FIRAuth.auth()?.addStateDidChangeListener { auth, user in
  if let _user = user {
    if _user.isAnonymous {
        print("User logged in as anonymous. Saving uid to user defaults storage.")
        UserDefaults.standard.setValue(_user.uid, forKey: "uid")
    } else {
         print("User logged in with email: (_user.email)")
    }
  } else {
     FIRAuth.auth()?.signInAnonymously() { (user, error) in
        if let _error = error {
           print("Anonymous signIn error: (_error)")
        }
     }
  }
}

let credential = FIREmailPasswordAuthProvider.credential(withEmail: emailField.text!, password: passwordField.text!)
FIRAuth.auth()?.currentUser?.link(with: credential, completion: { user, error in
    if error == nil {
        FIRAuth.auth()!.signIn(withEmail: emailField.text!,
                                       password: passwordField.text!)
     }
 })

if let prevUserUID = UserDefaults.standard.string(forKey: "uid"), prevUserUID != _user.uid {
    FIRDatabase.database().reference().child("todo-items").queryOrdered(byChild: "user").queryEqual(toValue: prevUserUID).observe(.value, with: { snapshot in
    for item in snapshot.children {
        var todoItem = TodoItem(snapshot: item as! FIRDataSnapshot)
        todoItem.user = _user.uid
        todoItem.ref?.setValue(todoItem.toAnyObject())
    }
  })
  print("Data migrated.")
}