Android编程实现popupwindow弹出后屏幕背景变成半透明效果

/**
* 设置添加屏幕的背景透明度
* @param bgAlpha
*/
public void backgroundAlpha(float bgAlpha)
{
    WindowManager.LayoutParams lp = getWindow().getAttributes();
    　　　　lp.alpha = bgAlpha; //0.0-1.0
        getWindow().setAttributes(lp);
}

popWin = new PopupWindow(popAddNoteType, mScreenWidth *8 /10, ViewGroup.LayoutParams.WRAP_CONTENT);
//在PopupWindow里面就加上下面代码，让键盘弹出时，不会挡住pop窗口。
popWin.setInputMethodMode(PopupWindow.INPUT_METHOD_NEEDED);
popWin.setSoftInputMode(WindowManager.LayoutParams.SOFT_INPUT_ADJUST_RESIZE);
//点击空白处时，隐藏掉pop窗口
popWin.setFocusable(true);
popWin.setBackgroundDrawable(new BitmapDrawable());
backgroundAlpha(1f);
//添加pop窗口关闭事件
popWin.setOnDismissListener(new poponDismissListener());

/**
* 添加新笔记时弹出的popWin关闭的事件，主要是为了将背景透明度改回来
* @author cg
*
*/
class poponDismissListener implements PopupWindow.OnDismissListener{
    @Override
    public void onDismiss() {
      // TODO Auto-generated method stub
      //Log.v("List_noteTypeActivity:", "我是关闭事件");
      backgroundAlpha(1f);
    }
}