在Swift闭包内更新的UILabel文本拒绝显示

您需要在主线程上执行UI更新

NSURLSession

dispatch_async

@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

尽管由于不保留闭包而在这里并不重要，但在某些情况下，显式声明捕获列表以避免保留周期很重要。