title: Determining if app is running in background
body: <p>How can I tell that my application has been put in the background (i.e. none of my app's activities are visible anymore)? I need to determine this so that I can release resources shared among several of my activities (graphics, sound clips and a socket connection).</p>
  
  <p>I've thought about keeping a global counter that's incremented in the activities' onStart() method, and decremented in onStop(). If the counter reaches zero, then all activities have been stopped and my app is running in the background. However I'm not sure if this is going to be 100% reliable. Also, I can't help but think that there must be a better way of doing this.</p>