title: how to prevent a touch event passed to a UIView's superview?
body: <p>I've made a program which contains pictures that can be dragged to move on the screen.
  also I used UIScrollView since the space is larger than the window.
  I subclassed UIView as a pictures view to display the picture and handle touches in this class.
  picture views are added as subviews to the scrollView.
  but now I have a problem:
  when I drag the picture, the scrollView moves together with the picture.
  I'm wondering if there is a way to consume the touches in the picture class to prevent them from passing to superview.</p>