How does the GMail app implement its selection summary in ActionBar?
<p>When an email is selected for deletion in the GMail app, the ActionBar looks like so:</p>

<p><img src="http://i.stack.imgur.com/TcYpX.png" alt="enter image description here"></p>

<p>There is a check mark and selection count displayed, with a subtle entry and exit animation. It appears it's an overlay that's on top of the ActionBar or replaces it.</p>

<p>Since I don't know the name of this widget I have trouble finding info about it. </p>

<ul>
<li>Is it part of the API? Google Calendar has a similar overlay when creating an event, which makes me think it's possibly part of the API.</li>
<li>If it is, what is it called?</li>
<li>How is the ActionBar updated with the row selection count? (No need for code, just a brief explanation would be helpful on how to write content to the the ActionBar overlay shown in the image).</li>
</ul>
