code: @Override
      protected View createOverlayView() {
          LinearLayout ll = new LinearLayout(getContext());
          ll.setLayoutParams(new FrameLayout.LayoutParams(FrameLayout.LayoutParams.MATCH_PARENT,FrameLayout.LayoutParams.MATCH_PARENT));
          ll.setGravity(Gravity.CENTER);
          View progressBar = createProgressBar();
          ll.addView(progressBar);
          return ll;
      }
docs: create loading mask
  @return