How to prevent the view from being viewed when a button is clicked (quick)

From your description, it sounds like it's seguealways being executed, but that's not what you want.

Connect the segue from the view controller itself, name it, and put the whole method inside the IBActionconnection with the button .

like this:

@IBAction func loginDidTouch(sender: AnyObject) {
    FIRAuth.auth()?.addStateDidChangeListener { auth, user in
        if let user = user {
            self.performSegue(withIdentifier: "AddDevice", sender: nil)
         }
         else {
             let alert = UIAlertController(title: "Sorry", message:"You Have to register", preferredStyle: .alert)
             alert.addAction(UIAlertAction(title: "Ok", style: .default) { _ in })
             self.present(alert, animated: true){}
    }
}

E.g:

• In the first image, the segue is connected to the button. So every time the button is pressed it will execute segue.

• In the second image, the segue is connected to the view controller, and the button is only connected to via IBAction. So the button only fires, actionbut the action fires segue.

• It is easy to check by selecting the segue in the storyboard