将值（使用 Firestore 数据计算）应用于标签触发器 "unwrapping an Optional value" 崩溃

Question

我有配料价目表，存储在 Cloud Firestore 中。我想计算特定收据的价格，将其传递给另一个 VC 并应用于标签，所以我写了这段代码：

extension RecepiesViewController: UICollectionViewDelegate {
    //Passing data to next VC
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        let destinationVC = segue.destination as! ResultViewController
        destinationVC.result = totalPrice
    }
    //Recepie cell pressed
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        print("You selected cell #\(indexPath.item)!")
        // Calculating price for recepie
        func recepieCalculation(_ completion: @escaping (Double) ->()) {     
            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)
                                self.totalPrice = self.totalPrice + priceForOne * Double(ingridients.count)
                            }
                        } else {
                            print("Document does not exist")
                        }
                        completion(self.totalPrice)
                }
            }
        }
        recepieCalculation {totalPrice in
            self.performSegue(withIdentifier: "goToResult", sender: self)
        }
    }
}

另一个VC代码：

import Foundation
import UIKit
import RealmSwift

class ResultViewController: UIViewController {

    @IBOutlet weak var resultLabel: UILabel!
    var result: Double = 0.0 {
        didSet {
            print(result)
            resultLabel.text = String(format: ".%f", result)
        }
    }
    @IBOutlet weak var recepieImage: UIImageView!
    override func viewDidLoad() {
        super.viewDidLoad()
    }
}

在第 resultLabel.text = String(format: ".%f", result) 行出现错误

Thread 1: Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value

有什么问题吗？

Answer 1

这是一个常见的错误。

目前 segue 的目标控制器将被呈现，出口尚未连接，因此将值分配给 didSet 观察者中的标签会崩溃。

解决方案：

删除观察者
```
var result = 0.0 
```

并在viewDidLoad

中赋值

override func viewDidLoad() {
    super.viewDidLoad()
    print(result)
    resultLabel.text = String(format: ".%f", result)
}

将值（使用 Firestore 数据计算）应用于标签触发器 "unwrapping an Optional value" 崩溃

Applying value (calculated with Firestore data) to label triggers "unwrapping an Optional value" crash

uilabel

ios

firebase

swift

google-cloud-firestore