title: Xcode Thread 1 : Is it always main UI thread?
body: <p>Xcode shows threads in Debug navigator. </p>
  
  <p>They are numbered: <code>Thread 1</code>, <code>Thread 2</code>, etc</p>
  
  <p>I notice that I am always in Thread 1. Is it correct assumption that it is main UI thread?
  I use libs/SDK like <code>Facebook</code> and <code>AFNetworking</code> they have callbacks and delegates. 
  But thread safety is not always properly documented.</p>