private function calculateAmount($productIds)
 {
     $this->amount = 0;
     foreach ($productIds as $id) {
         $this->amount += Product::findOrFail($id)->price;
     }
     return $this->amount;
 }
 /**
  * 顯示結帳頁面
  */
 public function index()
 {
     // 建立客戶 Account
     $account = Account::updateOrCreate(['id' => 1], ['name' => "梅宗主"]);
     $account->save();
     // 模擬加入購物車動作
     $cart = new MyCart();
     $cart->add(Product::updateOrCreate(['id' => 1], ['name' => '暖爐', 'price' => '4000']));
     $cart->add(Product::updateOrCreate(['id' => 2], ['name' => '護心丹', 'price' => '12000']));
     $cart->add(Product::updateOrCreate(['id' => 3], ['name' => '密道鐵門', 'price' => '8000']));
     return view('checkout', ['cart' => $cart, 'account' => $account]);
 }