How to save some data to the Firebase Database with a callback

gistfile1.txt

Database.database().reference().child("myFantasticUniqueKey").setValue("some data") { (error: Error?, reference: DatabaseReference) in
            if let error = error {
                print("Error: \(error.localizedDescription)")
            } else {
                print("Success!")
            }
        }
        
        
        
// Unique auto ID
Database.database().reference().child("myFantasticCollection").childByAutoId().setValue("some data") { (error: Error?, reference: DatabaseReference) in
            if let error = error {
                print("Error: \(error.localizedDescription)")
            } else {
                print("Success!")
            }
        }