title: zoomScale after resizing a MKMapView is not equal to zoomScale when draw MKOverlay
body: <p>I have an app, that uses MapKit.
  <br>I calculate current zoom after resizing a map.
  <br>By define (from <code>MKGeometry.h</code> file)</p>
  
  <pre><code>MKZoomScale provides a conversion factor between MKMapPoints and screen points.
  When MKZoomScale = 1, 1 screen point = 1 MKMapPoint.  When MKZoomScale is
  0.5, 1 screen point = 2 MKMapPoints.
  </code></pre>
  
  <p>So, I calculate it in this way:</p>
  
  <pre><code>- (void)mapView:(MKMapView *)mapView regionDidChangeAnimated:(BOOL)animated
  {
      CGSize screenSize = mapView.bounds.size;
      MKMapRect mapRect = mapView.visibleMapRect;
      MKZoomScale zoomScale = screenSize.width * screenSize.height / (mapRect.size.width * mapRect.size.height);
  }
  </code></pre>
  
  <p>The calculated <code>zoomScale</code> conforms to definition (I've checked).</p>
  
  <p>I also draw an overlay on my mapView in method </p>
  
  <pre><code>-(void)drawMapRect:(MKMapRect)mapRect zoomScale:(MKZoomScale)zoomScale inContext:(CGContextRef)context
  </code></pre>
  
  <p>The problem is, that my calculated <code>zoomScale</code> is not equal to that one, that passed to this method by system. I expect them to be equal, because <code>drawMapRect:</code> calls just after resizing (actually, resizing causes this method to be called).</p>
  
  <p>What is wrong here?</p>
  
  <p>I also tried to use </p>
  
  <pre><code>currentZoomScale = mapView.bounds.size.width / mapView.visibleMapRect.size.width;
  </code></pre>
  
  <p>, suggested <a href="https://stackoverflow.com/questions/2960294/mkmapview-getting-the-relative-zoom-ratio/4136791#4136791">here</a>, but this <code>currentZoomScale</code> is not equal to passed to <code>drawMapRect:</code> as well.</p>