我如何使用完成处理程序将值传递给另一个View Controller

Question

你好，我是Swift的新手，我遇到了问题。我有存储在Cloud Firestore中的配料价格表。我想计算特定Recepie的价格并将其传递给另一个VC，因此我编写了以下代码：

extension RecepiesViewController: UICollectionViewDelegate {
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        func recepieCalculation(_ completion: @escaping (Double) ->()) {
            var totalPrice = 0.0
            for ingridients in cakeRecepies![indexPath.item].ingridientList {
                db.collection("Okey")
                    .document(ingridients.name)
                    .getDocument() { (document, error) in
                        if let document = document, document.exists {
                            let data = document.data()
                            if let price = data!["price"] as? Double, let count = data!["count"] as? Int  {
                                let priceForOne = price / Double(count)
                                totalPrice = totalPrice + priceForOne * Double(ingridients.count)
                            }
                        } else {
                            print("Document does not exist")
                        }
                        completion(totalPrice)       
                }

            }
        }
        recepieCalculation {totalPrice in
            func prepare(for segue: UIStoryboardSegue, sender: Any?) {
                let destinationVC = segue.destination as! ResultViewController
                destinationVC.result = totalPrice
            }
            self.performSegue(withIdentifier: "goToResult", sender: self)
        }   
    }
}

第二个VC代码：

class ResultViewController: UIViewController {
    var result: Double? {
        didSet {
            print(result)
        }
    }
    @IBOutlet weak var recepieImage: UIImageView!
    override func viewDidLoad() {   
    }
}

但是控制台上没有任何内容。有什么问题吗？

Answer 1

获取

 func prepare(for segue: UIStoryboardSegue, sender: Any?) {
      let destinationVC = segue.destination as! ResultViewController
      destinationVC.result = totalPrice
 }