title: How to use SizeChangedLayoutNotifier?
body: <p>I got a <code>Wrap</code> inside my flexibleSpace in my <code>SliverAppBar</code>. Now when the size of my Wrap changes (dynamically) I need to get a notification, so I can change the height of my flexibleSpace in my SliverAppBar accordingly.</p>
  <p>I read the docs to the <a href="https://api.flutter.dev/flutter/widgets/SizeChangedLayoutNotifier-class.html" rel="noreferrer">SizeChangedLayoutNotifier</a> but I don't really understand how to use it. I wrapped my <code>Wrap</code> as a child in a <code>SizeChangedLayoutNotifier</code> but for me it is very unclear how to catch the notification with the <code>NotificationListener&lt;SizeChangedLayoutNotification&gt;</code>. I couldn't find any code example.</p>
  <p>I would really appreciate your help :) Thanks!</p>