如何使用完成处理程序将值传递给另一个视图控制器答案

【问题标题】：How could I pass value to another View Controller using completion handler如何使用完成处理程序将值传递给另一个视图控制器
【发布时间】：2020-09-19 04:33:14
【问题描述】：

您好，我是 Swift 新手，但遇到了问题。我有存储在 Cloud Firestore 中的成分价格表。我想计算特定recepie的价格并将其传递给另一个VC，所以我写了这段代码：

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() {   
    }
}

但是没有打印到控制台。有什么问题？

【问题讨论】：

标签： swift asynchronous google-cloud-firestore closures completionhandler

【解决方案1】：

获取

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

走出完成

extension RecepiesViewController: UICollectionViewDelegate {
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) { 
        recepieCalculation {totalPrice in
            self.performSegue(withIdentifier: "goToResult", sender: self)
        }   
    }
    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)       
            }

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

【讨论】：