UserSearchViewController.swift

import FirebaseAuth
import FirebaseUI

extension UserSearchViewController: FUIAuthDelegate {
    func showFUIAuthScreen() {
        let authUI = FUIAuth.defaultAuthUI()
        authUI?.delegate = self

        let providers: [FUIAuthProvider] = [
            FUIGoogleAuth(),
            FUIEmailAuth()
        ]
        authUI?.providers = providers

        if let authViewController = authUI?.authViewController() {
            self.present(authViewController, animated: false)
        }
    }

    func authUI(_ authUI: FUIAuth, didSignInWith authDataResult: AuthDataResult?, error: Error?) {

    }
}