title: What is invalid region in Android?
body: <p>Under "How Android Draws Views" topic, there is such a sentence :</p>
  
  <blockquote>
    <p>Drawing begins with the root node of
    the layout. It is requested to measure
    and draw the layout tree. Drawing is
    handled by walking the tree and
    rendering each View that intersects
    the <strong>invalid region</strong>.</p>
  </blockquote>
  
  <p>And I didn't quite understand the term "invalid region".</p>
  
  <p>Here is the <a href="http://developer.android.com/guide/topics/ui/how-android-draws.html" rel="noreferrer">source article of the quotation</a>.</p>
  
  <p>Can someone elaborate it?</p>
  
  <p>Thanks.</p>