UIscrollview的滑动 视图会出现和消失

//滚动
-(void)scrollViewDidScroll:(UIScrollView*)scrollView{
    if (scrollView.contentOffset.y>10) {
       
        [UIViewanimateWithDuration:0.4animations:^{
            self.DownView.frame= CGRectMake(0,self.view.bounds.size.height,self.view.bounds.size.width,50);
        }];
    }else{
    [UIViewanimateWithDuration:0.4animations:^{
        self.DownView.frame= CGRectMake(0,self.view.bounds.size.height-50,self.view.bounds.size.width,50);
       
    }];
   
    }

}