title: How do I determine if a specific tween has finished and destroyed?
body: <p>I need to hold a tween in a variable and then at some time determine if it's finished or not.</p>
  
  <p>I have tried to see <strong>progress</strong>() == 1 or <strong>_gc</strong>
  but firstly progress() doesn't exist and 
  secondly _gc I suppose it is a private/internal field that should not be used by me since it's not an extracted api and it could may return unexpected result or removed in future.</p>
  
  <pre><code>var tween = TweenMax.fromTo("body", 20, { x:-100 }, {x:0});
  // ... after a minute
  if (tween._gc)
  {
     console.log("tween is no longer alive");
  }
  </code></pre>