我为 UIViewController 创建了一个 extension，在这里创建了一个 alert 函数：
extension UIViewController {
func alert(message: String, title: String = "") {
let alertController = UIAlertController(title: title, message: message, preferredStyle: .Alert)
let OKAction = UIAlertAction(title: "OK", style: .Default, handler: nil)
alertController.addAction(OKAction)
self.presentViewController(alertController, animated: true, completion: nil)
}
}