title: How to use UIView's contentStretch
body: <p>I want to use an image of size 27x27 with the center 1 pixel stretchable as the backgroundImage of an UIButton. Here is my code:</p>
  
  <pre><code>[button setBackgroundImage:[UIImage imageNamed:@"button_image"] forState:UIControlStateNormal];
  button.frame = CGRectMake(0.f, 0.f, 27.f, 27.f);
  button.contentStretch = CGRectMake(13.f/27.f, 13.f/27.f, 1.f/27.f, 1.f/27.f);
  [button setTitle:NSLocalizedString(@"long title", @"long title") forState:UIControlStateNormal];
  [button sizeToFit];
  CGRect buttonFrame = button.frame;
  buttonFrame.size.width += 18.f * 2;
  button.frame = buttonFrame;
  </code></pre>
  
  <p>But the result is no different from that without setting contentStretch at all.</p>
  
  <p>So how to correctly use contentStretch to achieve what I want?</p>
  
  <p><em>Please don't tell me again and again that I can achieve the same effect with <code>stretchableImageWithLeftCapWidth:topCapHeight:</code>. I know that and it's apparently not what I'm asking for. I just want to know how to properly use <code>contentStretch</code>. Thanks.</em></p>