UserSearchViewController.swift

import UIKit
import Firebase

class UserSearchViewController: UIViewController, UITableViewDelegate, UITableViewDataSource, UISearchBarDelegate {

    @IBOutlet weak var tableView: UITableView!

    var userRef: DatabaseReference!
    var friendsRef: DatabaseReference!
    var resultsArray = [[String:String]]()

    var handle: AuthStateDidChangeListenerHandle?
    var currentUser: User?

    override func viewDidLoad() {
        super.viewDidLoad()

        // Set up our database refs
        userRef = Database.database().reference(withPath: "users")
        friendsRef = Database.database().reference(withPath: "friends")
    }

    override func viewWillAppear(_ animated: Bool) {
        handle = Auth.auth().addStateDidChangeListener { (auth, user) in
            self.currentUser = user
            // Return to the main screen if we lose our user
            if user == nil {
                DispatchQueue.main.async {
                    self.dismiss(animated: true, completion: nil)
                }
            }
        }
    }

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