Swift xcode 6 label.text在NSURLSession中没有变化

Question

This is a simple test app to demonstrate how to pull data from the web. Within it there is a NSURLSession which is triggered by a button action.

import UIKit

class ViewController: UIViewController {

@IBOutlet weak var city: UITextField!
@IBOutlet weak var message: UILabel!

@IBAction func buttonPressed(sender: AnyObject) {
    var urlString = "http://www.weather-forecast.com/locations/" + city.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))
        var contentArray = urlContent!.componentsSeparatedByString("<span class=\"phrase\">")
        var newContentArray = contentArray[1].componentsSeparatedByString("</span>")
        var messageContent = newContentArray[0] as String

        println(messageContent)
        self.message.text = messageContent
    }
    task.resume()
}

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}
}

The problem is the label "message" wont change to the "messageContent" variable. When I println(messageContent) it displays the data perfectly in the console but the GUI label remains unchanged.

If you place self.message.text = "it worked" outside of the NSURLSession the label changes.

It seems to me that the problem occurs within the NSURLSession not being able to change the label.text

Could someone please help?

Answer 1

NSURLSession's completion closure is invoked in sub thread. Updating UI should be done in main thread.

try this:

dispatch_async(dispatch_get_main_queue(), { () -> Void in
    self.message.text = messageContent
})