如果讓vs Guard讓Swift 1.2中的UIImage

Question

我正在通過iOS Apple Developer Tutorial進行工作，但是由於我的計算機的Swift版本是1.2，而Xcode是6.4，而不是Swift 3.0，因此我遇到了一個問題。 我已經能夠根據需要對代碼進行降級處理，但是遇到了以下“ Swift 2.0”中引入的“ guard let”錯誤：

// The info dictionary may contain multiple representations of the image. 
// You want to use the original.

guard let selectedImage = info[UIImagePickerControllerOriginalImage] as? UIImage else {
    fatalError("Expected a dictionary containing an image, but was provided the following: \(info)")
}

我將其轉換為以下內容：

if let selectedImage = info[UIImagePickerControllerOriginalImage] as? UIImage {
     return selectedImage
} else {
     fatalError("Expected a dictionary containing an image, but was provided the following: \(info)")
}

//Set photoImageView to display the image.
photoImageView.image = selectedImage

但出現以下錯誤：“使用未解決的已識別'selectedImage'”

任何幫助將不勝感激！

編輯：完整代碼如下

import UIKit

class ViewController: UIViewController, UITextFieldDelegate,
UIImagePickerControllerDelegate, UINavigationControllerDelegate {

//MARK: Properties
@IBOutlet weak var nameTextField: UITextField!
@IBOutlet weak var mealNameLabel: UILabel!
@IBOutlet weak var photoImageView: UIImageView!

override func viewDidLoad() {
    super.viewDidLoad()

    //Handle the text field's user input through the delegate callbacks
    nameTextField.delegate = self
}


//MARK: UITextFieldDelegate
func textFieldShouldReturn(textField: UITextField) -> Bool {
    //Hide the keyboard
    textField.resignFirstResponder()
    return true
}
func textFieldDidEndEditing(textField: UITextField) {
    //Set the lablel name as what the used typed
    mealNameLabel.text = textField.text
}

//MARK: UIImagePickerControllerDelegate
func imagePickerControllerDidCancel(picker: UIImagePickerController) {
    //Dismiss the picker if the user canceled
    dismissViewControllerAnimated(true, completion:nil)
}
func imagePickerController(picker: UIImagePickerController,
    didFinishPickingMediaWithInfo info: [String : Any]) {
        //The info dictionary may contain multiple representations of the image. You want to use the original.
        if let selectedImage = info[UIImagePickerControllerOriginalImage] as? UIImage {
            //Set photoImageView to display the image
            photoImageView.image = selectedImage
        } else {
            fatalError("Expected a dictionary containing an image, but was provided the following: \(info)")
        }

        //Set photoImageView to display the image
        //photoImageView.image = selectedImage

        //Dismiss the picker
        dismissViewControllerAnimated(true, completion: nil)
}


//MARK: Actions
@IBAction func selectImageFromPhotoLibrary(sender: UITapGestureRecognizer) {
    //Hide the keyboard
    nameTextField.resignFirstResponder()

    //UIImagePickerController is a view controller that lets a user pick media from their photo library.
    let imagePickerController = UIImagePickerController()


    //Only allow photos to be picked, not taken.
    imagePickerController.sourceType = .PhotoLibrary

    //Make sure ViewController is notified when the user picks and image.
    imagePickerController.delegate = self
    presentViewController(imagePickerController, animated: true, completion: nil)

}
@IBAction func setDefaultLabelText(_: UIButton) {
    mealNameLabel.text = "Default Text"
}
}

Answer 1

發生這種情況是因為您只有在if let范圍內定義了selectedImage變量

您必須改用此代碼

if let selectedImage = info[UIImagePickerControllerOriginalImage] as? UIImage{
     //Set photoImageView to display the image.
     photoImageView.image = selectedImage
     return selectedImage
} else {
     fatalError("Expected a dictionary containing an image, but was provided the following: \(info)")
}

我希望這有幫助

Answer 2

if let可選的綁定語句創建一個新變量，該變量僅存在於以下括號內：

if let foo = optionalFoo {
  //foo is only defined here
}

//foo has gone out of scope.

guard let創建一個新變量，該變量從當前作用域的結尾一直存在到代碼中：

func bar(optionalFoo: FooThing) {
  guard let foo = optionalFoo else {return}

  //foo exists from here until the end of the 
  //current scope (the end of the func)
}

由於Swift 1沒有保護聲明，因此您需要將guard let之后的代碼移動到if let的花括號內：

看來@ReinierMelian用他的答案擊敗了我。 請參閱他的答案以獲取Swift 1.x中正確的代碼形式。

請注意，Swift 1.0已經嚴重過時了，使用它會給您帶來很大的損害。 您應該升級到Xcode 8.3和Swift 3.1。 從Swift 1到Swift 2的變化是巨大的，從Swift 2到Swift 3的變化很大 。 Swift 1代碼無法在Xcode 8中編譯，因此您可能無法通過自動轉換運行它。 （Xcode 8將提供通過自動轉換為Swift 3來運行Swift 2.x的功能，但我不確定是否會將Swift 1轉換為Swift 3。