title: Where to find defined keys for setValue func in Swift
body: <p>While trying to setup email subject and content via <code>UIActivityViewController</code> I worked with the <code>.setValue</code> func in Swift. The key to set the subject was easy to find, but I couldn't find the key to set the content.</p>
  
  <pre><code>activityViewController.setValue("Subject", forKey: "Subject")
  
  //Key "Content" is not defined
  activityViewController.setValue("Content", forKey: "Content"
  </code></pre>
  
  <p>In this context I searched for a "list" with all possible keys, but I couldn't find anything (Even in the API doc from Apple <a href="https://developer.apple.com/documentation/objectivec/nsobject/1415969-setvalue" rel="nofollow noreferrer">https://developer.apple.com/documentation/objectivec/nsobject/1415969-setvalue</a>). So I would like to ask in general where to find such a list, independent of this particular case.</p>