我之前写过这个函数，它运行正常。经过一周的休息后，我再次开始研究该项目，同样的代码无效。我在这里错过了什么吗？基本上，如果按下按钮，checkCredentials将运行。 “else”部分中的所有内容都完全正常，但“if user！= nil”部分无效：
func checkCredentials (){
PFUser.logInWithUsernameInBackground(usernameLoginTxt.text!, password: passwordLoginTxt.text!){
(user:PFUser?, error:NSError?) -> Void in
if user != nil{
let storyboard: UIStoryboard = UIStoryboard(name: "Main", bundle: nil)
let vc: UIViewController = storyboard.instantiateViewControllerWithIdentifier("SearchViewController") as UIViewController
self.presentViewController(vc, animated: false, completion:nil)
NSLog("it was successfull dude")
}else{
let title = "Warning!"
let message = "You have entered the wrong username or password.\n Please try again!"
let okButton = "Ok"
let alert = UIAlertController(title: title, message: message, preferredStyle: UIAlertControllerStyle.Alert)
let okayButton = UIAlertAction(title: okButton, style: UIAlertActionStyle.Cancel, handler: nil)
alert.addAction(okayButton)
self.presentViewController(alert, animated: true, completion: nil)
}
}
}