title: How's the iOS core animation works for setting a image with animation
body: <p>I'm wondering how's the iOS core animation working, as the following code, it will performs well by setting the new image with animation.<br>
  I can understand how the animation works if I set the button's position in the animation block - the framework will calculate the each offset for each animation step, based on the button's original position and the target position, then update it's frame for each animation step.
  However this simple logic definitely can not apply for the following case.
  Experts please help me to understand, thanks! </p>
  
  <pre><code>[UIView transitionWithView:self.view
                    duration:0.35
                     options:UIViewAnimationOptionTransitionCrossDissolve
                  animations:^{
                        [myButton setImage:[UIImage imageNamed:@"demo_button_view"] forState:UIControlStateNormal];
                        [myButton setImage:[UIImage imageNamed:@"demo_button_pressed_icon"] forState:UIControlStateHighlighted];
                  }
  </code></pre>