将popUpView放在视图中间

Question

我想将创建的popUpView置于视图的中心。我希望它水平和垂直居中。我尝试了很多东西，但我一直坚持下去。 这是我的java代码：

public class Gevaar extends ActionBarActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main_activity);

        final Button btnOpenPopup = (Button)findViewById(R.id.button1);
        btnOpenPopup.setOnClickListener(new Button.OnClickListener(){

            @Override
            public void onClick(View arg0) {
                LayoutInflater layoutInflater
                        = (LayoutInflater)getBaseContext()
                        .getSystemService(LAYOUT_INFLATER_SERVICE);
                View popupView = layoutInflater.inflate(R.layout.popup, null);
                final PopupWindow popupWindow = new PopupWindow(
                        popupView,
                        LayoutParams.MATCH_PARENT,
                        LayoutParams.MATCH_PARENT);

                Button btnDismiss = (Button)popupView.findViewById(R.id.dismiss);
                btnDismiss.setOnClickListener(new Button.OnClickListener(){

                    @Override
                    public void onClick(View v) {
                        // TODO Auto-generated method stub
                        popupWindow.dismiss();
                    }});

                popupWindow.showAsDropDown(btnOpenPopup, 100, 100);

            }});



    }
}

这里是.xml：

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical"
    android:background="@android:color/background_light"
   >


        <LinearLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="vertical"
            android:layout_margin="1dp"
            >

            <ImageView
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:src="@drawable/blaar"
                />
            <Button
                android:id="@+id/dismiss"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:text="Terug" />
        </LinearLayout>
    </LinearLayout>