在Swift闭包内更新的UILabel文本拒绝显示

Question

在学习Swift时,我正在编写一个简单的练习iOS应用程序来从一个站点抓取给定城市的天气信息并在UILabel中显示它.

该代码使用"NSURLSession.sharedSession().dataTaskWithURL"闭包.虽然我能够正确地获取数据,并捕捉到"UILabel.text"相关的文字,我不能得到实际应用显示更新的UILabel.

我究竟做错了什么？这是相关代码:

@IBAction func buttonPressed(sender: AnyObject) {

   var urlString = "http://www.weather-forecast.com/locations/" + cityName.text.stringByReplacingOccurrencesOfString(" ", withString: "") + "/forecasts/latest"

   var url = NSURL(string: urlString)

   let task = NSURLSession.sharedSession().dataTaskWithURL(url) {(data, response, error) in

      var urlContent = NSString(data: data, encoding: NSUTF8StringEncoding) as String

      var contentArray = urlContent.componentsSeparatedByString("<span class=\"phrase\">")
      var weatherInfo = contentArray[1].componentsSeparatedByString("</span>")

      self.resultShow.text = weatherInfo[0] // Text does not show in the app

      println(weatherInfo[0]) // This works correctly
      println(self.resultShow.text) // This works correctly

   }

   task.resume()

}

Answer 1

您需要在主线程上执行UI更新

NSURLSession将始终在后台线程上调用完成处理程序.要更新您的UI dispatch_async,主线程的简单应该足够:)

@IBAction func buttonPressed(sender: AnyObject) {
   var urlString = "http://www.weather-forecast.com/locations/" + cityName.text.stringByReplacingOccurrencesOfString(" ", withString: "") + "/forecasts/latest"
   var url = NSURL(string: urlString)
   let task = NSURLSession.sharedSession().dataTaskWithURL(url) {(data, response, error) in

      var urlContent = NSString(data: data, encoding: NSUTF8StringEncoding) as String

      var contentArray = urlContent.componentsSeparatedByString("<span class=\"phrase\">")
      var weatherInfo = contentArray[1].componentsSeparatedByString("</span>")

      dispatch_async(dispatch_get_main_queue(), {
          //perform all UI stuff here        
          self.resultShow.text = weatherInfo[0] 
      })
   }
   task.resume()
}

编辑

虽然这里不重要,因为不保留闭包,但在某些情况下,明确声明捕获列表以避免保留周期非常重要.