title: What is PurpleEventCallback?
body: <p>In the stack trace of my iPhone application, I see a call to something called <code>PurpleEventCallback</code>. I wasn't able to find any documentation for it. What is this?</p>
  
  <p><img src="https://i.stack.imgur.com/x9uwi.png" alt="PurpleEventCallback"></p>