－1.用手拖动但是一直不离开scroller：

scrollViewWillBeginDragging:

停止拖动后：

scrollViewWillEndDragging:withVelocity:targetContentOffset:
scrollViewDidEndDragging:willDecelerate:

如果decelerate返回YES那么接着触发：

scrollViewWillBeginDecelerating
scrollViewDidEndDecelerating

如果在decelerate的时候按住scrollView，使其停止滑动，那么会触发：

scrollViewWillEndDragging:withVelocity:targetContentOffset:
scrollViewDidEndDragging:willDecelerate:

总结：

grag之后自然停止回调 scrollViewDidEndDecelerating； 如果grag之后，正在decelerating ，使用grag使其停止，那么回调 scrollViewDidEndDragging:willDecelerate:并且decelerate返回NO；

－2.相关属性

@property(nonatomic,readonly,getter=isTracking)     BOOL tracking;        // returns YES if user has touched. may not yet have started dragging
@property(nonatomic,readonly,getter=isDragging)     BOOL dragging;        // returns YES if user has started scrolling. this may require some time and or distance to move to initiate dragging
@property(nonatomic,readonly,getter=isDecelerating) BOOL decelerating;    // returns YES if user isn't dragging (touch up) but scroll view is still moving

开始grag之前，进入tracking 模式； dragging 意味着进入grag状态； decelerating 意味着进入惯性滑动模式；