android popupWindow 显示时如何让背景变暗，就像dialog那样？

就类似于android版qq

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setBackgroundDrawable(new BitmapDrawable());
setOutsideTouchable(true);
setFocusable(true);  
//消失的时候设置窗体背景变亮
 setOnDismissListener(new OnDismissListener() {
                @Override
                public void onDismiss() {
                    WindowManager.LayoutParams lp = getWindow().getAttributes();
                    lp.alpha = 1.0f;
                    getWindow().setAttributes(lp);

                }
            });