Implement animation in UIButton
<p>I'm new to cocoa and iphone programming and I want to implement an animation in UIButton. For example, I create a custom UIButton with a square image. Then when I press that UIButton, the square image will flip. Note that the square image is the image of the UIButton.</p>

<pre><code>[UIButton setImage:[UIImage imageNamed:@"square.png"]];
</code></pre>
