Update Shopping Carts in Real-time

# sneakers_23_admin/lib/sneakers_23_web/channels/shopping_cart_channel.ex
  def handle_info(:update_tracked_cart, socket = %{
    assigns: %{cart: cart, cart_id: id}
  }) do
    {:ok, _} = Sneakers23Web.CartTracker.update_cart(
      socket, %{cart: cart, id: id}
    )
    {:noreply, socket}
  end
  def handle_out("cart_updated", params, socket) do
    modify_subscriptions(params)
    cart = get_cart(params)
    socket = assign(socket, :cart, cart)
    push(socket, "cart", cart_to_map(cart))
    send(self(), :update_tracked_cart)
    {:noreply, socket}
  end
  
  defp broadcast_cart(cart, socket, opts) do
    send(self(), :update_tracked_cart)
    {:ok, serialized} = Checkout.export_cart(cart)
    broadcast_from(socket, "cart_updated", %{
      "serialized" => serialized,
      "added" => Keyword.get(opts, :added, []),
      "removed" => Keyword.get(opts, :removed, [])
    })
  end