addListenerForSingleValueEvent does not execute the code inside onDataChange()

问题

I am just trying to do a simple program (in Android Studio). I have used addListenerForSingleValueEvent(). But the code inside onDataChange() does not execute at all.

dbref.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot snapshot) {
        Log.d("TAG", "message");
    }
    public void onCancelled(DatabaseError error) {
        Log.d("TAG", error.toString());
    }
});

Nothing is displayed in Log.

来源：https://stackoverflow.com/questions/62774908/addlistenerforsinglevalueevent-does-not-execute-the-code-inside-ondatachange