title: how to apply animations for different views sequentially?
body: <p>There are four views on which I need to apply animation.
  If an user tap button, the animation is that <code>view1</code> should hide for 1 second, and then <code>view2</code>, then <code>view3</code>, then <code>view4</code>. </p>
  
  <p>The problem is that how to apply animations in different views with sequence? </p>
  
  <p>I need to use Core Animation to finish it.</p>