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);
}