title: How to center a subview of UIView
body: <p>I have a <code>UIView</code> inside a <code>UIView</code>m and I want the inner <code>UIView</code> to be always centered inside the outer one, without it having to resize the width and height.</p>
  
  <p>I've set the struts and springs so that it's on top/left/right/bottom without setting the resize. But it still doesn't center. Any idea?</p>