scrollViewDidScroll 和scrollViewDidEndScrollingAnimation的区别
UIScrollViewDelegate has got two delegate methods scrollViewDidScroll: and scrollViewDidEndScrollingAnimation: but neither of these tell you when scrolling has completed.
scrollViewDidScroll only notifies you that the scroll view did scroll not that it has finished scrolling.
scrollViewDidEndScrollingAnimation only seems to fire if you programmatically move the scroll view not if the user scrolls.
