UIScrollView 上拉隱藏 下拉顯示導航條

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
   //scrollView已經有拖拽手勢，直接拿到scrollView的拖拽手勢
   UIPanGestureRecognizer *pan = scrollView.panGestureRecognizer;
   //獲取到拖拽的速度 >0 向下拖動 <0 向上拖動
   CGFloat velocity = [pan velocityInView:scrollView].y;
   if (velocity <- 5) {
       //向上拖動，隱藏導航欄
       [self.navigationController setNavigationBarHidden:YES animated:YES];
   }else if (velocity > 5) {
       //向下拖動，顯示導航欄
       [self.navigationController setNavigationBarHidden:NO animated:YES];
   }else if(velocity == 0){
       //中止拖拽
   }
}
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