ios - Xcode 中关于 iOS 中的 fenceExemptQueue 的奇怪警告

Question

当我的应用进入后台时(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 上找不到任何相关信息，也不知道为什么会这样。

有人知道这是什么意思吗？谢谢!

Answer 1

我在 Swift 程序中收到了同样奇怪的错误消息，最终弄清楚我的代码出了什么问题。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两边的单引号，您会收到奇怪的运行时错误消息。如果文件名周围没有单引号，元数据查询将无法在 iCloud 上找到您的文件。