"uncaught exception" 是什么？

Question

我正在尝试构建一个简单的 iPhone 应用程序，使用 UITextView 和 UIButton 将用户输入的文本保存到 iCloud。我从来没有做过这样的事情，所以我正在关注 this tutorial。到目前为止，他们的教程对我来说是完美无缺的，直到这一个。该应用程序构建良好，并在我的 iPhone 上运行，显示了一个输入文本的位置，以及一个将文本保存到 iCloud 的按钮。但是当我点击屏幕输入文本时，应用程序崩溃并显示以下消息：

2017-03-24 08:45:56.093572 Gaethr2[6342:1419021] * 由于未捕获的异常终止应用程序 'NSInvalidArgumentException'，原因：'* -[NSMetadataQuery valueOfAttribute:forResultAtIndex:]: 索引 (0) 越界 (0)' *** 首先抛出调用栈： (0x184b611b8 0x18359855c 0x184b61100 0x1855f1460 0x100056ffc 0x10005857c 0x184afab10 0x184afa214 0x184af9f90 0x184b69b8c 0x184a3be64 0x185570e0c 0x1855ef268 0x184afab10 0x184afa214 0x184af9f90 0x184b69b8c 0x184a3be64 0x185570e0c 0x1968d93e4 0x1968db29c 0x184b0ea44 0x184b0e240 0x184b0c094 0x184a3a2b8 0x1864ee198 0x18aa817fc 0x18aa7c534 0x10005b720 0x183a1d5b8) libc++abi.dylib：以 NSException 类型的未捕获异常终止来自调试器的消息：未能发送第 k 个数据包

这是 ViewController 代码：

import UIKit

class ViewController: UIViewController
{
    @IBOutlet weak var textView: UITextView!

    var document: MyDocument?
    var documentURL: URL?
    var ubiquityURL: URL?
    var metaDataQuery: NSMetadataQuery?



    func metadataQueryDidFinishGathering(notification: NSNotification) -> Void
    {
        let query: NSMetadataQuery = notification.object as! NSMetadataQuery

        query.disableUpdates()

        NotificationCenter.default.removeObserver(self,
                                                  name: NSNotification.Name.NSMetadataQueryDidFinishGathering,
                                                  object: query)

        query.stop()

        let resultURL = query.value(ofAttribute: NSMetadataItemURLKey,
                                    forResultAt: 0) as! URL

        if query.resultCount == 1 {
            let resultURL = query.value(ofAttribute: NSMetadataItemURLKey,
                                        forResultAt: 0) as! URL

            document = MyDocument(fileURL: resultURL as URL)

            document?.open(completionHandler: {(success: Bool) -> Void in
                if success {
                    print("iCloud file open OK")
                    self.textView.text = self.document?.userText
                    self.ubiquityURL = resultURL as URL
                } else {
                    print("iCloud file open failed")
                }
            })
        } else {
            document = MyDocument(fileURL: ubiquityURL!)

            document?.save(to: ubiquityURL!,
                           for: .forCreating,
                           completionHandler: {(success: Bool) -> Void in
                            if success {
                                print("iCloud create OK")
                            } else {
                                print("iCloud create failed")
                            }
            })
        }
    }



    override func viewDidLoad()
    {
        super.viewDidLoad()

        let filemgr = FileManager.default

        ubiquityURL = filemgr.url(forUbiquityContainerIdentifier: nil)

        guard ubiquityURL != nil else {
            print("Unable to access iCloud Account")
            print("Open the Settings app and enter your Apple ID into iCloud settings")
            return
        }

        ubiquityURL = ubiquityURL?.appendingPathComponent(
            "Documents/savefile.txt")

        metaDataQuery = NSMetadataQuery()

        metaDataQuery?.predicate =
            NSPredicate(format: "%K like 'savefile.txt'",
                        NSMetadataItemFSNameKey)
        metaDataQuery?.searchScopes =
            [NSMetadataQueryUbiquitousDocumentsScope]

        NotificationCenter.default.addObserver(self,
                                               selector: #selector(
                                                ViewController.metadataQueryDidFinishGathering),
                                               name: NSNotification.Name.NSMetadataQueryDidFinishGathering,
                                               object: metaDataQuery!)

        metaDataQuery!.start()
    }

    @IBAction func saveDocument(_ sender: AnyObject)
    {

        document!.userText = textView.text

        document?.save(to: ubiquityURL!,
                       for: .forOverwriting,
                       completionHandler: {(success: Bool) -> Void in
                        if success {
                            print("Save overwrite OK")
                        } else {
                            print("Save overwrite failed")
                        }
        })
    }

    override func didReceiveMemoryWarning()
    {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


}

这是 MyDocument.Swift

的代码

import UIKit

class MyDocument: UIDocument
{
    var userText: String? = "Some Sample Text"

    override func contents(forType typeName: String) throws -> Any
    {
        if let content = userText
        {

            let length =
                content.lengthOfBytes(using: String.Encoding.utf8)
            return NSData(bytes:content, length: length)
        }
        else
        {
            return Data()
        }
    }

    override func load(fromContents contents: Any, ofType typeName: String?) throws
    {
        if let userContent = contents as? Data
        {
            userText = NSString(bytes: (contents as AnyObject).bytes,
                                length: userContent.count,
                                encoding: String.Encoding.utf8.rawValue) as? String
        }
    }
}

这是崩溃的屏幕截图：

感谢您的帮助！

Answer 1

未捕获的异常是

-[NSMetadataQuery valueOfAttribute:forResultAtIndex:]: index (0) out of bounds (0)'

删除第一次出现的

let resultURL = query.value(ofAttribute: NSMetadataItemURLKey,
                                forResultAt: 0) as! URL

这是多余的，如果 query.results 为空

，则会导致错误

"uncaught exception" 是什么？

What is the "uncaught exception"?

ios

swift

icloud