ListView scrolls to top after updating it

Question

I have a mChildEventListener that queries for the first n items from Firebase Database and another mChildEventListener2 (triggered when the user scroll up and clicks on a Load More button) that queries for the next n items and adds them to the ChatMessagesList.

When the user clicks on the Load More button: the ChatMessagesList is updated and notifyDataSetChanged is called. The mMessageListView is updated but it scrolls down to the buttom every time. I explained this behavior because of the xml attribute: android:transcriptMode="alwaysScroll".

When I remove transcriptMode="alwaysScroll" and the user presses on the Load More button, the mMessageListView scrolls this time to the top.

None of these behaviors are wanted. I would like that when the user presses the button, the mMessageListView would maintain its' scrolling position. (for obvious reasons)

I tried calling notifyDataSetChanged from a new thread. No results.

I tried using listView.setSelectionFromTop(lastViewedPosition, topOffset); or using list.smoothScrollToPosition(); but none achieved a smooth experience that I would want.

How can I maintain the scrolling position after the mMessageListView is updated?

This is the code:

private ChildEventListener mChildEventListener;
private ChildEventListener mChildEventListener2;

List<ChatMessage> ChatMessages = new ArrayList<>();

@Override
protected void onCreate(Bundle savedInstanceState) {
      ...
mMessageAdapter = new MessageAdapter(this, R.layout.item_message_sent, ChatMessages);
mMessageListView.setAdapter(mMessageAdapter);

 mChildEventListener = new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                if(i == 0) {
                    current_last_key = dataSnapshot.getKey();
                }
                else {
                    ChatMessage chatMessage = dataSnapshot.getValue(ChatMessage.class);
                    mMessageAdapter.add(chatMessage);
                }
                i++;
            }
          ...
        };
        Query messagesQuery =  mDatabaseReference.child("chat_messages").
                child(chatId).limitToLast(25);
        messagesQuery.addChildEventListener(mChildEventListener);
          ...
   }

 @Override
public void onClick(View v) {
    j = 0;
    mChildEventListener2 = new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
            if(!dataSnapshot.getKey().equals(current_last_key)) {
                if (j == 0) {
                    current_last_key = dataSnapshot.getKey();
                }
                else {
                    ChatMessage chatMessage = dataSnapshot.getValue(ChatMessage.class);
                    ChatMessages.add(j- 1, chatMessage);
                }

            }
            else
            {
                ChatMessage chatMessage = dataSnapshot.getValue(ChatMessage.class);
                ChatMessages.add(0 , chatMessage);
                loadMoreTextView.setVisibility(View.GONE);
            }
            mMessageAdapter.notifyDataSetChanged();
            j++;
        }

        ...        };
    Query query = mDatabaseReference.child("chat_messages").
            child(chatId).orderByKey().endAt(current_last_key).limitToLast(10);
    query.addChildEventListener(mChildEventListener2);
}

Answer 1

notifyDataSetChanged()

Notifies the attached observers that the underlying data has been changed and any View reflecting the data set should refresh itself.

Try to see if "onCreate()" is called again when updating

Answer 2

FirebaseUI lib handles all that for you and simplifies your code. BTW, you can use mAdapter.notifyItemInserted(ChatMessages.size() - 1); or mMessageAdapter.notifyItemRangeInserted(ChatMessages.size() - 1, ChatMessages.size() - 1 + numberOfItemsBeingInserted); and make the other required changes in order for this to work, instead of using mMessageAdapter.notifyDataSetChanged();