UIScrollView 上拉隐藏 下拉显示导航条

- (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){
       //中止拖拽
   }
}
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