title: Objective-C setNeedsUpdateConstraints vs. updateConstraintsIfNeeded
body: <p>I have read Apple's <a href="https://developer.apple.com/library/ios/documentation/uikit/reference/uiview_class/UIView/UIView.html#//apple_ref/occ/instm/UIView/" rel="nofollow">documentation</a> on these two methods, but I still don't quite understand.</p>
  
  <p>What is the difference and why would I use either <code>setNeedsUpdateConstraints</code> or <code>updateConstraintsIfNeeded</code>?</p>