title: Variable username is not updated after call to Firebase
body: <pre><code>var username: String? = nil
  self.ref.child("users").observeSingleEvent(of: .value, with: { (snapshot) in
      let usersProfile = snapshot.value as? NSDictionary
      let userProfile = usersProfile![userID] as? NSDictionary
      username = userProfile!["username"] as! String
  
  }) { (error) in
      print(error.localizedDescription)
  }
  print(username)
  </code></pre>
  
  <p>Why is the variable <code>username</code> not updated in the end? </p>