android中popupwindow弹出后，屏幕背景变成半透明

/**   

    * 创建PopupWindow   

    */     

    protected void initPopuptWindow() {     

     WindowManager windowManager = getActivity().getWindowManager();  

     Display display =  windowManager.getDefaultDisplay();  

     popupWindow = new PopupWindow(popupWindow_view, display.getWidth(),display.getHeight(), true);    

      // 设置动画效果     

     popupWindow.setAnimationStyle(R.style.Animation_ZoomLight);  

        WindowManager.LayoutParams params=getActivity().getWindow().getAttributes();  

        params.alpha=0.7f;  

            

     getActivity().getWindow().setAttributes(params);  

      //点击其他地方消失     

     popupWindow_view.setOnTouchListener(new OnTouchListener() {     

      @Override     

      public boolean onTouch(View v, MotionEvent event) {     

      // TODO Auto-generated method stub     

      closePopupWindow();  

      return false;     

      }     

      });     

    }  

   /***   

    * 获取PopupWindow实例   

    */     

   private void getPopupWindow() {     

         

    if (null != popupWindow) {     

     closePopupWindow();  

    return;     

    } else {     

    initPopuptWindow();     

    }     

    }      

   /** 

    * 关闭窗口 

    */  

   private void closePopupWindow()  

   {  

     if (popupWindow != null && popupWindow.isShowing()) {     

      popupWindow.dismiss();     

      popupWindow = null;     

      WindowManager.LayoutParams params=getActivity().getWindow().getAttributes();  

      params.alpha=1f;  

      getActivity().getWindow().setAttributes(params);  

      }     

   }