android PopupWindow 底部阴影外部空间取消

PopupWindow   po;public void init(){  //初始化    po=new PopupWindow(this);    View vs = LayoutInflater.from(this).inflate(R.layout.testlayout,null);    Button viewById = (Button) vs.findViewById(R.id.btn12);    viewById.setOnClickListener(new View.OnClickListener() {        @Override        public void onClick(View v) {            po.dismiss();        }    });    po.setOnDismissListener(new PopupWindow.OnDismissListener() {        @Override        public void onDismiss() {            Log.v("ta","setOnDismissListener");            is=false;            backgroundAlpha(1);        }    });    po.setContentView(vs);  //  po.setOutsideTouchable(false);    po.setWidth(ViewGroup.LayoutParams.MATCH_PARENT);    po.setHeight(ViewGroup.LayoutParams.WRAP_CONTENT);}
boolean  is =false;  //禁止分发事件    @Override    public boolean dispatchTouchEvent(MotionEvent ev) {        Log.v("ta","dispatchTouchEvent");        if (is){            return false;        }        return super.dispatchTouchEvent(ev);    }
public void backgroundAlpha(float bgAlpha)  //阴影改变{    WindowManager.LayoutParams lp = getWindow().getAttributes();    lp.alpha = bgAlpha; //0.0-1.0    getWindow().setAttributes(lp);}