title: why I can't change the transparency of a Drawable?
body: <p>I want to get the background <code>Drawable</code> of a <code>View</code>, change it's transparency, and then set this new drawable as background of another <code>View</code>. what I do:</p>
  
  <pre><code>Drawable d=view.getBackgroung();
  d.setAlpha(100);
  secondView.setBackgroundDrawable(d);
  </code></pre>
  
  <p>but the transparency doesn't change. what's wrong?</p>