title: ScrollviewDidScroll not called at each point
body: <p>There is a button at the bottom of my view controller. When the user scrolls down the button has to be attached to the scrollview at certain height.</p>
  
  <p>I need to attach a button to the scrollview, immediately when the <code>contentOffset.y</code> reaches a particular value. <code>-(void) scrollviewDidScroll</code> doesn't help me as there might be a jump in <code>contentOffset</code> when the user is scrolling fast. Any leads on this are helpful.</p>
  
  <p>Also, whenever I add a subview to the scrollview, <code>-(void) viewDidLayoutSubviews</code> is called. Which in turn sets the contentOffset to <code>{0,0}</code>. How can I achieve the functionality I need?</p>