ViewController.swift

var messages : [String] = []                                       //(1)

override func viewDidLoad() {
    ...
    messageListener()                                              //(2)
}
  
func messageListener(){
    Firestore.firestore().collection("messages")
      .whereField("created", isGreaterThan: Date.timeIntervalBetween1970AndReferenceDate - 1000) //(3)
      .order(by: "created")                                        //(4)
      .addSnapshotListener { documentSnapshot, error in
        guard let snap = documentSnapshot else {
          print("Error fetching document: \(error!)")
          return
        }
        self.messages = []
        for doc in snap.documents{                                  //(5)
          self.messages.insert(doc.data()["message"] as! String, at: 0)
        }
        self.messagesTableView.reloadData()                         //(6)
    }
}

@IBAction func sendButtonPressed(_ sender: Any) {                   //(7)
    if let message = messagerTextField.text {                       //(8)
      self.messagerTextField.text = ""                              //(9)
      Firestore.firestore().collection("messages").addDocument(data: [
        "message": message,
        "created": Date().timeIntervalSince1970                     //(10)
      ]) { (err) in
        if let err = err {
          self.messagerTextField.text = message                     //(11)
          print("Error writing document: \(err)")
        } else {
          print("Document successfully written!")
        }
      }
    }
}

func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    return messages.count                                            //(12)
}

func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
  let cell = UITableViewCell()
  let label = UILabel(frame: cell.frame)
  label.text = messages[indexPath.row]
  cell.addSubview(label)                                             //(13)
  return cell
}