Woocommerce如何从function.php获取自定义字段值

function custom_woocommerce_field() {
    woocommerce_form_field( 'custom_field', array(
        'type'        => 'text',
        'class'       => array('my-field-class form-row-wide'),
        'label'       => __('Custom Field', 'woocommerce'),
        'placeholder' => __('Enter custom field value', 'woocommerce'),
        ), get_post_meta( get_the_ID(), 'custom_field', true ));
}
add_action( 'woocommerce_before_add_to_cart_button', 'custom_woocommerce_field' );

function get_custom_field_value() {
    global $product;
    $custom_field_value = get_post_meta( $product->get_id(), 'custom_field', true );
    return $custom_field_value;
}

echo get_custom_field_value();