Xcode中有关于iOS中的fenceExemptQueue的怪异警告

当我的应用程序进入后台(在applicationDidEnterBackground之后),出现以下警告.

2015-12-11 20:54:43.661 AppName[759:124891]  *|synchronize-skip|* a fence was started inside of a snapshot block - skipping the workspace synchronize because it may dequeue messages from the fenceExemptQueue and snapshots expect that not to happen
2015-12-11 20:54:44.084 AppName[759:124891]  *|synchronize-skip|* a fence was started inside of a snapshot block - skipping the workspace synchronize because it may dequeue messages from the fenceExemptQueue and snapshots expect that not to happen

我在Google上找不到任何有关信息,不知道为什么会这样.

有谁知道这是什么意思？谢谢！

解决方法

我在Swift程序中收到了同样奇怪的错误消息,最后弄清楚我的代码有什么问题.正确的代码是：

override func viewDidLoad() {
    super.viewDidLoad()

    let fileMgr = NSFileManager.defaultManager()
    ubiquityURL = fileMgr.URLForUbiquityContainerIdentifier(nil)

    guard ubiquityURL != nil else {
        print("Dave: Unable to access iCloud account")
        return
    }
    ubiquityURL = ubiquityURL?.URLByAppendingPathComponent("Documents/savefile.txt")

    metaDataQuery = NSMetadataQuery()
    metaDataQuery?.predicate = NSPredicate(format: "%K like 'savefile.txt'",NSMetadataItemFSNameKey)
    metaDataQuery?.searchScopes = [NSMetadataQueryUbiquitousDocumentsScope]
    NSNotificationCenter.defaultCenter().addObserver(self,selector: "metadataQueryDidFinishGathering:",name: NSMetadataQueryDidFinishGatheringNotification,object: metaDataQuery!)
    metaDataQuery!.startQuery()
}

在上面的代码中,正确的格式是“％K like”savefile.txt’“,但是如果您忘记了文件名中的单引号,则会收到奇怪的运行时错误消息.没有文件名的单引号,元数据查询无法在iCloud上找到您的文件.