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

问题现象：想在页面初始化的时候，使用self.presentViewController方法弹出个告警提示框UIAlertController。但行后报了个如下告警，同时告警框也出不来。

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!

解决办法：原来的调用代码是写在viewDidLoad方法中，这个表示视图加载完毕。我们应该把方法调用放到viewDidApper中，这个是UIViewController对象的视图已经加入到窗口时调用。

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.
    }
}