以下是其中的代码，如果userNameTF或passwordTF已满或为空，则会显示警报。

@IBAction func LoginBtn(sender: AnyObject) {
    let userName = userNameTF.text
    let password = passwordTF.text

    if ((userName?.isEmpty) != nil) {

        displayMyAlertMessage ("Forget to fill your user name")
        return        
    }

    if ((password?.isEmpty) != nil){

        displayMyAlertMessage ("Forget to fill your password")
        return    
    }        
}

func displayMyAlertMessage(userMessage:String){

    let myAlert = UIAlertController(title: "WOF", message: userMessage, preferredStyle: UIAlertControllerStyle.Alert)  
    let okAction = UIAlertAction(title: "Ok", style: UIAlertActionStyle.Default, handler: nil)
    myAlert.addAction(okAction)
    self.presentViewController(myAlert, animated: true, completion: nil)
 }