Swift - whose view is not in the window hierarchy 问题解决方法

2015-03-10 09:55:34.197 Test[1140:29622] Warning: Attempt to present <UIAlertController: 0x7c95ca20> on <Test.ViewController: 0x7a6afc60> whose view is not in the window hierarchy!

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import UIKit

class ViewController: UIViewController ,UIActionSheetDelegate {

    override func viewDidLoad() {

        super.viewDidLoad()

    }

    override func viewDidAppear(animated: Bool){

        super.viewDidAppear(animated)

        var alertController = UIAlertController(title: "系统提示",

            message: "您确定要离开hangge.com吗？", preferredStyle: UIAlertControllerStyle.Alert)

        var cancelAction = UIAlertAction(title: "取消", style: UIAlertActionStyle.Cancel, handler: nil)

        var okAction = UIAlertAction(title: "好的", style: UIAlertActionStyle.Default, handler: nil)

        alertController.addAction(cancelAction)

        alertController.addAction(okAction)

        self.presentViewController(alertController, animated: true, completion: nil)

    }

    override func didReceiveMemoryWarning() {

        super.didReceiveMemoryWarning()

        // Dispose of any resources that can be recreated.

    }

}