變量初始化錯誤

Question

我有一個包含4個單元格的表（縮略圖要從URI中顯示）。 如果URI爲空，我想顯示一個佔位符圖像。 如果不爲空，我想在下載時顯示一個活動指示器。

什麼我是一個非常「快速和骯髒」的代碼 - 但它的工作原理：

func setCell(previewView1: String, previewView2: String, previewView3: String, previewView4: String, id: String) { 
    self.loadPreview1(previewView1) 
    self.loadPreview2(previewView2) 
    self.loadPreview3(previewView3) 
    self.loadPreview4(previewView4) 
    self.cellID = id; 
} 

func loadPreview1(urlString: String) { 
    if urlString == "" { 
     self.previewView1.image = UIImage(named: "imagePlatzhalter") 
     self.activityIndicator1.stopAnimating(); // Animation stoppen 
    } 
    else { 
     self.activityIndicator1.startAnimating() // Animation Start 
     var imgURL = NSURL(string: urlString); 
     let request: NSURLRequest = NSURLRequest(URL: imgURL!); 
     let mainQueue = NSOperationQueue.mainQueue(); 
     NSURLConnection.sendAsynchronousRequest(request, queue: mainQueue, completionHandler: { (response, data, error) -> Void in 
      if error == nil { 
       // Convert the downloaded data in to a UIImage object 
       let image = UIImage(data: data) 
       // Update the cell 
       self.activityIndicator1.stopAnimating(); // Animation stoppen 
       self.previewView1.image = image; 
      } 
      else { 
       println("Error: \(error.localizedDescription)") 
       self.previewView1.image = UIImage(named: "imagePlatzhalter") 
       self.activityIndicator1.stopAnimating(); // Animation stoppen 
      } 
     }) 
    } 
} 

func loadPreview2(urlString: String) { 
    if urlString == "" { 
     self.previewView2.image = UIImage(named: "imagePlatzhalter") 
     self.activityIndicator2.stopAnimating(); // Animation stoppen 
    } 
    else { 
     self.activityIndicator2.startAnimating() // Animation Start 
     var imgURL = NSURL(string: urlString); 
     let request: NSURLRequest = NSURLRequest(URL: imgURL!); 
     let mainQueue = NSOperationQueue.mainQueue(); 
     NSURLConnection.sendAsynchronousRequest(request, queue: mainQueue, completionHandler: { (response, data, error) -> Void in 
      if error == nil { 
       // Convert the downloaded data in to a UIImage object 
       let image = UIImage(data: data) 
       // Update the cell 
       self.activityIndicator2.stopAnimating(); // Animation stoppen 
       self.previewView2.image = image; 
      } 
      else { 
       println("Error: \(error.localizedDescription)") 
       self.previewView2.image = UIImage(named: "imagePlatzhalter") 
       self.activityIndicator2.stopAnimating(); // Animation stoppen 
      } 
     }) 
    } 
} 
func loadPreview3(urlString: String) { 
: same as 1 and 2 with references on self.previewView3 and self.activityIndicator3... 
} 

func loadPreview4(urlString: String) { 
: same as 1 and 2 with references on self.previewView4 and self.activityIndicator4... 
} 

這個解決方案工作正常，但現在我要重構代碼，在一個更好的解決方案。這是我的方法：

func previewImage (urlString: String, controlIndex: Int) { 
    var previewViewImage : UIImage; 
    var activityIndicator : UIActivityIndicatorView; 

    if controlIndex == 1 { 
     previewViewImage = self.previewView1.image!; 
     activityIndicator = self.activityIndicator1; 
    } else if controlIndex == 2 { 
     previewViewImage = self.previewView2.image!; 
     activityIndicator = self.activityIndicator2; 
    } else if controlIndex == 3 { 
     previewViewImage = self.previewView3.image!; 
     activityIndicator = self.activityIndicator3; 
    } else if controlIndex == 4 { 
     previewViewImage = self.previewView4.image!; 
     activityIndicator = self.activityIndicator4; 
    } 

    if urlString == "" { 
     // Set image to placeholder image: 
     previewViewImage = UIImage(named: "imagePlatzhalter")!; 
    } 
    else { 
     activityIndicator.startAnimating() // Animation Start 
     var imgURL = NSURL(string: urlString); 

     // Check ob Image gecacht ist/TODO 
     let request: NSURLRequest = NSURLRequest(URL: imgURL!); 
     let mainQueue = NSOperationQueue.mainQueue(); 
     NSURLConnection.sendAsynchronousRequest(request, queue: mainQueue, completionHandler: { (response, data, error) -> Void in 
      if error == nil { 
       // Convert the downloaded data in to a UIImage object 
       let image = UIImage(data: data) 
       // Store the image in to our cache 
       //self.imageCache[urlString] = image 

       // Update the cell 
       previewViewImage = image!; 
      } 
      else { 
       println("Error: \(error.localizedDescription)") 

       previewViewImage = UIImage(named: "imagePlatzhalter")!; 
      } 
     }) 

    } 
    // Stop activity indicator: 
    activityIndicator.stopAnimating(); 
} 

但Xcode的位置拋出3個錯誤：在

activityIndicator.stopAnimating(); 

activityIndicator.startAnimating() 

Error: "Variable activityIndicator used before initialized

相同，回調中我得到了錯誤：

"Variable previewViewImage captured by a closure before being initialized"

我是新來的斯威夫特，不明白，爲什麼我的代碼將無法正常工作。任何人都可以幫助我重構上面的代碼嗎？

Answer 1

Swift發現一條可能的路徑，其中previewViewImage和activityIndicator未初始化。這裏是你的代碼：如果controlIndex是5

func previewImage (urlString: String, controlIndex: Int) { 
    var previewViewImage : UIImage; 
    var activityIndicator : UIActivityIndicatorView; 

    if controlIndex == 1 { 
     previewViewImage = self.previewView1.image!; 
     activityIndicator = self.activityIndicator1; 
    } else if controlIndex == 2 { 
     previewViewImage = self.previewView2.image!; 
     activityIndicator = self.activityIndicator2; 
    } else if controlIndex == 3 { 
     previewViewImage = self.previewView3.image!; 
     activityIndicator = self.activityIndicator3; 
    } else if controlIndex == 4 { 
     previewViewImage = self.previewView4.image!; 
     activityIndicator = self.activityIndicator4; 
    } 

會發生什麼？這兩個變量都不會被初始化。所以，Swift把它們視爲未初始化的，這就是爲什麼你會得到錯誤。

您可以通過使最後的else if只是一個else來解決此問題。那樣的話，你想assert那controlIndex == 4。或者，您可以在if之前將previewViewImage和activityIndicator初始化爲一些合理的默認值。