title: How Autolayout constraint First Item and Second Item works?
body: <p>If I have UIView with top 100 left 120 right 120 how exactly First Item and Second Item relationship work. After going through Apple document each relationship has a linear equation with an expression as.</p>
  
  <pre><code>firstItem.firstItemAttribute == secondItem.secondItemAttribute * multiplier + constant
  </code></pre>
  
  <p>With this equation for <strong>top leading</strong> constraint with relation </p>
  
  <ul>
  <li>First Item <strong>Safe Area.Leading</strong> </li>
  <li>Relation <strong>=</strong></li>
  <li>Second Item = <strong>MyView.Leading</strong></li>
  <li>Constant <strong>120</strong></li>
  </ul>
  
  <p>with equation the First Item And Second Item I understood. But Problem for me in understanding when I do the <strong>reverse</strong> the second item constant changes to <strong>- 120</strong> and layout has no effect after the change. Why negative has no effect and what is its use for?
  <strong>Xcode</strong> provides a set of properties for FirstItem &amp; SecondItem like leading, center, trailing when we need to switch this properties.</p>