Android 排序z order,How to android Z order?

In my app, I want to draw on top of the background image. I have the following xml:

android:orientation="vertical"

android:layout_width="match_parent"

android:layout_height="match_parent"

android:background="@drawable/bg2"

>

android:layout_width="fill_parent"

android:layout_height="fill_parent"

android:id="@+id/drawingSurface"

/>

android:orientation="horizontal"

android:background="@drawable/bg2"

android:layout_width="match_parent"

android:layout_height="match_parent">

android:layout_height="wrap_content"

android:layout_width="wrap_content"

android:text="OK"

android:onClick="onClick"

android:id="@+id/colorBlueBtn"

/>

android:layout_height="wrap_content"

android:layout_width="wrap_content"

android:text="Save"

android:onClick="onClick"

android:id="@+id/saveBtn"

/>

No, the problem is, my drawing is not showing when I try to draw on the drawing surface. The background image and buttons were shown. And once I saved it, the image file generated by my app is shown. I think the problem is the Z order of my layout.

Any ideas? Thanks for any help! :)