title: meaning of format reference in android attribute
body: <p>Let's start with an example</p>
  
  <pre><code>&lt;attr name="spinnerDropDownItemStyle" format="reference" /&gt;
  </code></pre>
  
  <p>How do I take this apart and understand it? </p>
  
  <p>I watch the <a href="https://www.youtube.com/watch?v=TIHXGwRTMWI" rel="noreferrer">Android themes &amp; styles demystified - Google I/O 2016</a> talk multiple times and I still don't understand how this reference thing works.</p>