ios - stop scroll when reach to expected object

ios - stop scroll when reach to expected object

The image https://www.dropbox.com/s/e7b189mzl168efk/scrollview.png is my
scrollview and 4views. What I want is when I pulldown the scrollview it
display the views base on the offset. And I want to stop the scroll when
it reach to end view. Currently it still continue to scroll.
its like
if(scrollView.contentOffset.y == -155)
then scroll will not continue to scroll but i can still scroll back to the
bottom to highlight the view3 or until view1. Please refer on the image.
example code is very helpful.