI: http://thecartpress.com
Description: Order by Stock for TheCartPress
Version: 1.1
Author: TheCartPress team
Author URI: http://thecartpress.com
License: GPL
Parent: thecartpress
*/

class OrderByStock {

	function __construct() {
		add_action( 'init', array( $this, 'init' ) );
	}

	function init() {
		add_filter( 'tcp_sorting_fields', array( $this, 'tcp_sorting_fields' ) );
		if ( ! is_admin() ) {
	add_filter( 'tcp_sort_main_loop', array( $this, 'tcp_sort_main_loop' ), 10, 3 );</span>
		}
	}

	//adds the new sorting method: 'stock'
	function tcp_sorting_fields( $sorting_fields ) {
		$sorting_fields[] = array( 'value' => 'stock', 'title' => 'By Stock' );
		return $sorting_fields;
	}

private function compare_by_stock_asc( $s1, $s2 ) {</span>
		$p1 = tcp_get_the_stock( $s1->ID );
		$p2 = tcp_get_the_stock( $s2->ID );
		$v  = $p1 > $p2 ? 1 : ( $p1 < $p2 ? -1 : 0 );
		return $v;
	}

	private function compare_by_stock_desc( $s1, $s2 ) {
		$p1 = tcp_get_the_stock( $s1->ID );
		$p2 = tcp_get_the_stock( $s2->ID );
		$v  = $p1 > $p2 ? -1 : ($p1 < $p2 ? 1 : 0 );
		return $v;
	}

	function tcp_sort_main_loop( $query, $order_type, $order_desc ) {
		if ( $order_type == 'stock' ) {
			$query['orderby']	= 'meta_value_num';
			$query['meta_key']	= 'tcp_stock';
			//$query_string['order'] = $order_desc();
		}
		return $query;
	}
}

new OrderByStock();
?>

Usage

Since 1.1

Related

tcp_the_sort_panel | tcp_get_sorting_fields | tcp_get_sorting_fields