title: Swift 3 Firebase: current user login success, event already delete this user in the Firebase Console
body: <p>After create the Firebase user successful by code in login form. I deleted this new user in Firebase console, quit the app. When open the app again, it still inform login success with the user already deleted by admin. Why is this, and how to know in the login form that user already deleted?</p>
  
  <p>Refer to my below code of login form:</p>
  
  <pre><code>override func viewDidLoad() {
      super.viewDidLoad()
  
      // get the current user is by setting a listener on the Auth object:
      FIRAuth.auth()!.addStateDidChangeListener() { auth, user in
          if user != nil {
              // User is signed in.
              print("start login success: " + (user?.email)! )
              //self.performSegue(withIdentifier: loginToList, sender: nil)
          } else {
              // No user is signed in.
              print("No user is signed in.")
          }
  
      }
  
      //get the currently signed-in user by using the currentUser property. If a user isn't signed in, currentUser is nil:
      if let curUser = FIRAuth.auth()?.currentUser {
          // User is signed in.
          print("start current user: " + curUser.email! )
      } else {
          // No current user is signed in.
          print("Currently, no user is signed in.")
      }
  }
  }
  </code></pre>
  
  <p>sample code from <a href="https://firebase.google.com/docs/auth/ios/manage-users" rel="nofollow noreferrer">https://firebase.google.com/docs/auth/ios/manage-users</a></p>