title: Observe child changed in realtime database?
body: <p>What is the correct way to listen for a child changed in realtime database? </p>
  
  <p>This is what I try, but nothing prints when I manually update the value of billingUpdated (Int) from the web console.</p>
  
  <pre><code>
      let userID = Auth.auth().currentUser?.uid as! String
      let ref = Database.database().reference()
  
      override func viewDidLoad() {
  
          super.viewDidLoad()
  
          self.ref.child("users").child(userID).child("billingUpdated").observe(.childChanged, with: { (snapshot) in
  
  
              print("child changed")
  
          })
  
      }
  </code></pre>