导航栏随着scrollview的滑动改变颜色透明度

- (void)scrollViewDidScroll:(UIScrollView *)scrollView{

NSLog(@"offset---scroll:%f",scrollView.contentOffset.y);

UIColor *color=blue_jump_color; //blue_jump_color这个是定义的颜色宏

if (scrollView == self.tableView) { //这一步是为了保证滑动tableview时候才让导航栏变色

CGFloat offset=scrollView.contentOffset.y;  //获取y值

      if (offset<=0) {

      _navBackV.backgroundColor =  [color colorWithAlphaComponent:0]; // _navBackV是导航栏   

}else if(offset>0){

                   CGFloat alpha=1-((64-offset)/64);

             _navBackV.backgroundColor =  [color colorWithAlphaComponent:alpha];

            }

      }

}