WooCommerce: Add stock filter function to builtin shortcodes.

wc-builtin-shortcode-stock-filter.php

<?php
/**
 * Description:  WooCommerce: Add stock filter function to builtin shortcodes.
 * Author:       chrono-meter@gmx.net
 * Version:      v20200727
 *
 * @link https://docs.woocommerce.com/document/woocommerce-shortcodes/
 *
 * Example:
 *      [products stock="instock"]
 *      [products stock="instock,onbackorder"]
 */


class WC_Builtin_Shortcode_Stock_Filter {
	public $shortcodes = [];

	/**
	 * @see \WC_Shortcodes::init()
	 */
	public function __construct() {
		$this->shortcodes = apply_filters( 'WC_Builtin_Shortcode_Stock_Filter/enabled_shortcodes', [
			'products',
			'product_category',
			'recent_products',
			'sale_products',
			'best_selling_products',
			'top_rated_products',
			'product',
			'sale_products',
			'featured_products',
			'product_attribute',
		] );

		foreach ( $this->shortcodes as $shortcode ) {
			add_filter( "shortcode_atts_{$shortcode}", [ $this, 'shortcode_atts' ], 10, 4 );
		}

		add_filter( 'woocommerce_shortcode_products_query', [ $this, 'woocommerce_shortcode_products_query' ], 10, 3 );
	}

	/**
	 * @param array $out
	 * @param array $pairs
	 * @param array $atts
	 * @param string $shortcode
	 *
	 * @return array
	 *
	 * @see \WC_Shortcode_Products::parse_attributes()
	 */
	function shortcode_atts( $out, $pairs, $atts, $shortcode ) {
		if ( isset( $atts['stock'] ) ) {
			$out['_stock_status'] = $atts['stock'];
		}

		return $out;
	}

	/**
	 * @param array $query_args
	 * @param array $atts
	 * @param string $shortcode
	 *
	 * @return array
	 *
	 * @link https://stackoverflow.com/a/45967535/3622941
	 */
	function woocommerce_shortcode_products_query( $query_args, $atts, $shortcode ) {
		if ( in_array( $shortcode, $this->shortcodes ) && ! empty( $atts['_stock_status'] ) ) {
			if ( ! isset( $query_args['meta_query'] ) ) {
				$query_args['meta_query'] = [];
			}

			$query_args['meta_query'][] = [
				'key'     => '_stock_status',
				'compare' => 'IN',
				'value'   => explode( ',', $atts['_stock_status'] ),
			];
		}

		return $query_args;
	}
}

new WC_Builtin_Shortcode_Stock_Filter();