title: How to test if a fragment view is visible to the user?
body: <p>I have a ViewPager, each page is a Fragment view. I want to test if a fragment is in a visible region. the Fragment.isVisible only test</p>
  
  <ul>
  <li>the fragment is attached to a activity</li>
  <li>the fragment is set to visible</li>
  <li>the fragment has been added to a view</li>
  </ul>
  
  <p>The ViewPager will create 3 (by default) fragment and all three of them meet the above criteria, but only one is actually visible to the user (the human <strong>eyes</strong>)</p>