WooCommerce Product Sample uses the Sample object class which extends the WooCommerce WC_Data class in the same way that the WC_Product class does. Since a product sample, ultimately, is itself a product, this is the most obvious approach. This way, every property and method can be inherited and, eventually, adjusted to describe the sample properties, relative to the regular product it relates to.

The main difference with a regular product is that a sample is not stored as a separate row in the wp_posts table. Instead, the information about the product sample is a set of metadata stored in the wp_postmeta table with the same ID as the regular product. For this reason, from the perspective of the interaction with the database, the Sample class doesn't use a WC_Data_Store class. It rather defines its own read, save, update or delete methods that take care of the retrieval, update and deletion of the sample-related metadata from the wp_postmeta table.

Still, the Sample class is designed to closely replicate the same properties and methods of the WC_Product class. This allows developers to extend the plugin using the knowledge they already have about the WC_Product class.

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For example, consider the following code snippet:

global $product;
echo $product->get_price();
// output: 10

This can easily be recognized as a fragment that outputs the price of a product. Similarly, the following code snippet does the same thing but with the sample price instead:

global $product;
$sample = new Barn2\Plugin\WC_Product_Sample\Sample( $product );
echo $sample->get_price();
// output: 1

In this code snippet, you can see that a sample object can be instantiated from the $product object it refers to. This can be done by passing either the $product object itself or its ID (although the former method, when viable, is preferred as it prevents an unnecessary call to the wc_get_product function). Then, every property and method can be accessed the same way developers are already used to with the $product object. The only difference is that the sample object will be returning the product sample properties instead of the regular product ones (when they are different, of course).

Whether the $sample object will return the same result the $product object does is just a matter of how the sample is configured. For example, if a sample is set to come from the same stock of the regular product, then all the inventory properties and methods (stock quantity, stock status, weight and dimensions) are going to return the exact same values the regular product does (and no additional, unnecessary data will be stored in the database).

The product property

Every sample can be instantiated from the product object it refers to. Because of the tight connection between a sample and the regular product it represents, a reference of the regular product is stored as a property of the sample object so that it can always be referred to when needed. This way, even for those properties or methods that are not replicated in the Sample class design, developers can always refer to the connected product and use its properties and methods instead.

Let's consider the following example. A store owner wants to disable the possibility to order samples of all the products that are currently on sale. This can be easily done with the following code snippet:

add_filter( 'wsa_sample_should_add_button', 'my_wsa_sample_should_add_button', 10, 2 );
function my_wsa_sample_should_add_button( $is_button_visible, $sample ) {
    $product = $sample->get_product();
    return $is_button_visible && ! $product->is_on_sale();
}

It is important to notice that, in the case of product variations, the product property of a sample refers to the variation. If it is necessary to reference the variable product that variation is a child of, it will be possible to use the parent_product property instead, which is going to be instantiated only if the current product is a variation.

Use of the sample object in action and filter callback functions

Similar to what happens with properties and methods, the Sample class replicates also many of the action and filter hooks associated with a WC_Product class. For example, when retrieving a sample property, the value can be filtered using the wsa_sample_get_{$prop} filter hook the exact same way one can use the woocommerce_product_get_{$prop} filter hook for the same purpose. Let's consider the following code snippet:

add_filter( 'wsa_sample_get_price', 'my_wsa_sample_get_price', 10, 2 );
function my_wsa_sample_get_price( $price, $sample ) {
    if ( ! $sample->is_virtual() ) {
        $price += 2;
    }
    return $price;
}

The fragment above increases the price of samples of non-virtual products by 2 (e.g. $5 becomes $7). As you can see, the $sample object has the same is_virtual() conditional method a regular $product object does and we don't need to refer to the connected product object.