UICollectionView滾動和選擇單元格後崩潰

Question

我有一個Horizontal CollectionView其中有一個image view裏面的單元格。我想要做的是設置一個默認選定的單元格。選定的單元格應該更改其圖像顏色。如果用戶選擇其他單元格，則應該選擇它，並且應該取消選擇默認選定的單元格。目前所有這些工作。但有時當我滾動並選擇一個單元格時，它的崩潰說如下。

ERROR: unexpectedly found nil while unwrapping an Optional value

這裏是我完整的代碼

override func viewDidLoad() { 
      super.viewDidLoad() 
      self.collectionView.delegate = self 
      self.locationManager.delegate = self 
      collectionView.allowsMultipleSelection = false 
     } 

    func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int { 
      return CatNames.count 
     } 


     func collectionView(collectionView: UICollectionView, didSelectItemAtIndexPath indexPath: NSIndexPath) { 

       let cell = collectionView.cellForItemAtIndexPath(indexPath) as! CategoryCell 
       cell.CarImage.image = UIImage(named: CatImages[indexPath.row])?.maskWithColor(UIColorFromRGB(0xEA7C6A)) 
       cell.Name.textColor = UIColorFromRGB(0xEA7C6A) 

     } 

     func collectionView(collectionView: UICollectionView, didDeselectItemAtIndexPath indexPath: NSIndexPath) { 

       let cell = collectionView.cellForItemAtIndexPath(indexPath) as! CategoryCell 
       cell.CarImage.image = UIImage(named: CatImages[indexPath.row]) 
       cell.Name.textColor = UIColor.darkGrayColor() 

     } 

     func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell { 
      let cell = collectionView.dequeueReusableCellWithReuseIdentifier("cell", forIndexPath: indexPath) as! CategoryCell 

      cell.Name.text = CatNames[indexPath.row] 


      // setup selectedBackgroundView 
      if(cell.selected == true){ 
       cell.CarImage.image = UIImage(named: CatImages[indexPath.row])!.maskWithColor(UIColorFromRGB(0xEA7C6A)) 
       cell.Name.textColor = UIColorFromRGB(0xEA7C6A) 
      } 
      else{ 
       cell.CarImage.image = UIImage(named: CatImages[indexPath.row]) 
       cell.Name.textColor = UIColor.darkGrayColor() 
      } 


      return cell 
     } 


     override func viewWillAppear(animated: Bool) { 
      super.viewWillAppear(animated) 
      let indexPath = NSIndexPath(forRow: 0, inSection: 0) 
      self.collectionView.selectItemAtIndexPath(indexPath, animated: false, scrollPosition: .None) 
     } 

extension UIImage { 
    func maskWithColor(color: UIColor) -> UIImage? { 

     let maskImage = self.CGImage 
     let width = self.size.width 
     let height = self.size.height 
     let bounds = CGRectMake(0, 0, width, height) 

     let colorSpace = CGColorSpaceCreateDeviceRGB() 
     let bitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.PremultipliedLast.rawValue) 
     let bitmapContext = CGBitmapContextCreate(nil, Int(width), Int(height), 8, 0, colorSpace, bitmapInfo.rawValue) //needs rawValue of bitmapInfo 

     CGContextClipToMask(bitmapContext, bounds, maskImage) 
     CGContextSetFillColorWithColor(bitmapContext, color.CGColor) 
     CGContextFillRect(bitmapContext, bounds) 

     //is it nil? 
     if let cImage = CGBitmapContextCreateImage(bitmapContext) { 
      let coloredImage = UIImage(CGImage: cImage) 

      return coloredImage 

     } else { 
      return nil 
     } 
    } 
} 

Answer 1

 func collectionView(collectionView: UICollectionView, didSelectItemAtIndexPath indexPath: NSIndexPath) { 
      guard collectionView.cellForItemAtIndexPath(indexPath) != nil else { 
        return 
      } 
      let cell = collectionView.cellForItemAtIndexPath(indexPath) as! CategoryCell 
      cell.CarImage.image = UIImage(named: CatImages[indexPath.row])?.maskWithColor(UIColorFromRGB(0xEA7C6A)) 
      cell.Name.textColor = UIColorFromRGB(0xEA7C6A) 

    } 

    func collectionView(collectionView: UICollectionView, didDeselectItemAtIndexPath indexPath: NSIndexPath) { 
      guard collectionView.cellForItemAtIndexPath(indexPath) != nil else { 
        return 
      } 
      let cell = collectionView.cellForItemAtIndexPath(indexPath) as! CategoryCell 
      cell.CarImage.image = UIImage(named: CatImages[indexPath.row]) 
      cell.Name.textColor = UIColor.darkGrayColor() 

    } 

這基本上回答你的問題，因爲你的這部分代碼

let cell = collectionView.cellForItemAtIndexPath(indexPath) as! CategoryCell 

你基本上很難用as!澆鑄如果collectionView.cellForItemAtIndexPath(indexPath)的值爲零，應用程序將崩潰

另外，您還可以使用if let語法而不是guard else

if let cell = collectionView.cellForItemAtIndexPath(indexPath) { 
    let aCell = cell as! CategoryCell 
    // do stuff here 
}