title: how to get frame of 'screen viewport' in zoomed uiview?
body: <p>I have a UIImageView which can be zoomed and panned via gesture controls.  (I am not using an UIScrollView for this, for no reason other than that I was building from a tutorial and didn't learn about Scroll Views until later.)</p>
  
  <p>I want to add another view as a subview of the main Image View, and I want the frame of the new view to be the region shown on the screen.   i.e. if the main image is zoomed in to very fine detail so that only a tiny piece of the image is showing, then I want the frame of the new view to be only that visible piece.</p>
  
  <p>Similarly, if the main view is zoomed way out, so that it appears very small on the screen, I want the frame of the new view to be comparatively very large, encompassing everything shown on the screen.</p>
  
  <p>Would prefer an answer in terms of Swift language, but I can convert from Objective C.</p>
  
  <p>In case it's relevant: The ultimate application for all this is for hand-gesture drawing, so that I'm allocating the appropriate amount of space &amp; resources for the region the user is likely to try &amp; draw.  It'd be memory-wasteful to make the frame of the new "UIBezierCurveView" be the frame of the entire (hi res) image if they're zoomed way in.  </p>