斯威夫特 1.2 Xcode 6.3 NSString？ to String 给出线程 1：EXC_BAD_ACCESS (code=1, address=0x20)

Question

现在苹果已经轻松地摆脱了NSString 和String 的自动兼容性，我在两者之间有点噩梦。我从字典中得到了几个NSStrings，但我无法将它们转换为常规的Strings...

我试过了：

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)")
}

但都产生错误。

我已经在转换点插入并且看不到 NSStrings 都不是 nil：

但两者都不高兴。获取Thread 1: EXC_BAD_ACCESS (code=1, address=0x20)。我在这里遗漏了什么明显的东西吗？

即使只是在转换前尝试打印 NSString filename 也会导致相同的错误..

在转换之前发布代码尝试查看是否与它有关...

// 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NSString，那么您需要做的就是filename as String（不是!）。但听起来问题是你的filename，可选类型NSString?，是nil。 （option-clickfilename确认其类型）

如果有合理的默认值（例如，空字符串），请尝试

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

或者如果您需要使用特定代码处理nil：

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，但想改变可能值内的类型，你可以这样做

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，你应该尽量减少! 的使用，因为它会导致这样的问题。 ! 仅适用于当您从代码中知道该值绝对不能是 nil 的时候。

编辑：根据您的示例代码，尝试以下操作：

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

这里的所有内容都是可选的，但安全，因此您可以在各个点测试 nil 以用于日志记录/错误报告。