异步加载本地 JSON 文件答案

【问题标题】：Load local JSON file async异步加载本地 JSON 文件
【发布时间】：2018-05-08 14:02:29
【问题描述】：

我不熟悉 swift 和同步/异步加载文件的方式。我在本地有一个很大的 JSON 文件，用于一个关于足球的 iPad 应用程序，其中包含足球运动员的名单和统计数据。

目前，我将整个玩家列表加载到字典数组中我让用户搜索特定的播放器

func loadJSON() {
    /// Load Json File
    if let path = Bundle.main.path(forResource: "players", ofType: "json") {
        do {
            let data = try Data(contentsOf: URL(fileURLWithPath: path), options: .alwaysMapped)
            let jsonObj = try JSON(data: data)

            /// For Player in JSON Serialize values
            for (_,subJson):(String, JSON) in jsonObj["PackData"]["PlayerData"]["P"] {

                let firstName = subJson["_f"].stringValue
                let lastName = subJson["_s"].stringValue
                let id = subJson["_id"].stringValue
                let dateOfBirth = subJson["_d"].stringValue
                let height = subJson["_h"].stringValue
                let weight = subJson["_w"].stringValue
                let image = subJson["_i"].stringValue


                let player = Player(id: id, firstName: firstName, lastName: lastName, dateOfBirth: dateOfBirth, height: height, weight: weight, image: image)

                /// Append Player in players Array
                players.append(player)

            }

由于我在ViewDidLoad 中使用了loadJSON()，因此当我转到此视图时，应用程序会冻结几秒钟并占用大量内存。

在数据库中异步处理/实现类似搜索的正确方法是什么？

编辑： 我已经尝试使用 dispatch DispatchQueue.global(qos: .background).async 但我收到错误：indexPath.row out of range on player = filteredPlayers[indexPath.row]

 // create a cell for each table view row
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {

    // create a new cell if needed or reuse an old one

    let cell:UITableViewCell = self.searchTableView.dequeueReusableCell(withIdentifier: cellReuseIdentifier) as UITableViewCell!
    let player: Player

    /// Return a different table if is searching or not
    if isFiltering() {
        player = filteredPlayers[indexPath.row]
    } else {
        player = players[indexPath.row]
    }
    cell.textLabel?.text = player.firstName! + " " + player.lastName!

    cell.textLabel?.textColor = UIColor.white

    return cell

}

【问题讨论】：

尝试使用DispatchQueue.global().async { }

标签： ios swift dispatch

【解决方案1】：

你需要在后台使用DispatchQueue，

func loadJSON() {
    /// Load Json File
    DispatchQueue.global(qos: .background).async{
        if let path = Bundle.main.path(forResource: "players", ofType: "json") {
            do {
                let data = try Data(contentsOf: URL(fileURLWithPath: path), options: .alwaysMapped)
                let jsonObj = try JSON(data: data)

                /// For Player in JSON Serialize values
                for (_,subJson):(String, JSON) in jsonObj["PackData"]["PlayerData"]["P"] {

                    let firstName = subJson["_f"].stringValue
                    let lastName = subJson["_s"].stringValue
                    let id = subJson["_id"].stringValue
                    let dateOfBirth = subJson["_d"].stringValue
                    let height = subJson["_h"].stringValue
                    let weight = subJson["_w"].stringValue
                    let image = subJson["_i"].stringValue


                    let player = Player(id: id, firstName: firstName, lastName: lastName, dateOfBirth: dateOfBirth, height: height, weight: weight, image: image)

                    /// Append Player in players Array
                    players.append(player)

                }
            }
        }
    }
}

【讨论】：

我尝试使用它，但是当我在 tableView 中显示该数据时，在 [indexPath.row] 上出现超出范围错误，我是否也应该更改 tableView 以异步显示数据？
你如何声明你的数组 players ？ @丹
我在问题中添加了额外的代码。我有两个数组，一个用于过滤玩家，当用户开始输入 searchBar 时，另一个用于通用玩家列表。
@dan 在您将播放器添加到播放器数组后直接过滤它。
能详细点吗？我不明白你说的直接过滤是什么意思。谢谢:)

【解决方案2】：

您需要将冗长的任务分派到后台队列并将结果分派回主队列。

我的简化 json 示例：

{
    "person": "Bob"
}

创建加载json方法

func loadJSON(completion: @escaping (_ data: String?, _ error: Error?) -> ())  {
    var person: String?
    var receivedError: Error?

