Detect when UITableViewCell goes off the screen

Question

Hi,

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

I think I would try periodically checking the indexPathsForVisibleRows property of the UITableView. From the largest index path, you can deduce that all previous rows have been scrolled past.

Answer 2

I think you could use the

- (NSArray *)visibleCells

method for your UITableView. This returns an array of all cells that are visible. You can then mark any data that is "not visible" (i.e. not in this array) in the way you want, such that when you scroll back to it, it has been updated.

Hope that helps

Answer 3

Are you sure a cell going offscreen is exactly what you want to catch? If you want to mark items as read, this does not seem like a proper way to do it. For example, I might scroll though the table really fast, and I would be very surprised if you marked all of the stuff as read.

As for the technical part, simply keep a list of cells that are on screen (cellForRowAtIndexPath should add cells to that list), and in scrollViewDidScroll delegate method check if any of them are no longer visible.

Another possible idea: I remember there is prepareForReuse method on the cell. Not sure when it is called, though.