读取 Json 后 iOS PickerView 为空

Question

我正在 iOS 中开发一个应用程序，一切都很顺利，但有一个错误我无法修复。当用户第一次启动应用程序时，应用程序会从我的服务器请求一个 json。读取 json 时，我会在选择器 View 中显示结果。问题是 pickerview 总是显示为空，直到用户触摸屏幕。我已经尝试了很多东西，但没有任何效果。理论上它是空的，因为尚未读取 json，但事实并非如此，因为在控制台中我可以看到 json 已准备就绪。

相关代码如下:

override func viewDidLoad() {

    super.viewDidLoad()

    warning.isHidden = true
    self.codeInput.delegate = self;

    DispatchQueue.main.async {
        self.readJson()
        self.picker.reloadAllComponents()
    }
 }

以及我读取 json 的部分

  func readJson(){
    let urlRequest: NSMutableURLRequest = NSMutableURLRequest(url: requestURL)
    let session = URLSession.shared
    let task = session.dataTask(with: urlRequest as URLRequest, completionHandler: {
        (data, response, error) -> Void in

        let httpResponse = response as! HTTPURLResponse
        let statusCode = httpResponse.statusCode


        if (statusCode == 200) {
            print("Everyone is fine, file downloaded successfully.")
            do{

                let json = try JSONSerialization.jsonObject(with: data!, options:.allowFragments) as! [String:AnyObject]

                if let events = json["events"] as? [[String: AnyObject]] {

                    for event in events {

                        //here I read the json and I save the data in my custom array
                        }
                        self.picker.reloadAllComponents()
                    }
                    print(self.eventsArray)
                }

            }catch {
                print("Error with Json: \(error)")
            }
        }
        else{
            print(statusCode)
        }
    })
    picker.reloadAllComponents()
    task.resume()
}

Answer 1

你需要做几件事:

您需要将重新加载选择器 View 的调用移动到数据任务的完成处理程序中。加载数据后将调用该闭包。

但是，URLSession 任务的完成方法在后台线程上执行。因此，您需要将调用包装在对主线程的 GCD 调用中。将此代码添加为完成闭包中的最后一行，就在右大括号之前:

DispatchQueue.main.async{
    picker.reloadAllComponents()
}     

(这是 Swift 3 语法。)

编辑:

代码看起来像这样:

函数 readJson(){ 让 urlRequest: NSMutableURLRequest = NSMutableURLRequest(url: requestURL) 让 session = URLSession.shared let task = session.dataTask(with: urlRequest as URLRequest, completionHandler: { (data, response, error) -> Void in

    let httpResponse = response as! HTTPURLResponse
    let statusCode = httpResponse.statusCode


    if (statusCode == 200) {
        print("Everyone is fine, file downloaded successfully.")
        do{

            let json = try JSONSerialization.jsonObject(with: data!, options:.allowFragments) as! [String:AnyObject]

            if let events = json["events"] as? [[String: AnyObject]] {

                for event in events {

                    //here I read the json and I save the data in my custom array
                    }
                    //Delete this call to reloadAllComponents()
                    //self.picker.reloadAllComponents()
                }
                print(self.eventsArray)
            }
            //------------------------------------
            //This is where the new code goes
            DispatchQueue.main.async{
                picker.reloadAllComponents()
            }     
            //------------------------------------
        }catch {
            print("Error with Json: \(error)")
        }
    }
    else{
        print(statusCode)
    }
})
//Delete this call to reloadAllComponents()
//picker.reloadAllComponents()
task.resume()