Hooks around add to cart for Woocommerce

Question

I am running a WooCommerce store with WC Marketplace. What I am trying to achieve with the hook below is to prevent a new item being added to a basket if there is already a product in the basket from a different vendor. e.g. If a shopper adds product x from vendor y to his basket, if they were to add product a from vendor b, then the item will not be added and the user will be informed of the error.

I have 2 questions:
- firstly when does a hook run, is it before the main function fired or after? I have a hook for the function woocommerce_add_to_cart. So I want to know will the hook fire after the function woocommerce_add_to_cart runs or before.
- My second question is, I have attached the hook below, in your opinion would this work?

function action_woocommerce_add_to_cart( $cart_item_key, $product_id, $quantity, $variation_id, $variation, $cart_item_data ) { 
    $same_vendor = 1;

    $empty = WC_Cart::is_empty();

    //If there is an item in the cart then,
    if (!$empty) {
        //Get the VendorId of the product being added to the cart.
        $vendor = get_wcmp_product_vendors($product_id);
        $vendor_id = $vendor->id;

        foreach( WC()->cart->get_cart() as $cart_item ) {
            //Get the vendor Id of the item
            $cart_product_id = $cart_item['product_id'];
            $cart_vendor = get_wcmp_product_vendors($product_id);
            $cart_vendor_id = $cart_vendor->id;

            //If two products do not have the same Vendor then set $same_vendor to 0
            if($vendor_id !== $cart_vendor_id) {
                $same_vendor = 0;
            }
        }

        if ($same_vendor === 0) {
            WC()->cart->remove_cart_item( $cart_item_key );
            //How do I show a message to tell the customer.
        }
    }
}

Regards

Answer 1

Here below are the hooks involved in WC_Cart add_to_cart() method:

A) Before an item is added to cart:

1. Validation filter hook woocommerce_add_to_cart_validation
2. Item Quantity change filter hook woocommerce_add_to_cart_quantity (not with ajax)
3. Item Data change filter hook woocommerce_add_cart_item_data (not with ajax)
4. and some others related to "sold individually" products (see here)

A) After an item is added to cart:

1. Change cart Item filter hook woocommerce_add_cart_item
2. Add an event, action hook woocommerce_add_to_cart

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To be clear in your case:

1. As you can see now woocommerce_add_to_cart is not a function but only an action hook.
2. Hook location: It's located inside WC_Cart add_to_cart() method (at the end of the source code).
3. When the hook is fired: It's fired once the WC_Cart add_to_cart() method is executed.
4. What is the purpose: To execute some custom code when this method is executed (event).

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Regarding your code:
It should be better to use the dedicated filter hook woocommerce_add_to_cart_validation that will stop customer that want to add a new item to the cart if there is already a product in cart from a different vendor, displaying optionally a custom message:

add_filter( 'woocommerce_add_to_cart_validation', 'filter_add_to_cart_validation', 10, 3 );
function filter_add_to_cart_validation( $passed, $product_id, $quantity ) { 
    if ( WC()->cart->is_empty() ) return $passed;

    // Get the VendorId of the product being added to the cart.
    $current_vendor = get_wcmp_product_vendors($product_id);

    foreach( WC()->cart->get_cart() as $cart_item ) {
        // Get the vendor Id of the item
        $cart_vendor = get_wcmp_product_vendors($cart_item['product_id']);

        // If two products do not have the same Vendor
        if( $current_vendor->id != $cart_vendor->id ) {
            // We set 'passed' argument to false
            $passed = false ;

            // Displaying a custom message
            $message = __( "This is your custom message", "woocommerce" );
            wc_add_notice( $message, 'error' );
            // We stop the loop
            break; 
        }
    }
    return $passed;
}

Code goes in function.php file of your active child theme (or active theme) or in any plugin file.

Tested and works.

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