let rootRef = Database.database().reference()
let user = Auth.auth().currentUser

userRef.child(user!.uid).observeSingleEvent(of: DataEventType.value,
with: {
  (snapshot: DataSnapshot) in
    print("ExistsUser:childrenCount: %@", snapshot.childrenCount)  // ①
      ret = true
})
if ( ret == false ) {
  userRef.child(user!.uid).setValue(["username": user?.displayName])  // ②
}