Question

I have a listview which is obviously scrollable. The listview contains some form questions. Once the user submitts the form we put a stamp-like looking custom view on top of the listview (and prevent user from answering questions, but it doesn’t matter here).

The stamp should appear at the top of the listview (as in top of the screen) and be scrollable at the same speed as the listview, i.e. it should disappear when the listview items which were initially underneath it disappear at the top of the screen when scrolled.

Any idea how to achieve this?

Answer 1

I found a solution. Not sure if it is too elegant, but it works so will leave it unless somebody comes up with something better.

The view which I want to scroll together with the list is a custom view, which has to know about the list view. So I implement a setListView(ListView listView) method on it:

private int scrollY; //1
private Map<Integer, Integer> listViewItemHeights = new Hashtable<>();
public void setListView(final ListView listView) {
    listView.setOnScrollListener(new OnScrollListener() {  //2
        @Override
        public void onScrollStateChanged(AbsListView view, int scrollState) {} //3

        @Override
        public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) { //4
            View c = listView.getChildAt(0);
            if (c != null) {
                int oldScrollY = scrollY;
                scrollY = -c.getTop();

                listViewItemHeights.put(listView.getFirstVisiblePosition(), c.getHeight());
                for (int i = 0; i < listView.getFirstVisiblePosition(); ++i) {
                    if (listViewItemHeights.get(i) != null)
                        scrollY += listViewItemHeights.get(i);
                }
                scrollBy(0, scrollY - oldScrollY);
            }
        }
    });
}

Comment No.1: this is a variable letting me keep track of the current scroll position.

Comment No.2: setting a new on scroll listener to let my custom view know when the list

Comment No.3: this does not need to be implemented in this case.

Comment No.4: this is where magic happens. Note in the end I scroll my view by scrollY – oldScrollY, let me start with this bit first. oldScrollY is the kept scroll position, scrollY is the new one. I need to scroll by the difference between them. As for how scrollY is calculated I refer you to my answer here: Android getting exact scroll position in ListView, it is how the scroll position is calculated in list view.