Adding items to ListView, maintaining scroll position and NOT seeing a scroll jump

Question

I\'m building an interface similar to the Google Hangouts chat interface. New messages are added to the bottom of the list. Scrolling up to the top of the list will trigger a lo

Answer 1

The code suggested by the question author works, but it's dangerous.
For instance, this condition:

listView.getFirstVisiblePosition() == positionToSave

may always be true if no items were changed.

I had some problems with this aproach in a situation where any number of elements were added both above and below the current element. So I came up with a sligtly improved version:

/* This listener will block any listView redraws utils unlock() is called */
private class ListViewPredrawListener implements OnPreDrawListener {

    private View view;
    private boolean locked;

    private ListViewPredrawListener(View view) {
        this.view = view;
    }

    public void lock() {
        if (!locked) {
            locked = true;
            view.getViewTreeObserver().addOnPreDrawListener(this);
        }
    }

    public void unlock() {
        if (locked) {
            locked = false;
            view.getViewTreeObserver().removeOnPreDrawListener(this);
        }
    }

    @Override
    public boolean onPreDraw() {
        return false;
    }
}

/* Method inside our BaseAdapter */
private updateList(List newItems) {
    int pos = listView.getFirstVisiblePosition();
    View cell = listView.getChildAt(pos);
    String savedId = adapter.getItemId(pos); // item the user is currently looking at
    savedPositionOffset = cell == null ? 0 : cell.getTop(); // current item top offset

    // Now we block listView drawing until after setSelectionFromTop() is called
    final ListViewPredrawListener predrawListener = new ListViewPredrawListener(listView);
    predrawListener.lock();

    // We have no idea what changed between items and newItems, the only assumption
    // that we make is that item with savedId is still in the newItems list
    items = newItems; 
    notifyDataSetChanged();
    // or for ArrayAdapter:
    //clear(); 
    //addAll(newItems);

    listView.post(new Runnable() {
        @Override
        public void run() {
            // Now we can finally unlock listView drawing
            // Note that this code will always be executed
            predrawListener.unlock();

            int newPosition = ...; // Calculate new position based on the savedId
            listView.setSelectionFromTop(newPosition, savedPositionOffset);
        }
    });
}