//  设置UIPageViewController的scrollView的代理为self
for (id view in self.pageController.view.subviews) {
    if ([view isKindOfClass:[UIScrollView class]]) {
      UIScrollView *scrollView = (UIScrollView *)view;
      scrollView.delegate = self;
    }
 }

- (void)setOffset:(CGFloat)offset {
    _offset = offset;
    //  Do something with offsetX
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    CGFloat width = scrollView.bounds.size.width;
    CGFloat offset = self.offset;
    NSInteger childCount = self.childControllers.count;
    // 将当前滑动显示的view 的坐标 => self.view 的坐标
    for (UIViewController *vc in self.childControllers) {
      CGPoint p = [vc.view convertPoint:CGPointMake(0, 0) toView:self.view];
      if (p.x > 0 && p.x < width) {
        NSInteger index = [self.childControllers indexOfObject:vc];
        offset = index * width - p.x;
      }
    }
    if (offset >= (childCount - 1) * width) {
      CGPoint p = [self.childControllers.lastObject.view convertPoint:CGPointMake(0, 0) toView:self.view];
      offset = (childCount - 1) * width - p.x;
    }
    self.offset = offset;
}

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView {
    CGFloat width = scrollView.bounds.size.width;
    NSInteger index = round(self.offset / width);
    if (index < 0) {
      index = self.childControllers.count - 1;
    }
    self.offset = index * width;
}