Question

我正在努力了解GCD，尤其是DispatchGroup，以通过FMDB包装器组织对SQLite数据库的下载。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。将这些保存在本地的SQLite db中，以备将来使用，并通过UITableViewController展示可用的内容
如果选择了主题，则其内容将从服务器下载并本地保存以供以后的会话使用。我这样做不是在启动时一次完成，因为这是应用程序内购买的先驱。我也在这里下载其他一些东西。然后选择主题内容的新表格。
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，以便将来可以使用wait(timeout:)函数。

但是，在实现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]

还有

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

代码如下：

didSelectRow

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)
    }       
}

DownloadModel，downloadCaseBundle

downloadToken函数大致相同

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

结果证明这些方法无关，我需要在我的FlatList单例中实现FMDatabaseQueue而不是FMDatabase。

使用SQLite数据库的DispatchGroup

didSelectRow

DownloadModel，downloadCaseBundle

下载模式，parseJSON

1 个答案: