WooCommerce - Restriction of one category per order - php

Question

I searched in the different messages but I can't find exactly what I am looking for ... Could you help me?
I found this code which almost matches what I need.
It prevents the addition of new products if a product is already present in the cart.
I would also like to add the additional condition which is that if I have products in my cart, it is not possible to add the product. And show an alert message.
// Add custom Theme Functions here
// Custom conditional function that checks for parent product categories
function has_parent_term( $product_id ) {
// HERE set your targeted product category SLUG
$category_slug = 'box-dejeuner'; // <==== <==== <==== <==== <==== <==== <====
// Convert category term slug to term id
$category_id = get_term_by('slug', $category_slug, 'product_cat')->term_id;
$parent_term_ids = array(); // Initializing
// Loop through the current product category terms to get only parent main category term
foreach( get_the_terms( $product_id, 'product_cat' ) as $term ){
if( $term->parent > 0 ){
$parent_term_ids[] = $term->parent; // Set the parent product category
} else {
$parent_term_ids[] = $term->term_id;
}
}
return in_array( $category_id, $parent_term_ids );
}
// Avoid add to cart others product categories when "box-dejeuner" is in cart
add_filter( 'woocommerce_add_to_cart_validation', 'specific_category_avoid_add_to_cart_others', 20, 3 );
function specific_category_avoid_add_to_cart_others( $passed, $product_id, $quantity) {
if( WC()->cart->is_empty() || has_parent_term( $product_id ) ) {
return $passed;
}
foreach( WC()->cart->get_cart() as $cart_item ){
if( has_parent_term( $cart_item['product_id'] ) ) {
wc_add_notice( __('Si une box déjeuner est présente dans votre panier, vous ne pouvez pas commander un autre produit. Veuillez finaliser votre commande de la box et repasser une nouvelles commandes pour vos autres produits. ', 'woocommerce' ), 'error' ); // Display a custom error notice
return false; // Avoid add to cart
}
}
return $passed;
}