UserSearchViewController.swift

var handle: AuthStateDidChangeListenerHandle?
var currentUser: User?

override func viewWillAppear(_ animated: Bool) {
    handle = Auth.auth().addStateDidChangeListener { (auth, user) in
        self.currentUser = user
        if user == nil {
            self.showFUIAuthScreen()
        }
    }
}

override func viewWillDisappear(_ animated: Bool) {
    if let handle = handle {
        Auth.auth().removeStateDidChangeListener(handle)
    }
}