title: Is there an easy way to observe the visible cells in a UITableView?
body: <p>I need to keep track of the visible cells in a <code>UITableView</code>, since certain events need to occur when cells that represent specific pieces of data become visible. <code>UITableView</code> doesn't seem to be KVO compliant for <code>-indexPathsForVisibleRows</code> or <code>-visibleCells</code>, and there's no <code>UITableViewDelegate</code> method that gives a hook for being notified that the visible cells have changed.</p>
  
  <p>Is there any way of doing this, short of manually keeping track of my own array, and adding/removing objects every time I insert/remove a row, reload the table, or methods like <code>-tableView:cellForRowAtIndexPath:</code> are called?</p>