title: Flutter: Change text when FlexibleSpaceBar is collapsed
body: <p>I have looked through the Flutter documentation to try and find an event, callback or even a state that I could hook into when the FlexibleSpaceBar is collapsed or expanded.</p>
  
  <pre><code>return new FlexibleSpaceBar(
    title: new Column(
      crossAxisAlignment: CrossAxisAlignment.end,
      mainAxisAlignment: MainAxisAlignment.end,
      children: &lt;Widget&gt;[
          new Text(_name, style: textTheme.headline),
          new Text(_caption, style: textTheme.caption)
      ]),
    centerTitle: false,
    background: getImage());`
  </code></pre>
  
  <p>When the FlexibleSpaceBar is snapped in (collapsed), I want to hide the <strong>_caption</strong> text and only display the <strong>_name</strong> text. When it is expanded fully, I obviously want to display both <strong>_name</strong> &amp; <strong>_caption</strong>.</p>
  
  <p>How do I go about doing that?</p>
  
  <p>Im new to flutter, so I am somewhat lost on this.</p>
  
  <p>Also reported at <a href="https://github.com/flutter/flutter/issues/18567" rel="noreferrer">https://github.com/flutter/flutter/issues/18567</a></p>