title: Android ProgressBar.setProgressDrawable only works once?
body: <p>In a color picker, I have 3 SeekBars for Hue, Saturation, and Value.  Calling setProgressDrawable on these SeekBars only works once -- at initialization from onCreate.  When the user updates the Hue SeekBar, I want to call setProgressDrawable for the Saturation and Value SeekBars, to show the user their color choices for the new Hue.</p>
  
  <p>But all calls to setProgressDrawable (after the initial ones from <code>onCreate</code>) cause the SeekBar to be blanked.</p>
  
  <p>How can I update the background gradient of my SeekBars based upon user input?</p>