变量初始化错误

Question

我有一个包含4个单元格的表（缩略图要从URI中显示）。 如果URI为空，我想显示一个占位符图像。 如果不为空，我想在下载时显示一个活动指示器。

什么我是一个非常“快速和肮脏”的代码 - 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... 
} 

这个解决方案工作正常，但现在我要重构代码，在一个更好的解决方案。这是我的方法：

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的位置抛出3个错误：在

activityIndicator.stopAnimating(); 

activityIndicator.startAnimating() 

Error: "Variable activityIndicator used before initialized

相同，回调中我得到了错误：

"Variable previewViewImage captured by a closure before being initialized"

我是新来的斯威夫特，不明白，为什么我的代码将无法正常工作。任何人都可以帮助我重构上面的代码吗？

Answer 1

Swift发现一条可能的路径，其中previewViewImage和activityIndicator未初始化。这里是你的代码：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; 
    } 

会发生什么？这两个变量都不会被初始化。所以，Swift把它们视为未初始化的，这就是为什么你会得到错误。

您可以通过使最后的else if只是一个else来解决此问题。那样的话，你想assert那controlIndex == 4。或者，您可以在if之前将previewViewImage和activityIndicator初始化为一些合理的默认值。