@Override
protected void onStart() {
    super.onStart();
    fetchData();
//        mDatabaseReference.child("UserData").child(UID).child("Online").setValue("True");
}
@Override
protected void onStop() {
    super.onStop();
    fetchData();
//        mDatabaseReference.child("UserData").child(UID).child("Online").setValue(false);
}

DatabaseRef presenceRef = FirebaseDatabase.getInstance().getReference("disconnectmessage");
// Write a string when this client loses connection
presenceRef.onDisconnect().setValue("I disconnected!");

protected void onStart() {
    super.onStart();
    fetchData();
    DatabaseReference onlineRef = mDatabaseReference.child("UserData").child(UID).child("Online");
    onlineRef.setValue("True");
    onlineRef.onDisconnect().setValue("False");
}