Question

我不知道为什么我无法在我的UILabel中打印“pocetDatabaze3”。

这是我的代码：

@IBOutlet weak var aktualniPocet: UILabel!
var pocetDatabaze2 = 0
var pocetDatabaze3 = "dfh"

override func viewDidLoad() {
    super.viewDidLoad()

    if let url = NSURL(string: "http://servis.dronysitmp.cz/cteni_baterie.php") {
        let task = URLSession.shared.dataTask(with: url as URL, completionHandler: { (data, response, error) -> Void in
            //print(data)

            if let pocetDatabaze = NSString(data: data!, encoding: String.Encoding.ascii.rawValue){

                self.pocetDatabaze2 = Int(pocetDatabaze as String)!
                //self.pocetDatabaze3 = String(describing: pocetDatabaze)
            }

        }

        );
        task.resume()
    }

    pocetDatabaze3 = String(pocetDatabaze2 as Int)
    aktualniPocet.text = pocetDatabaze3

}

在UILabel中它显示“0”但它应该是“25”。加载此视图后，我还有两个操作。当我使用它们时，它会显示正确的值。其余的是：

@IBAction func odecteni(_ sender: UIButton) {
    pocetDatabaze2 = pocetDatabaze2 - 1
    self.pocetDatabaze3 = String(pocetDatabaze2 as Int)
    aktualniPocet.text = pocetDatabaze3
}

Answer 1

这是一个线程问题。 URLSession是异步的，因此您必须更新completionHandler。

中的用户界面

let task = URLSession.shared.dataTask(with: url as URL, completionHandler: { (data, response, error) -> Void in
    //print(data)

    if let pocetDatabaze = NSString(data: data!, encoding: String.Encoding.ascii.rawValue) {

        self.pocetDatabaze2 = Int(pocetDatabaze as String)!
        self.pocetDatabaze3 = String(pocetDatabaze2 as Int)
        DispatchQueue.main.async {
            self.aktualniPocet.text = pocetDatabaze3
        }
    }

})

您需要调用DispatchQueue.main，因为您应该更新主线程中的UI。然后你可以注释掉其他两个电话。

Answer 2

做这样的事情

 if let url = NSURL(string: "http://servis.dronysitmp.cz/cteni_baterie.php") {
    let task = URLSession.shared.dataTask(with: url as URL, completionHandler: { (data, response, error) -> Void in
        if let pocetDatabaze = NSString(data: data!, encoding: String.Encoding.ascii.rawValue){
                 DispatchQueue.main.async {
                       self.aktualniPocet.text = pocetDatabaze as! String
                   }
              }
         }
    );
    task.resume()
}

Answer 3

您应该只从主线程更新UI。

    if let url = NSURL(string: "http://servis.dronysitmp.cz/cteni_baterie.php") {
    let task = URLSession.shared.dataTask(with: url as URL, completionHandler: { (data, response, error) -> Void in
        //This block executes on a background thread

        if let pocetDatabaze = NSString(data: data!, encoding: String.Encoding.ascii.rawValue){

            DispatchQueue.main.async{ // Add this to your code
              // This block executes on the Main Thread and the UI updates properly
              self.pocetDatabaze2 = Int(pocetDatabaze as String)!
            }
        }

    }
    );
    task.resume()
}

有关Swift中多线程和GCD的更多信息，您可以阅读以下文章： https://developer.apple.com/documentation/dispatch

https://www.raywenderlich.com/148513/grand-central-dispatch-tutorial-swift-3-part-1

无法在UILabel中显示我的文字

3 个答案: