具有 SQLite 数据库的调度组

Question

我试图了解 GCD，特别是 DispatchGroup 以通过 FMDB 包装器组织下载到 SQLite 数据库。 我的应用程序执行以下操作：

• 使用 SQL db 从远程服务器下载应用程序启动时可用主题的信息。 将这些本地保存在 SQLite 数据库中以供将来的会话使用，并通过 UITableViewController 显示可用的内容
• 如果选择了一个主题，它的内容将从服务器下载并保存在本地以供将来的会话使用。 我这样做而不是在启动时一次完成，因为这是应用内购买的前兆。 我还在这里下载了一些其他的东西。 然后转到主题内容的新表格视图。
• 我可以通过将下载和保存功能与完成处理程序链接在一起来实现上述目的，但是我想利用 DispatchGroup 以便将来可以利用wait(timeout:) function。

但是，在我实施 DispatchGroup（如下）时，我收到以下错误。

API call with NULL database connection pointer
[logging] misuse at line 125820 of [378230ae7f]

并且

BUG IN CLIENT OF libsqlite3.dylib: illegal multi-threaded access to database connection

代码如下：

选择行

override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {

//Download from server
if availableSubjects[indexPath.row].isDownloaded == 0 {
    //CHAINING THIS WAY WORKS
    /* downloadModel.downloadCaseBundle(withSubjectID: indexPath.row, completion: {
       self.downloadModel.downloadToken(forSubject: indexPath.row, completion: {
       self.caseBundle = DBManager.sharedDBManager.getCaseBundle(forSubject: indexPath.row)
       self.availableSubjects[indexPath.row].isDownloaded = 1
       DispatchQueue.main.async {
           self.performSegue(withIdentifier: "showCaseList", sender: self)
           }
       })
  })*/

    let dispatchGroup = DispatchGroup()

    //Download content
    dispatchGroup.enter()
    downloadModel.downloadCaseBundle(withSubjectID: indexPath.row) {
        dispatchGroup.leave()
    }

    //Download token
    dispatchGroup.enter()
    downloadModel.downloadToken(forSubject: indexPath.row) {
        dispatchGroup.leave()
    }

    //Execute
    dispatchGroup.notify(queue: .main) {
    self.caseBundle = DBManager.sharedDBManager.getCaseBundle(forSubject: indexPath.row)
    self.availableSubjects[indexPath.row].isDownloaded = 1
    self.performSegue(withIdentifier: "showCaseList", sender: self)
    }

} else { //Already downloaded, just retrieve from local db and present
    caseBundle = DBManager.sharedDBManager.getCaseBundle(forSubject: indexPath.row)
    self.performSegue(withIdentifier: "showCaseList", sender: self)
    }       
}

下载模型，下载案例包

downloadToken function 或多或少相同

func downloadCaseBundle(withSubjectID subjectID: Int, completion: @escaping () -> Void) {
    let urlPath = "someStringtoRemoteDB"
    let url: URL = URL(string: urlPath)!
    let defaultSession = Foundation.URLSession(configuration: URLSessionConfiguration.default)

    let task = defaultSession.dataTask(with: url) { (data, response, error) in

        if error != nil {
            print("Error")
        } else {
            print("cases downloaded")
            self.parseCasesJSON(data!, header: self.remoteMasterTable, forSubject: subjectID)
            completion()
        }
    }
    task.resume()
}

下载模式，parseJSON

func parseCasesJSON(_ data:Data, header: String, forSubject subjectID: Int) {
        var jsonResult = NSArray()
        var jsonElement = NSDictionary()
        let cases = NSMutableArray()

        do {
            jsonResult = try JSONSerialization.jsonObject(with: data, options:JSONSerialization.ReadingOptions.allowFragments) as! NSArray
        } catch let error as NSError {
            print(error)
            print("error at serialisation")
        }

        //Iterate through JSON result (i.e. case), construct and append to cases array
        for i in 0 ..< jsonResult.count {
            jsonElement = jsonResult[i] as! NSDictionary
            var caseObject = CaseModel()

            //The following insures none of the JsonElement values are nil through optional binding
            if let uniqueID = jsonElement["id"] as? Int,
                let subjectTitle = jsonElement["subjectTitle"] as? String,
                let subjectID = jsonElement["subjectID"] as? Int,
                let questionID = jsonElement["questionID"] as? Int,
                //And so on
            {
                caseObject.uniqueID = uniqueID
                caseObject.subjectTitle = subjectTitle
                caseObject.subjectID = subjectID
                caseObject.questionID = questionID
                //And so on
            }
            cases.add(caseObject)
        }

        DBManager.sharedDBManager.saveCasesLocally(dataToSave: cases as! [CaseModel])
        DBManager.sharedDBManager.setSubjectAsDownloaded(forSubjectID: subjectID)
    }

Answer 1

事实证明这与这些方法无关，我需要在我的DBManager singleton 中实现FMDatabaseQueue而不是FMDatabase 。