Question about CCScrollView

I’m wondering whether CCScrollView is a complete class.

There are two delegate functions:

virtual void scrollViewDidScroll(cocos2d::extension::CCScrollView* view);
virtual void scrollViewDidZoom(cocos2d::extension::CCScrollView* view);

The scrollViewDidScroll is fine. But scrollViewDidZoom is never called. I found the problem is actually :

else if (m_pTouches->count() == 2)

This one is never been true. How can I trigger the scrollViewDidZoom? Thanks