title: Detect the current top cell in a UITableView after scrolling
body: <p>a simple question but I don't seem to have the right terminology to search Stackoverflow by it.</p>
  
  <p>I have a UITableView with no sections, the user can scroll up and down a long list of data (rows) presented inside this tableview.</p>
  
  <p><strong>Question:</strong> how can I detect the most top cell row number after the user scrolled.
  (for example, if the user is scrolling 30 cells down, the top cell after the scroll is = 30)</p>