How does woocommerce_loop_add_to_cart_link filter hook work in depth?

Question

I have some basic Woocommerce questions and cannot find anywhere online.

• from the code below, where are $args come from?
• how $product and $args assign to %s ?

thanks!

global $product;

echo apply_filters( 'woocommerce_loop_add_to_cart_link', // WPCS: XSS ok.
    sprintf( '<a href="%s" data-quantity="%s" class="%s" %s>%s</a>',
        esc_url( $product->add_to_cart_url() ),
        esc_attr( isset( $args['quantity'] ) ? $args['quantity'] : 1 ),
        esc_attr( isset( $args['class'] ) ? $args['class'] : 'button' ),
        isset( $args['attributes'] ) ? wc_implode_html_attributes( $args['attributes'] ) : '',
        esc_html( $product->add_to_cart_text() )
    ),
$product, $args );

Answer 1

1) Explanations for $args variable in template file loop/add_to_cart.php:

The code in your question come from the template file loop/add_to_cart.php.
It is called by content-product.php template file on woocommerce_after_shop_loop_item hook:

/**
 * Hook: woocommerce_after_shop_loop_item.
 *
 * @hooked woocommerce_template_loop_product_link_close - 5
 * @hooked woocommerce_template_loop_add_to_cart - 10
 */
do_action( 'woocommerce_after_shop_loop_item' );

As you can see the template function woocommerce_template_loop_add_to_cart() does this job and it is located in WooCommerce plugin under includes/wc-template-functions.php.

So the default arguments are:

$defaults = array(
    'quantity'   => 1,
    'class'      => implode(
        ' ',
        array_filter(
            array(
                'button',
                wc_wp_theme_get_element_class_name( 'button' ), // escaped in the template.
                'product_type_' . $product->get_type(),
                $product->is_purchasable() && $product->is_in_stock() ? 'add_to_cart_button' : '',
                $product->supports( 'ajax_add_to_cart' ) && $product->is_purchasable() && $product->is_in_stock() ? 'ajax_add_to_cart' : '',
            )
        )
    ),
    'attributes' => array(
        'data-product_id'  => $product->get_id(),
        'data-product_sku' => $product->get_sku(),
        'aria-label'       => $product->add_to_cart_description(),
        'aria-describedby' => $product->add_to_cart_aria_describedby(),
        'rel'              => 'nofollow',
    ),
);

They are parsed through the filter hook woocommerce_loop_add_to_cart_args:

$args = apply_filters( 'woocommerce_loop_add_to_cart_args', wp_parse_args( $args, $defaults ), $product );

… allowing to make changes on this arguments.

The $args variable in woocommerce_loop_add_to_cart_link is made of that.

---

2. How $product and $args are assigned to %s:

This is specific to PHP printf() and sprintf() function where each %s are a placeholder.

In '<a href="%s" data-quantity="%s" class="%s" %s>%s</a>':

• the 1st placeholder %s will be replaced by esc_url( $product->add_to_cart_url() )
• the 2nd placeholder %s by esc_attr( isset( $args['quantity'] ) ? $args['quantity'] : 1 )
• and so on…

Documentation for printf() and sprintf() php functions.