ios - swift NSURLSession : trouble when running multiple requests at the same time

Question

我目前正在构建一个天气应用程序，其中我有一个容器 ViewController 管理多个子 ViewControllers，其主要 View 位于容器 ViewController 的 UIScrollView。

每个子 VC 都有一个位置属性，它是保存所有天气信息的类。此类具有以下用于从 OpenWeatherMaps API 加载数据的函数:

func refresh(completionHandler: () -> ()) {
    let session = NSURLSession.sharedSession()
    session.dataTaskWithURL(self._apiUrl) { (responseData: NSData?, response:NSURLResponse?, error: NSError?) -> Void in
        if let data = responseData {
            do {
                let json: AnyObject = try NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.AllowFragments)
                // parse json
                completionHandler()
            } catch let err as NSError {
                print(err.debugDescription)
            }
        }
    }.resume()
}

然后子 VC 有一个调用另一个函数的函数，completionHandler 在主线程上运行(因为我正在更新用户界面):

func refreshWeatherData() {
    location.refresh {
        dispatch_async(dispatch_get_main_queue(), {
            print("\(self.location.name)")
            self.forecastCollectionView.reloadData()
        })
    }
}

最后，我从 ContainerVC 调用了我所有 LocationVC 上的 refreshWeatherData 函数。

override func viewDidLoad() {
    super.viewDidLoad()
    let zurich = WALocation(name: "Zürich", id: "7287650", country: "CH", longitude: 8.53071)
    let shanghai = WALocation(name: "Shanghai", id: "1796236", country: "CN", longitude: 121.45)
    let boston = WALocation(name: "Boston", id: "4183849", country: "US", longitude: -83.78)
    let vancouver = WALocation(name: "Vancouver", id: "6173331", country: "CA", longitude: -123.11)

    addLocationControllerForLocation(shanghai)
    locationControllers[0].refreshWeatherData()
    addLocationControllerForLocation(boston)
    locationControllers[1].refreshWeatherData()
    addLocationControllerForLocation(zurich)
    locationControllers[2].refreshWeatherData()
    addLocationControllerForLocation(vancouver)
    locationControllers[3].refreshWeatherData()
}

现在我遇到的问题是有时(不总是)，一两次或三次，JSONSerialisation 抛出错误(在位置的刷新函数中): Error Domain=NSCocoaErrorDomain Code=3840“无值。” UserInfo ``{NSDebugDescription=No value.} 相反，在我的 ContainerVC 中，我这样做:

override func viewDidLoad() {
    super.viewDidLoad()
    let zurich = WALocation(name: "Zürich", id: "7287650", country: "CH", longitude: 8.53071)
    let shanghai = WALocation(name: "Shanghai", id: "1796236", country: "CN", longitude: 121.45)
    let boston = WALocation(name: "Boston", id: "4183849", country: "US", longitude: -83.78)
    let vancouver = WALocation(name: "Vancouver", id: "6173331", country: "CA", longitude: -123.11)

    addLocationControllerForLocation(shanghai)
    locationControllers[0].refreshWeatherData()
    sleep(1)
    addLocationControllerForLocation(boston)
    locationControllers[1].refreshWeatherData()
    sleep(1)
    addLocationControllerForLocation(zurich)
    locationControllers[2].refreshWeatherData()
    sleep(1)
    addLocationControllerForLocation(vancouver)
    locationControllers[3].refreshWeatherData()
}

NSJSONSerialisation 永远不会失败。这让我觉得只有当多个请求异步运行时它才会失败。

如有任何帮助，我们将不胜感激。

Answer 1

这听起来像是一个线程问题。您是否尝试过将刷新调用(或刷新本身)包含在 GCD 调用中？例如:

func refresh(completionHandler: () -> ()) {
    dispatch_async(dispatch_get_main_queue()){
        let session = NSURLSession.sharedSession()
        session.dataTaskWithURL(self._apiUrl) { (responseData: NSData?, response:NSURLResponse?, error: NSError?) -> Void in
            if let data = responseData {
                do {
                    let json: AnyObject = try NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.AllowFragments)
                    // parse json
                    completionHandler()
                } catch let err as NSError {
                    print(err.debugDescription)
                }
            }
            }.resume()
    }
}

如果需要，您可以使用dispatch_sync。

无论哪种方式，请确保 completionHandler 在正确的线程上回调它应该调用的任何内容。如果是 UI 更新，那么它就是主线程。

编辑 作为我下面帖子的延续，这里更新了刷新以在 completionHandler 中返回数据。

func refresh(completionHandler: (AnyObject) -> ()) {  //Anyobject being your JsonData or whatever so could be NSString, String, etc...
    let session = NSURLSession.sharedSession()
    session.dataTaskWithURL(self._apiUrl) { (responseData: NSData?, response:NSURLResponse?, error: NSError?) -> Void in
        if let data = responseData {
            do {
                let json: AnyObject = try NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.AllowFragments)
                // parse json
                completionHandler(json)
            } catch let err as NSError {
                print(err.debugDescription)
            }
        }
    }.resume()
}