Add to cart button altering doesn't work with AJAX product variations switching

Ok we don't need another hook, this seems to be the right one, but

$variation = $orderItem->getPurchasedEntity();

always returns the first variation of the current product!

Okay I think we need to do it differently:

From AddToCartForm:

  public function buildForm(array $form, FormStateInterface $form_state) {
    $original_form = $form;
    $form = parent::buildForm($form, $form_state);
    /** @var \Drupal\commerce_order\Entity\OrderItemInterface $order_item */
    $order_item = $this->entity;
    // The widgets are allowed to signal that the form should be hidden
    // (because there's no purchasable entity to select, for example).
    if ($form_state->get('hide_form')) {
      $form['#access'] = FALSE;
    }
    else {
      $selected_variation = $form_state->get('selected_variation');
      // If the order item references a different variation than the one
      // currently selected, and the selected variation is supposed to use
      // a different order item type, rebuild the form.
      if ($selected_variation && $order_item->getPurchasedEntityId() != $selected_variation) {
        /** @var \Drupal\commerce_product\Entity\ProductVariationInterface $selected_variation */
        $selected_variation = $this->entityTypeManager->getStorage('commerce_product_variation')->load($selected_variation);
        if ($selected_variation->getOrderItemTypeId() !== $order_item->bundle()) {
          /** @var \Drupal\commerce_order\OrderItemStorageInterface $order_item_storage */
          $order_item_storage = $this->entityTypeManager->getStorage('commerce_order_item');
          $order_item = $order_item_storage->createFromPurchasableEntity($selected_variation);
          $this->prepareOrderItem($order_item);
          $this->setEntity($order_item);
          $form_display = EntityFormDisplay::collectRenderDisplay($order_item, $this->operation);
          $this->setFormDisplay($form_display, $form_state);
          $form = $original_form;
          $form = parent::buildForm($form, $form_state);
        }
      }
    }

    return $form;
  }

Found really really helpful documentation here btw: 🐛 Document how to alter forms when hook_form_alter won't work Needs work

Found the solution in \Drupal\commerce_cart\Form\AddToCartForm::buildForm()

Works as expected now!

Automatically closed - issue fixed for 2 weeks with no activity.