如何更新uilabel文本以响应swift 4

Question

我尝试使用服务器响应更新 label.text。但标签仅在第二次调用方法后更新。怎么了？

dispatch_async(dispatch_get_main_queue()) 已弃用。我将其更改为 DispatchQueue.main.async，但它不起作用。

快速 2 的示例： Swift changing text of a label after downloading content of a web page

不工作的代码：

@IBAction func buttonAction(_ sender: Any) {

    let url = URL(string: "https://api.nasa.gov/planetary/apod?date=2005-2-22&api_key=DEMO_KEY")!
    let task = URLSession.shared.dataTask(with: url) { [weak self] (data, response, error) in
        if let data = data,
            let stringData = String(data: data, encoding: .utf8) {
            print("stringData: \(stringData)") // prints every time

            DispatchQueue.main.async {
                // change label text after second calling.
                // ??? How to change label text at first calling ???
                self?.mainLabel.text = stringData
            }
        }
    }

    task.resume()
}

Answer 1

试试这个

var labelString : String = "" {
        didSet {
            self.mainLabel.text = labelString
        }
    }

在 buttonAction 中编写以下代码

DispatchQueue.main.async {
        // change label text after second calling.
       // ??? How to change label text at first calling ???
        self?.labelString = stringData
  }