Custom Product Type in WooCommerce

<?php
//
// Setting Custom Product Type for Yarn and Needles, with appropriate custom fields
//


// Register the custom product type after init

    function register_yarn_product_type() {

        class WC_Product_Yarn extends WC_Product_Simple {

            public function __construct( $product ) {

                $this->product_type = 'yarn';

                parent::__construct( $product );

            }

        }

    }
    add_action( 'init', 'register_yarn_product_type' );

    // Register custom product type in "Product Data -" drop down menu

    function add_yarn_product( $types ){

        // Key should be exactly the same as in the class product_type parameter
        $types[ 'yarn' ] = __( 'Yarn' );

        return $types;

    }
    add_filter( 'product_type_selector', 'add_yarn_product' );


}


// Show pricing fields for yarn product.

function yarn_custom_js() {
    if ( 'product' != get_post_type() ) : return; endif;
?>
    <script>

        jQuery(document).ready(function () {

            // Check which product type is selected

            var selectedProductType = jQuery('#product-type').val();

            // If it's 'yarn', make sure selected tabs are shown

            if (selectedProductType == 'yarn') {

                jQuery("li.inventory_options.inventory_tab,li.general_options.general_tab,div.options_group.pricing,#general_product_data > div.options_group:not(.show_if_downloadable,.show_if_external)").addClass('show_if_yarn').show();

            }
        });

    </script>

    <?php
}

add_action( 'admin_footer', 'yarn_custom_js' );