导航栏颜色渐变

1、导航栏开始时是没有导航栏的感觉，往上滚动后又出现导航栏，采用下面的方法

- (void)scrollViewDidScroll:(UIScrollView *)scrollView

{

    CGFloat minAlphaOffset = - 88;

    CGFloat maxAlphaOffset = 200;

    CGFloat offset = scrollView.contentOffset.y;

    CGFloat alpha;

    if (offset <= 0) {

        alpha = 0.0;

    } else {

        alpha = (offset - minAlphaOffset) / (maxAlphaOffset - minAlphaOffset);

    }

    UIView *barImageView = self.navigationController.navigationBar.subviews.firstObject;

    barImageView.alpha = alpha;

}

//设置透明导航栏

    UIView *barImageView = self.navigationController.navigationBar.subviews.firstObject;

    barImageView.alpha = 0.0;