title: How to create a custom UIView that defines a baseline?
body: <p>Using the <code>NSLayoutConstraint</code> class, it is possible to create a constraint that is based on a view's baseline (<code>NSLayoutAttributeBaseline</code>). However, I haven't seen any documentation that describes how a <code>UIView</code> actually provides a baseline value to the auto layout system. </p>
  
  <p>If I wanted to create a custom <code>UIView</code> subclass that defines a baseline, how would I do it? <code>NSView</code> defines a <code>baselineOffsetFromBottom</code> method that I assume is involved somehow on OS X, but how does this work in iOS?</p>