title: Android: Back button event in FragmentActivity
body: <p>I have a FragmentActivity.
  The Activity contains one fragment.<br>
  In the fragment, there is a button, when click a imageview will appear (View.VISIBLE) at the center.<br>
  I want to have a back button event to check that if the imageview is visible, then hide it, otherwise, continue the default back button event.<br>
  As the FragmentActivity and the Fragment is separate class. And there are no onBackPressed() in Fragment. So how can I do it? I want to handle the back event in the Fragment class.</p>