    /// Load json file and parse in background queue
    DispatchQueue.global(qos: .background).async {
        let path = Bundle.main.path(forResource: "myJSON", ofType: "json")!
        do {
            let data = try Data(contentsOf: URL(fileURLWithPath: path), options: .alwaysMapped)
            let json = try JSONSerialization.jsonObject(with: data, options: .allowFragments)
            let jsonDictionary =  json as! Dictionary<String, Any>

            person = jsonDictionary["person"] as? String
        } catch {
            receivedError = error
        }

        // Dispatch the found value to main queue
        DispatchQueue.main.async {
            completion(person, receivedError)
        }
    }
}

并在您的代码中调用它：

loadJSON { (data, error) in
    if let retrievedData = data {
        print(retrievedData)
        // It is safe to assign the value to UI objects 
        // because the callback is on the main thread
    }
}

【讨论】：

【解决方案3】：

在后台线程中执行loadJSON()，然后通过回调或属性赋值将结果推送回main。代码：

DispatchQueue(label: "jsonLoading", qos: .background).async {
    let players = self.loadJSON()
    DispatchQueue.main.async {
        // Handle data in the main thread in whatever way you need, eg:
        self.players = players
    }
}

【讨论】：

【解决方案4】：

func loadJSON(completion: @escaping ()->()) {
    /// Above Parameter is a completion handler which informs user that some task have been done.
    //Do such heavy operations like json loading from local file in a back ground thread so your main thread doesn't get affected.
    DispatchQueue.global(qos: .background).async {
        if let path = Bundle.main.path(forResource: "players", ofType: "json") {
            do {
                let data = try Data(contentsOf: URL(fileURLWithPath: path), options: .alwaysMapped)
                let jsonObj = try JSON(data: data)

                /// For Player in JSON Serialize values
                for (_,subJson):(String, JSON) in jsonObj["PackData"]["PlayerData"]["P"] {

                    let firstName = subJson["_f"].stringValue
                    let lastName = subJson["_s"].stringValue
                    let id = subJson["_id"].stringValue
                    let dateOfBirth = subJson["_d"].stringValue
                    let height = subJson["_h"].stringValue
                    let weight = subJson["_w"].stringValue
                    let image = subJson["_i"].stringValue


                    let player = Player(id: id, firstName: firstName, lastName: lastName, dateOfBirth: dateOfBirth, height: height, weight: weight, image: image)

                    /// Append Player in players Array
                    players.append(player)
                }
                completion()
            }
            catch _ {

            }
        }
    }
}

现在在您的viewController 类或您想要的任何其他类中调用上述方法。

func yourViewControllerMethod() {
    loadJSON {
        // This Block will execute when your json is loaded and parsed completely.
        DispatchQueue.main.async{
             // This is the main thread now you are again here on your main thread after your journey from background thread.

        }
    }
}

永远不要对 background thread 中的 UI 元素做任何事情

【讨论】：

请注意，因为完成块不是从主线程执行的。

【解决方案5】：

移动到后台线程做一些长时间运行的工作

func loadJSON(_ completion:@escaping (_ jsonObj:JSON?) -> Void){
        DispatchQueue.global(qos: .background).async {
            if let path = Bundle.main.path(forResource: "players", ofType: "json") {
               if  let data = try? Data(contentsOf: URL(fileURLWithPath: path), options: .alwaysMapped), let jsonObj = try? JSON.init(data: data){
                    completion(jsonObj)
               }else{
                 completion(nil)
                }
            }else{
                 completion(nil)
            }
        }
    }

然后这样调用

   self.loadJSON { (jsonObj) in

            DispatchQueue.main.async {
                guard let jsonObj = jsonObj else {return}
                /// For Player in JSON Serialize values
                for (_,subJson):(String, JSON) in jsonObj["PackData"]["PlayerData"]["P"] {

                    let firstName = subJson["_f"].stringValue
                    let lastName = subJson["_s"].stringValue
                    let id = subJson["_id"].stringValue
                    let dateOfBirth = subJson["_d"].stringValue
                    let height = subJson["_h"].stringValue
                    let weight = subJson["_w"].stringValue
                    let image = subJson["_i"].stringValue


                    let player = Player(id: id, firstName: firstName, lastName: lastName, dateOfBirth: dateOfBirth, height: height, weight: weight, image: image)

                    /// Append Player in players Array
                    players.append(player)
            }
        }

【讨论】：