title: How do you change UIButton image alpha on disabled state?
body: <p>I have a UIButton with an image and on its disabled state, this image should have .3 alpha.</p>
  
  <pre><code>UIButton *button = [UIButton buttonWithType:UIButtonTypeCustom];
  UIImage *arrowImage = [UIImage imageNamed:@"arrow.png"];
  [button setImage:arrowImage forState:UIControlStateNormal];
  
  // The arrow should be half transparent here
  [button setImage:arrowImage forState:UIControlStateDisabled];
  </code></pre>
  
  <p>How do I accomplish this?</p>
  
  <p><strong>UPDATE</strong>: I noticed, by default UIButton does reduce the alpha of the image on disabled state (probably at .5?). But I'd like to know how to fine-tune this value.</p>