- (void)scrollViewDidScroll:(UIScrollView *)scrollView {   //scrollView已经有拖拽手势，直接拿到scrollView的拖拽手势   UIPanGestureRecognizer *pan = scrollView.panGestureRecognizer;   //获取到拖拽的速度 >0 向下拖动 <0 向上拖动   CGFloat velocity = [pan velocityInView:scrollView].y;   if (velocity <- 5) {       //向上拖动，隐藏导航栏       [self.navigationController setNavigationBarHidden:YES animated:YES];   }else if (velocity > 5) {       //向下拖动，显示导航栏       [self.navigationController setNavigationBarHidden:NO animated:YES];   }else if(velocity == 0){       //停止拖拽   }}复制代码