How do I make a drawable from multiple images?

I have three 9-patch PNG's which together make up the background for a button (left side, middle, right side). I would like to combine these three images together in a drawable which I can specify as the background for a button in XML, something along the lines of:

res/drawable/button_background.xml:

<?xml version="1.0" encoding="utf-8"?>
<nine-patch xmlns:android="http://schemas.android.com/apk/res/android"
android:src="@drawable/button_left_side" />

<nine-patch android:src="@drawable/button_middle" />

<nine-patch android:src="@drawable/button_right_side" />

res/layout/main.xml:

<button android:background="@drawable/button_background" />

Is this possible?

Solution

After some trial-and-error, I was able to solve the problem in a satisfactory way. I simply implemented a Layer-List drawable as follows:

<?xml version="1.0" encoding="utf-8"?>
<layer-list
    xmlns:android="http://schemas.android.com/apk/res/android" >
    <item android:drawable="@drawable/btn_left" android:left="0px" />
    <item android:drawable="@drawable/btn_middle" 
        android:left="26px" android:right="26px" />
    <item android:drawable="@drawable/btn_right" android:right="0px" />
</layer-list>

Where the 26px values are the width of the two button side images in pixels.

To use this drawable, simply call it like any other drawable:

<bitmap android:src="@drawable/button_background" />

This works exactly how you would expect it to, with the middle expanding as normal to fit the desired width, and all three images expanding to fit the desired height. I hope others can benefit from this!