title: In Objective C, can you check if an object has a particular property or message?
body: <p>I wat to do something like this:</p>
  
  <pre><code>if (viewController.mapView) [viewController.mapView someMethod];
  </code></pre>
  
  <p>However, if mapView is not a class variable, this crashes. How do I check if mapView exists?</p>