title: Android: Set alpha on button on click. Preferably xml only
body: <p>I want to introduce a 'down' style on some buttons. I've created a style, and a state list. </p>
  
  <pre><code>&lt;selector xmlns:android="http://schemas.android.com/apk/res/android"&gt;
  
      &lt;item android:drawable="@drawable/inactive_button_background" android:state_enabled="false"/&gt;
      &lt;item android:drawable="@drawable/active_button_background" android:state_enabled="true"/&gt;
      &lt;item android:drawable="@drawable/pressed_button_background" android:state_pressed="true"&gt;&lt;/item&gt;
  &lt;/selector&gt;
  </code></pre>
  
  <p>The problem is that I want to be able to change just the alpha of the background when the button gets clicked (I will have variable backgrounds, so setting a sold color with alpha channel is not a solution). </p>
  
  <p>And I want to do this from the declarative xml only (don't want to polute my code with layout stuff).</p>
  
  <p>Problem is I don't know how to apply this alpha blending to the button from a xml drawable.
  I am pretty sure there's a way though.</p>