XML drawable composed of png and overlay

In my_button_bg.xml:

<selector xmlns:android="http://schemas.android.com/apk/res/android">    <item android:drawable="@drawable/button_pressed" android:state_pressed="true"/>    <item android:drawable="@drawable/button_normal"/></selector>

button_normal is a png

button_pressed is an xml:

<?xml version="1.0" encoding="utf-8"?><layer-list xmlns:android="http://schemas.android.com/apk/res/android" >    <item android:drawable="@drawable/button_normal"/>    <item android:drawable="@color/btn_bg_pressed_mask"/></layer-list>

where btn_bg_pressed_mask is a color:

<color name="btn_bg_pressed_mask">#19000000</color>

This should work

<layer-list xmlns:android="http://schemas.android.com/apk/res/android">    <item        android:state_enabled="true"        android:drawable="@drawable/my_button" />    <item>        <selector>            <item                android:state_pressed="true"                android:state_enabled="true">                <color android:color="#00000088" />            </item>        </selector>    </item></layer-list>

The order of the items in the selector XML make a difference. The first match is what is going to get displayed. As you see in marmor's answer, the normal state of the button is listed at the end.

Another thing to keep in mind is that if you are using 9-patch images (.9.png), the color will only be applied to the content area. So if you want the color to be overlayed over the whole image, make sure to mark the whole image as the content area.