title: When and why is updateConstraints called?
body: <p>When and why is updateConstraints called? And when overriding, what kind work is meant to be done in the override?</p>
  
  <p>I've read and watched lots of information on how Auto Layout works, but i can't seem wrap my head around it completely.</p>
  
  <p>Do you update the constant of any special interest constraints you saved references to, or to do customising do you remove all constraints from your view and recreate them entirely like one guy did in a WWDC video?</p>