iphone - Detect when UITableViewCell goes off the screen

Question

I'm implementing a rich UITableView with custom created UITableViewCell, I show these on the screen in one fashion, but once they go off the screen I want to take a note of that, since the second time they come on I would like them to get displayed in a different manner. Think auto "mark as read" when going off the screen.

I've been looking for some way to detect when a cell goes off the screen (get's dealloc'ed or dequeued or equivalent), preferably in the :UITableViewController class to make a quick note of the [indexPath row], but in the :UITableViewCell is equally as good.

I haven't been able to do this in any standard way ... counting the times it appeared seems out of the question as I do multiple reloadData calls on the table.

Anyone any ideas? This seems a bit tricky :)

Answer 1

This is an old question, but in case anyone is looking, in iOS6, a new UITableViewDelegate function was introduced that does just this:

- (void)tableView:(UITableView *)tableView didEndDisplayingCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath

It does a great job at telling you whenever a cell is removed, however, it is very thorough and thus if you did a reload cell, even the old cell that's being replaced will trigger this delegate function. In my implementation I simply check to see if the indexPath passed is still within the array tableView.indexPathsForVisibleRows. Something like:

- (void)tableView:(UITableView *)tableView didEndDisplayingCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath
{
    if ([tableView.indexPathsForVisibleRows indexOfObject:indexPath] == NSNotFound)
    {
        // This indeed is an indexPath no longer visible
        // Do something to this non-visible cell...
    }
}