【android】让popupwindow显示在view的上方并与该view水平居中对齐

public class MainActivity extends Activity implements OnClickListener{

private Button showBtn1;
private Button showBtn2;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

showBtn1 = (Button) findViewById(R.id.showBtn1);
showBtn2 = (Button) findViewById(R.id.showBtn2);
showBtn1.setOnClickListener(this);
showBtn2.setOnClickListener(this);
}

@Override
public void onClick(View v) {
// TODO Auto-generated method stub
View popupView = LayoutInflater.from(this).inflate(R.layout.popup, null);
PopupWindow popupWindow = new PopupWindow(popupView, ViewGroup.LayoutParams.WRAP_CONTENT,
ViewGroup.LayoutParams.WRAP_CONTENT, true);
popupWindow.setBackgroundDrawable(new BitmapDrawable());
popupView.measure(MeasureSpec.UNSPECIFIED, MeasureSpec.UNSPECIFIED);
int popupWidth = popupView.getMeasuredWidth();
int popupHeight =  popupView.getMeasuredHeight();
int[] location = new int[2];
switch (v.getId()) {
case R.id.showBtn1:
v.getLocationOnScreen(location);
popupWindow.showAtLocation(v, Gravity.NO_GRAVITY, (location[0]+v.getWidth()/2)-popupWidth/2,
location[1]-popupHeight);
break;
case R.id.showBtn2:
v.getLocationOnScreen(location);
popupWindow.showAtLocation(v, Gravity.NO_GRAVITY, (location[0]+v.getWidth()/2)-popupWidth/2,
location[1]-popupHeight);
break;

default:
break;
}
}
}