Swift 1.2 Xcode 6.3 NSString？给字符串给线程1：EXC_BAD_ACCESS（代码= 1，地址= 0x20）

Question

现在，苹果已轻松摆脱NSString和String自动兼容性，我有一点噩梦在两者之间。我得到几个NSString出去了一本字典的，我无法将它们转换为普通String小号...

我已经试过：

let fileNameString: String = String(format: "%@", filename!) 

let fileNameString: String = (filename as! String) 

let fileNameString = filename as? String 

let fileNameString = (filename as? String) ?? "" 

if let fileNameString = filename as? String { 
    println("\(fileNameString)") 
} 

但所有产生错误。

我在转换点打破了，可以看到无论是NSString的零：

但随着要么没有喜悦。获取Thread 1: EXC_BAD_ACCESS (code=1, address=0x20)。我在这里错过了很明显的东西吗

即使只是试图打印​​转换导致相同的错误面前..

之前发布的代码转换的尝试，看看是否有什么关系呢？

// First we create a head request as the info I need is in the headers 
var newRequest: NSMutableURLRequest = NSMutableURLRequest(URL: request.URL!) 
newRequest.HTTPMethod = "HEAD" 
var response: NSURLResponse? 
NSURLConnection.sendSynchronousRequest(newRequest, returningResponse: &response, error: nil) 

// Unwrap response as httpResponse in order to access allHeaderFields   
if let httpResponse = response as? NSHTTPURLResponse { 

    let headerString = "sfn-Document-Filename" 
    let headerNSString = headerString as NSString 
    let filetypeString = "Content-Type" 
    let filetypeNSString = filetypeString as NSString 

    // This is a dictionary where the keys are NSCFStrings 
    // (NSStrings, hence creating the NSStrings above) 
    var allHeaders = httpResponse.allHeaderFields 

    // Getting the filename out here only works with as? NSString. as? String creates the same error as converting. 
    let filename = allHeaders[headerNSString] as? NSString 

    // This is a string which contains the type as 'application/pdf' for example. We only need the part after the /. 
    // Again, trying to get this out as a String fails 
    let typeString = allHeaders[filetypeNSString] as? NSString 
    var typeArray = typeString?.componentsSeparatedByString("/") as! [NSString] 
    let filetype = typeArray[1] 
} 

Answer 1

如果这是一个NSString，第所有你需要做的是filename as String（no !）。但这听起来像问题是您的filename，可选类型NSString?，是nil。 （选项单击filename，以确认它的类型）

如果有一个合理的默认值（比方说，一个空字符串），尝试

let fileNameString = (filename as? String) ?? "" 

或者，如果你需要处理nil特定代码：

if let fileNameString = filename as? String { 
    // use fileNameString, which will be unwrapped and of type String 
} 
else { 
    // log error or similar 
} 

或者，如果你想推迟展开，但要更改可能值里面的类型，你可以做

let fileNameString = filename as? String 
// which is a less long-winded way of saying 
let fileNameString = filename.map { $0 as String } 

一般来说，您应该尝试减少!的用法，因为它会导致类似的问题。 !仅适用于从代码中知道值绝对肯定不能为nil的那些时间。

编辑：基于您的示例代码，请尝试以下操作：这里

let url = NSURL(string: "http://www.google.com") 
let request = url.map { NSMutableURLRequest(URL: $0) } 
request?.HTTPMethod = "HEAD" 

let response: NSHTTPURLResponse? = request.flatMap { 
    var response: NSURLResponse? 
    NSURLConnection.sendSynchronousRequest($0, returningResponse: &response, error: nil) 
    return response as? NSHTTPURLResponse 
} 

let headers = response?.allHeaderFields as? [String:String] 

// google.com has no such header but still... 
let filename = headers?["sfn-Document-Filename"] 

// bear in mind someArray[1] will also crash if there's no such entry, 
// first/last are better if that's what you want 
let type = headers?["Content-Type"]? 
       .componentsSeparatedByString(";").first? 
       .componentsSeparatedByString("/").last 

一切都是可选的，但安全的，所以你可以在不同的点测试零的记录/错误报告的目的。