title: Adding constraints programmatically in Objective-C
body: <p>I need to add some constraints to a <code>UIButton</code> programmatically.
  I need it to have it centred horizontally to superview, centred vertically to superview, aspect ratio to its superview and aspect ratio to self.</p>
  
  <p>Can anyone help me please?</p>
  
  <p>Thank you.</p>