title: Change image in unfocussed ViewPager Fragment
body: <p>So I have a view pager implemented like this
  <img src="https://i.stack.imgur.com/cjppM.png" alt="enter image description here"></p>
  
  <p>In the main view I have button at the top right of the screen thats similar to a navigation bar. When I click it or swipe it I want it to disappear and when it refocuses on the main screen I want it to appear again. I've been trying it for hours and have had no luck.</p>
  
  <p>I know I need to use <code>setVisibility(View.GONE)/VISIBLE</code> I just keep getting null pointers or the image just wont change.</p>
  
  <p>Ive implemented <code>OnPageChangeListener()</code> and tried using otto events to change the button state as well, but the event is never being received.</p>
  
  <p>In short the main goal is to be able to interact with elements on any view not visibly shown in the ViewPager</p>