@IBAction func didTapSignIn(sender: AnyObject) {
    let email = emailField.text
    let password = passwordField.text
    FIRAuth.auth()?.signInWithEmail(email!, password: password!) { (user, error) in
        if let error = error {
            print(error.localizedDescription)
            return
        }
        self.signedIn(user!)
    }
}

func signedIn(user: FIRUser?) {
    AppState.sharedInstance.displayName = user?.displayName ?? user?.email
    AppState.sharedInstance.signedIn = true
    NSNotificationCenter.defaultCenter().postNotificationName(Constants.NotificationKeys.SignedIn, object: nil, userInfo: nil)
    performSegueWithIdentifier(Constants.Segues.SignInToHome, sender: nil)
}

override func viewDidAppear(animated: Bool) {
    super.viewDidAppear(true)
    //Synchronously gets the cached current user, or null if there is none.
    if let user = FirebaseConfigManager.sharedInstance.currentUser {
        self.signedIn(user)
    }
}

@IBAction func didTapSignOut(sender: UIBarButtonItem) {
    print("sign out button tapped")
    let firebaseAuth = FIRAuth.auth()
    do {
        try firebaseAuth?.signOut()
        AppState.sharedInstance.signedIn = false
        dismissViewControllerAnimated(true, completion: nil)
    } catch let signOutError as NSError {
        print ("Error signing out: (signOutError)")
    } catch {
        print("Unknown error.")
    }
}

po FIRAuth.auth()?.currentUser
▿ Optional<FIRUser>
  - Some : <FIRUser: 0x7fde43540f50>

try! FIRAuth.auth()!.signOut()

GIDSignIn.sharedInstance().signOut()

var currentUser: User? = Auth.auth().currentUser
var handle: AuthStateDidChangeListenerHandle!

init() {

    handle = Auth.auth().addStateDidChangeListener { (auth, user) in
        self.currentUser = user

        if user == nil {
             UserDefaults.standard.setValue(false, forKey: UserDefaults.loggedIn)
        } else {
             UserDefaults.standard.setValue(true, forKey: UserDefaults.loggedIn)
        }

    }

}