/**
 * 获取 raw 下 指定视频文件的缩略图
 * @param aVideoName 
 * @return
 */
private Drawable getThumbnail(String aVideoName)
{
  Drawable ret = null;

  int id = 0;
  try
  {
    id = R.raw.class.getDeclaredField(aVideoName).getInt(this);

    Uri videoURI = Uri.parse("android.resource://" + this.getPackageName() + "/" + id);

    ret = getThumbnail(videoURI);

  } catch ( IllegalAccessException aE )
  {
    aE.printStackTrace();
  } catch ( NoSuchFieldException aE )
  {
    aE.printStackTrace();
  }

  return ret;
}

private Drawable getThumbnail(Uri aVideoUri)
{
  MediaMetadataRetriever retriever = new MediaMetadataRetriever();
  retriever.setDataSource(this, aVideoUri);
  Bitmap bitmap = retriever
      .getFrameAtTime(1*1000*1000, MediaMetadataRetriever.OPTION_PREVIOUS_SYNC);
  Drawable drawable = new BitmapDrawable(getResources(), bitmap);

  return drawable;
}