發現零而展開的可選值 - 斯威夫特2

Question

我得到這個錯誤，我無法解釋：

@IBOutlet weak var licencePlateLabel: UILabel! 

var editItem: CarValues? { 
     didSet { 
      // Update the view. 
      self.configureView() 
     } 
    } 

func configureView() { 
     // Update the user interface for the detail item. 

     if let editedCar = self.editItem { 

      if let licencePlate = self.licencePlateLabel { 
       licencePlate.text? = editedCar.licencePlate!//this gives me nil 
      } 
      else { 
       print("value was nil") 
       } 

      print(editedCar.licencePlate!)//this is giving me the correct value 

} 

如果我更換

if let licencePlate = self.licencePlateLabel { 
        licencePlate.text! = editedCar.licencePlate! 
       }//this throws an error "found nil......" 

即使我做到這一點我還在原地得到在「發現零...」

func configureView() { 

licencePlateLabel.text = "test" 
     [...] 
} 

，但如果我把上面的viewDidLoad中然後正常工作

override func viewDidLoad() { 

licencePlateLabel.text = "test" 
      [...] 

} 

這段代碼是怎麼回事？

編輯

我傳遞的editItem的值從的DetailView到EditView中是這樣的：

@IBAction func editButtonTapped(sender: AnyObject) { 

     let storyBoard = UIStoryboard(name: "Main", bundle:nil) 
     let editScreen = storyBoard.instantiateViewControllerWithIdentifier("ID_EditViewController") 
     self.navigationController?.pushViewController(editScreen, animated: true) 

     let controller = EditViewController() 
     controller.editItem = detailItem 
     controller.navigationItem.leftBarButtonItem = self.splitViewController?.displayModeButtonItem() 
     controller.navigationItem.leftItemsSupplementBackButton = true 
    } 

Answer 1

你不拆開包裝性能設置他們，爲只讀從他們。所以，你的代碼可以簡單地是：

if let licencePlate = self.licencePlateLabel { 
    licencePlate.text = editedCar.licencePlate 
} 

注意，因爲licencePlate.text是一個可選值，無論如何，也沒有必要解開editedCar.licencePlate。無論是否爲零或包含字符串，都可以使用它的值。