popupWindow弹出来后，背景变暗

/**
	 * 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。