title: how to use [UIImage resizableImageWithCapInsets:]
body: <p>I'd like to stretch a image with size 170x50 and show it in an image view with size 240x140.
  The original image looks like:</p>
  
  <p><img src="https://i.stack.imgur.com/z6wN8.png" alt="enter image description here"></p>
  
  <p>I want to keep the four corners and only stretch the center part. I use the following code:</p>
  
  <pre><code>
      UIImage *originalImg = [UIImage imageNamed:@"ImageNamed"];
      UIImage *resizeImg = [originalImg resizableImageWithCapInsets:UIEdgeInsetsMake(20 ,10, 10, 10)];
      self.originalImgV.image = originalImg;
      self.resizedImgV.image = resizeImg;
  </code></pre>
  
  <p>Both originalImgV and resizedImgV are set to "aspect fill". I run it on the simulater, and the result is :</p>
  
  <p><img src="https://i.stack.imgur.com/bQjqY.png" alt="enter image description here"></p>
  
  <p>What I can't understand is: the resizedImgV has 2 arrows! 
  Could any one tell me why and how can I use it correctly?
  Thanks</p>