@IBAction func addToCart(_ sender: UIBarButtonItem) {
    let itemToAdd = product
    let existingIndex = cart.containsAtIndex(product!)
    // If item from catalogue exist in cart, then modify
    if (existingIndex >= 0) {
            cart.changeItem(existingIndex, newItem: itemToAdd!)
            modifyCart(product: itemToAdd!)
        }
    // If item is not found in cart, then add a new one
    else {
            let storyboard = UIStoryboard(name: "Main", bundle: nil)
            if let controller = storyboard.instantiateViewController(withIdentifier: "CartTableView") as? CartTableViewController {
                let newIndexPath = IndexPath(row: cart.numberOfItems(), section: 0)
                cart.addItem(itemToAdd!)
                controller.tableView.insertRows(at: [newIndexPath], with: .automatic)
                saveToCart(product: itemToAdd!, update: true)
            }
        }
        self.navigationController?.popViewController(animated: true)
        self.dismiss(animated: true, completion: nil)
    }



private func saveToCart(product: Product, update: Bool) {
    let realm = try! Realm()
    print(product)
    try! realm.write {
        realm.add(product, update: true)
    }
}

private func modifyCart(product: Product) {
    let prodName = product.itemName
    let realm = try! Realm()
    try! realm.write {
        let item = realm.objects(Product.self).filter("itemName = %@", prodName).first
        item!.quantity = product.quantity
    }
}

class Cart: NSObject {


static let myCart = Cart()
private var products: [Product]

private override init() {
    self.products = []
}

func getCartItems() -> [Product] {
    return products
}

func getItem(_ index: Int) -> Product? {
    if products.indices.contains(index) {
        return products[index]
    } else {
        return nil
    }
}

func addItem(_ item: Product) {
    products.append(item)
}

func removeItem(_ index: Int) {
    products.remove(at: index)
}

func numberOfItems() -> Int {
    return products.count
}

func changeItem(_ index: Int, newItem: Product) {
    if products.indices.contains(index) {
        products[index] = newItem
    }
}

func containsAtIndex(_ item: Product) -> Int {
    for i in (0..<products.count) {
        if (products[i].itemName == item.itemName) {
            return i
        }
    }
    return -1
}
}