popupWindow弹出来后，背景变暗,半透明

 /**
      * 点击评分，如果评分后，显示的弹出框
      */
     private void showMakeGradeMarkedWindow() {
         View view = LayoutInflater.from(DetailActivity.this).inflate(
                 R.layout.grade_screen_marked, null);
         final PopupWindow mPopupWindow = new PopupWindow(view, 469, 280);
         mPopupWindow.setFocusable(true);
         ColorDrawable dw = new ColorDrawable(0xb0000000);
         mPopupWindow.setBackgroundDrawable(dw);
         mPopupWindow.showAtLocation(install, Gravity.CENTER, 0, 0);

         final RatingBar mraRatingBar = (RatingBar) view
                 .findViewById(R.id.grade_screen);
         mraRatingBar.setRating(4.5f);
 //      mraRatingBar.setEnabled(false);
         // 设置背景颜色变暗
         WindowManager.LayoutParams lp = getWindow().getAttributes();
         lp.alpha = 0.7f;
         getWindow().setAttributes(lp);
         mPopupWindow.setOnDismissListener(new OnDismissListener() {

             @Override
             public void onDismiss() {
                 WindowManager.LayoutParams lp = getWindow().getAttributes();
                 lp.alpha = 1f;
                 getWindow().setAttributes(lp);
             }
         });

     }

当popupWindow消失后，监听其消失的方法，让背景恢复原样。