ListView (Adapter) item visibility listener

Question

Is it possible to have a visibility listener on each listview's item? Like: on user sees the item, do something. On item is hidden, do something else.

I want to check when a item "enters" or "exits" the scroll so as to update a second list.

Additionaly my ListView might expand like:

adapter.addAll(  (Collection<? extends DBObject>) events);
adapter.notifyDataSetChanged();

I think a way might be to use a global listener to check the items but I am afraid it will get messed up if I add more items to the ListView (above). I have not accomplished anything with that yet

eventList.setOnScrollListener(new OnScrollListener() {

        public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {

        }

        @Override
        public void onScrollStateChanged(AbsListView view, int scrollState) {}
});

Answer 1

You can use the onScroll function to calculate that.

eventList.setOnScrollListener(new OnScrollListener() {
    int oldFirstVisibleItem = 0;
    int oldLastVisibleItem = 0;

    public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {
        if (firstVisibleItem > oldFirstVisibleItem) {
            for(int i = oldFirstVisibleItem; i < firstVisibleItem; i++) {
                onExit(i);
            }
        }
        if (firstVisibleItem < oldFirstVisibleItem) {
            for(int i = firstVisibleItem; i < oldFirstVisibleItem; i++) {
                onEnter(i);
            }
        }

        int lastVisibleItem = firstVisibleItem + visibleItemCount - 1;
        if (lastVisibleItem < oldLastVisibleItem) {
            for(int i = oldLastVisibleItem+1; i <= lastVisibleItem; i++) {
                onExit(i);
            }
        } 
        if (lastVisibleItem > oldLastVisibleItem) {
            for(int i = oldLastVisibleItem+1; i <= lastVisibleItem; i++) {
                onEnter(i);
            }
        } 

        oldFirstVisibleItem = firstVisibleItem;
        oldLastVisibleItem = lastVisibleItem;
    }

    @Override
    public void onScrollStateChanged(AbsListView view, int scrollState) {}
});

public void onEnter(int position) {
    // Handle an item coming into view.
}

public void onExit(int position) {
    // Handle an item going out of view.
}

Answer 2

The answer is in your question.

One of the methods that you override in the onScrollListener() is

public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {

        }

The parameters passed include the firstVisibleItem, and the visibleItemCount. you can use these two numbers to figure out which items are currently visible.

Just keep in mind that onScroll is called very often while scrolling.