對於循環的結果有時EXC_BAD_ACCESS

Question

有時我得到一個EXC_BAD_ACCESS錯誤在此代碼：

internal func downloadMultiple(files: NSMutableArray, remoteBaseUrl: NSURL, completion: (result: Int)->()) -> Void { 
    self.filesToDownload = files 
    self.cb = completion 

    for item in files { // this line gets marked, but why this line? 
     print("file ", item["file"] as! String) 
     self.download(remoteBaseUrl.URLByAppendingPathComponent(item["file"] as! String)!) 
    } 
} 

但它只是有時會發生，任何想法如何找出是什麼原因造成的？

Answer 1

你所得到的崩潰，因爲item[file]是nil和你正在使用武力展開，一起去選購這樣

for item in files { 
    // this line gets marked, but why this line? 
    if let file = item["file"] as String { 
     print("file ", file) 
     self.download(remoteBaseUrl.URLByAppendingPathComponent(file) 
    } else { 
     print("file not available") 
    } 

}