title: ios how to change alpha of popover presentation controller layout
body: <p>I'm quite new in ios programming so sorry if I ask a dumb questin.</p>
  
  <p>I want to develop an application for <strong>ipad using ios 7.1</strong>.</p>
  
  <p><strong>My problem:</strong>
  I used popoover presentation from storyboard to link a button to a view controller. I gave a custom class for the view controller that will be poppedover. But initially I want to <strong>set its layout alpha value</strong> and afterwards to make an animation. I did something like this:</p>
  
  <pre><code>self.view.alpha = 0.2;
  self.popoverPresentationController.containerView.alpha = 0.2;
  </code></pre>
  
  <p>But I'm getting an <strong>error</strong> when I use the second line: 
  <strong>[Popover popoverPresentationController]: unrecognized selector sent to instance..</strong>.</p>
  
  <p>So how can I change my popover alpha value at runtime? Thank you :)</p>