UIScrollViewDelegate

1、滚动时调用（只要offset改变就会调用）
- (void)scrollViewDidScroll:(UIScrollView *)scrollView;// any offset changes
2、即将拖拽
- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView;
3、拖拽结束时调用 decelerate:是否有减速
- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate;
4、将要开始减速
- (void)scrollViewWillBeginDecelerating:(UIScrollView *)scrollView;
5、结束减速
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView;
6、 滚动动画完成后调用（调用下面两个方法（animated=YES）且动画完成时会调用
/*scrollView scrollRectToVisible:<#(CGRect)#> animated:<#(BOOL)#>
scrollView setContentOffset:<#(CGPoint)#> animated:<#(BOOL)#> ）*/
- (void)scrollViewDidEndScrollingAnimation:(UIScrollView *)scrollView;
//下面的方法平时很少使用
7、缩放时调用
- (void)scrollViewDidZoom:(UIScrollView *)scrollView; // any zoom scale changes
8、返回一个将被缩放的view，如果返回nil，则没有效果
- (nullable UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView;
9、scrollView的content缩放之前调用
- (void)scrollViewWillBeginZooming:(UIScrollView *)scrollView withView:(nullable UIView *)view ;
10、缩放结束之后调用 (minimum <=scale <=maximum called after any 'bounce' animations)
- (void)scrollViewDidEndZooming:(UIScrollView *)scrollView withView:(nullable UIView *)view atScale:(CGFloat)scale;
11、是否滑动到最顶部，默认是YES
- (BOOL)scrollViewShouldScrollToTop:(UIScrollView *)scrollView;
12、滑动到顶部后调用 （如果已经在顶部，则无效）
- (void)scrollViewDidScrollToTop:(UIScrollView *)scrollView;
13、called on finger up if the user dragged. velocity is in points/millisecond. targetContentOffset may
be changed to adjust where the scroll view comes to rest
- (void)scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset;