gistfile1.m

@interface UIScrollView (willBeginScrollingAnimation)
@end

@implementation UIScrollView (willBeginScrollingAnimation)

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wobjc-method-access"
+(void)load {
    Method setContentOffsetAnimatedMethod = class_getInstanceMethod([UIScrollView class], @selector(setContentOffset:animated:));
    IMP oldImplementation = method_getImplementation(setContentOffsetAnimatedMethod);
    method_setImplementation(setContentOffsetAnimatedMethod, imp_implementationWithBlock(^(UIScrollView *_self, CGPoint offset, BOOL animated) {
        if ([_self.delegate respondsToSelector:@selector(scrollViewWillBeginScrollingAnimation:)] && animated &&
            CGPointEqualToPoint(_self.contentOffset, offset) == NO) {
            [_self.delegate scrollViewWillBeginScrollingAnimation:_self];
        }
        oldImplementation(_self, @selector(setContentOffset:animated:), offset, animated);
    }));
}
#pragma clang diagnostic pop
@end

i guess you can just observe 'contentOffset' of scrollview