title: Constrain button within a view so that they are all evenly spaced
body: <p>I'm struggling constraining a set of button (see below), so that they the vertical space between them is equal.  I am trying to create a subview with buttons that mimics a keypad.  I constrain the top center button to the horizontal center and top space to view.  I constrain the bottom center button to the horizontal center and bottom space to view.  How do I get those middle 2 buttons to be evenly spaced?  On android I would accomplish this by using <code>dp</code> (density independent pixels), however, if i try to constrain these buttons by vertical spacing, it will be different on every display because the pixel densities are different.</p>
  
  <p><a href="https://i.stack.imgur.com/eDEtf.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/eDEtf.png" alt="enter image description here"></a></p>