title: Android: raise AlertDialog from background thread
body: <p>In my activity there's some stuff going on in a background thread, which gets started in Activity_1. The processing of the background thread takes a while and I want to notify the user when it's completed via an <code>AlertDialog</code>. However, the user might have changed to Activity_2 or Activity_3 in the meantime and I would like to pop up the AlertDialog always in the current Activity.</p>
  
  <p>Any idea how to realize this?</p>