How to Sign Out of Google After Being Authenticated

Question

So my app has the option to sign in with Google. Upon clicking the button that Google provides, a web view opens and has the user input their credentials. After allowing the app to access their information the app then signs the user in and changes the SignInViewController to the TabBarController (where they can now interact accordingly).

When the user presses a Signout button they are directed to the login screen as one would expect. But the odd thing is, if the user presses the google button again they are automatically signed in with no further authentication at all and no option to remove their account. Is their a way to clear the google account credentials as to protect the users from accidental theft?

Sign in function:

func signIn(signIn: GIDSignIn!, didSignInForUser user: GIDGoogleUser!, withError error: NSError!) {
    if let error = error {
        print(error.localizedDescription)
        return
    }
    let authentication = user.authentication
    let credential = FIRGoogleAuthProvider.credentialWithIDToken(authentication.idToken, accessToken: authentication.accessToken)
    FIRAuth.auth()?.signInWithCredential(credential) { (user, error) in
        // ...
        SignInViewController().signedIn(user)
    }
    // ...
}

Sign out function:

func signOutOverride() {
    do {
        try! FIRAuth.auth()!.signOut()
        CredentialState.sharedInstance.signedIn = false
        // Set the view to the login screen after signing out
        let storyboard = UIStoryboard(name: "SignIn", bundle: nil)
        let loginVC = storyboard.instantiateViewControllerWithIdentifier("SignInVC") as! SignInViewController
        let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
        appDelegate.window?.rootViewController = loginVC
    } catch let signOutError as NSError {
        print ("Error signing out: \(signOutError)")
    }
}

Answer 1

Swift

try GIDSignIn.sharedInstance().signOut()

objective - c

[[GIDSignIn sharedInstance] signOut];