title: Event that would "setState" when div becomes visible in amp-html
body: <p>To begin, I understand how to <code>setState</code> in amp-html using amp-bind like so:</p>
  
  <pre><code>&lt;amp-selector on="select:AMP.setState({ guess: event.targetOption })"&gt;
      &lt;div class="button" option="a"&gt;Answer 1&lt;/div&gt;
      &lt;div class="button" option="b"&gt;Answer 2&lt;/div&gt;
      &lt;div class="button" option="c"&gt;Answer 3&lt;/div&gt;
  &lt;/amp-selector&gt;
  </code></pre>
  
  <p>Now say I select the button <code>Answer 2</code>, then the state upon <code>printState()</code> will look like <code>{"guess":"b"}</code> and this div will no longer be hidden:</p>
  
  <pre><code>&lt;div hidden [hidden]="!guess"&gt;
      &lt;h4&gt;You have the correct answer!&lt;/h4&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p><strong>My question is</strong>: How do I set another key/value to state once the hidden div becomes visible? In other words, how can I get this to work?</p>
  
  <pre><code>&lt;div hidden [hidden]="!guess" on="WhenThisIsVisible:AMP.setState({"text":"hi!"})"&gt;&lt;/div&gt;
  </code></pre>
  
  <p>I can use <code>tap:</code> instead of my made-up event, but I'd like to not have the user have to click on the div to <code>setState</code>.</p>
  
  <p>So once the div is visible, <code>printState()</code> would return <code>{"guess":"b","text":"hi!"}</code></p>