title: iOS autolayout constraints refresh
body: <p>Main question : I have three labels (label1, label2, label3) one below the other.
  How do I align label3 below label1 if, during runtime, label2 is set to hidden or if removed from superview.</p>
  
  <p>Initial View:</p>
  
  <pre class="lang-none prettyprint-override"><code>Label1
  vertical spacing 
  Label2
  vertical spacing 
  Label3
  </code></pre>
  
  <p>Action:</p>
  
  <pre><code>set Label2.hidden = yes;
  </code></pre>
  
  <p>Output:</p>
  
  <pre class="lang-none prettyprint-override"><code>Label1
  vertical spacing
  Label3
  </code></pre>
  
  <p>What I have tried is to apply vertical spacing constraints:</p>
  
  <ol>
  <li>between label1 and label2 with some priority</li>
  <li>between label2 and label3 with high priority wrt to 3rd constraint</li>
  <li>between label1 and label3 with lowest priority </li>
  </ol>
  
  <p>Now when I remove the label2 how do I refresh the constraints or should I keep track of all constraints and remove the 2nd constraint when label2 is hidden?</p>